Review Questions. Module 1

Transcription

1 Review Questions This material is provided for the sole use of students in the 2016 Winter Semester course P4P03 Nuclear Plant Systems and Operation. This material is not to be copied or shared in either electronic or paper form. Module 1

2 1-1. WHIH OF TH FOLLOWING STTMNTS IS INORRT: Far more people die and are injured annually from either smoking, or driving a car, or flying in an airplane than because of the radiation from nuclear power plants. The principal source of radiation hazard in a nuclear plant is due to the highly radioactive fission products that accumulate in the fuel. The LR Principle states that all hazards must be reduced to a level that is s Low s Reasonably chievable. The accident at hernobyl proved that nuclear power plants can blow up just like an atomic bomb. ONTROL, OOL and ONTIN is the short form of the Golden Rule of Reactor Safety.

3 1-1. WHIH OF TH FOLLOWING STTMNTS IS INORRT: Far more people die and are injured annually from either smoking, or driving a car, or flying in an airplane than because of the radiation from nuclear power plants. The principle source of radiation hazard in a nuclear plant is due to the highly radioactive fission products that accumulate in the fuel. The LR Principle states that all hazards must be reduced to a level that is s Low s Reasonably chievable. The accident at hernobyl proved that nuclear power plants can blow up just like an atomic bomb. ONTROL, OOL and ONTIN is the short form of the Golden Rule of Reactor Safety. is incorrect: the explosion of the unit 3 reactor at hernobyl was not a fast fission explosion as in an atomic bomb. lthough the reactor went prompt critical, the explosion was due to the rapid increase in steam pressure.

4 1-2. WHIH OF TH FOLLOWING STTMNTS IS INORRT: Workers at a nuclear power plant are typically ten times more safe while on the job than away from it. No worker at any of the anadian nuclear plant has been killed or permanently injured by radiation. Nuclear plant workers are usually twice as safe at their work than the average employee anywhere else in the same utility. There are internationally recognized standards against which safety performance needs to be measured. mployees have more control over what they do at work than in their private lives.

5 1-2. WHIH OF TH FOLLOWING STTMNTS IS INORRT: Workers at a nuclear power plant are typically ten times more safe while on the job than away from it. No worker at any of the anadian nuclear plant has been killed or permanently injured by radiation. Nuclear plant workers are usually twice as safe at their work than the average employee anywhere else in the same utility. There are internationally recognized standards against which safety performance needs to be measured. mployees have more control over what they do at work than in their private lives. is incorrect: employees usually have to follow procedures and the directions of their supervisors, so they have less control over what they do than in their private lives. Yet, because of strict safety standards at nuclear power plants, workers are typically ten times safer at work than away from it.

6 1-3. WHIH OF TH FOLLOWING STTMNTS IS INORRT: ngineered safety systems provide back-up to the process and control systems to assure reactor safety. Process systems must be reliable as there are the first line of defense in reactor safety. Safety systems are poised to act in order to mitigate and if necessary accommodate the consequences of a reactor accident. Process systems should operate continuously, if they stop, accidents will occur. Highly reliable safety systems provide the second level of protection to ensure reactor safety.

7 1-3. WHIH OF TH FOLLOWING STTMNTS IS INORRT: ngineered safety systems provide back-up to the process and control systems to assure reactor safety. Process systems must be reliable as there are the first line of defense in reactor safety. Safety systems are poised to act in order to mitigate and if necessary accommodate the consequences of a reactor accident. Process systems should operate continuously, if they stop, accidents will occur. Highly reliable safety systems provide the second level of protection to ensure reactor safety. is incorrect: shutting down a process system is not a cause of accidents, although operating procedures may be different from when the system is under normal operating conditions.

8 1-4. WHIH OF TH FOLLOWING STTMNTS IS INORRT: The containment system provides a relatively leak tight envelope that is maintained slightly below atmospheric pressure. The uranium fuel is molded into ceramic pellets which have a high melting point and lock in most of the fission products. n exclusion zone of at least two kilometer radius around the reactor is required to minimize public exposure to radiation. The heat transport system is constructed of high strength pressure tubes, piping and vessels and contains the fuel bundles. The fuel sheath that contains the ceramic fuel is made of high integrity welded metal (zircaloy)

9 1-4. WHIH OF TH FOLLOWING STTMNTS IS INORRT: The containment system provides a relatively leak tight envelope that is maintained slightly below atmospheric pressure. The uranium fuel is molded into ceramic pellets which have a high melting point and lock in most of the fission products. n exclusion zone of at least two kilometer radius around the reactor is required to minimize public exposure to radiation. The heat transport system is constructed of high strength pressure tubes, piping and vessels and contains the fuel bundles. The fuel sheath that contains the ceramic fuel is made of high integrity welded metal (zircaloy). is incorrect. xclusion zones are typically 0.5 to 1 km in radius.

10 1-1. WHIH OF TH FOLLOWING STTMNTS IS INORRT: Under normal operating conditions the heat generated by the fuel in the Reactor is removed by the Heat Transport System and is transferred in the Steam Generators to the Feedwater. The alandria of a NU Reactor is a stainless steel horizontal cylindrical vessel that holds the heavy water moderator & reflector. The heat transport coolant is under high pressure, so the coolant will not boil during normal operations. The Steam Generators transfer the heat from the heavy water coolant of the heat transport system on the primary side to the light water on the secondary side to form steam. The average residency time of a fuel bundle in the reactor is approximately one year.

11 1-1. WHIH OF TH FOLLOWING STTMNTS IS INORRT: Under normal operating conditions the heat generated by the fuel in the Reactor is removed by the Heat Transport System and is transferred in the Steam Generators to the Feedwater. The alandria of a NU Reactor is a stainless steel horizontal cylindrical vessel that holds the heavy water moderator & reflector. The heat transport coolant is under high pressure, so the coolant will not boil during normal operations. The Steam Generators transfer the heat from the heavy water coolant of the heat transport system on the primary side to the light water on the secondary side to form steam. The average residency time of a fuel bundle in the reactor is approximately one year. is incorrect: some boiling of the heat transport coolant is permitted in NU reactors.

12 1-2. WHIH OF TH FOLLOWING STTMNTS IS INORRT: The Reactor Regulating System computes the signals that position the reactivity control devices. In-core flux detectors are distributed spatially in a NU Reactor, and measure both the total flux and its spatial distribution. Measurements of thermal power are used to calibrate the bulk and spatial neutron flux signals. ll the control logic required for Reactor Regulation is implemented by digital computer programs. hanges in bulk reactor power and in the spatial flux distribution are achieved by altering the level of light water in 14 compartments that are distributed in the reactor core in the form of vertical zone control units.

13 1-2. WHIH OF TH FOLLOWING STTMNTS IS INORRT: The Reactor Regulating System computes the signals that position the reactivity control devices. In-core flux detectors are distributed spatially in a NU Reactor, and measure both the total flux and its spatial distribution. Measurements of thermal power are used to calibrate the bulk and spatial neutron flux signals. ll the control logic required for Reactor Regulation is implemented by digital computer programs. hanges in bulk reactor power and in the spatial flux distribution are achieved by altering the level of light water in 14 compartments that are distributed in the reactor core in the form of vertical zone control units. is incorrect: there are some hardware interlocks in the control logic of the Reactor Regulating System.

14 1-3. WHIH OF TH FOLLOWING STTMNTS IS INORRT: Self-powered in-core detectors are used to measure the neutron flux below about 1%FP. Ion chambers are located on the outside of the calandria shell and measure the neutron leakage flux. Platinum flux detectors have fast response to changes in neutron flux, and can be used to control both the spatial flux distribution and the total reactor power level of the Reactor. Vanadium flux detectors are sensitive to neutrons and not to gammas, but have a slow response. The thermal power produced by the Reactor can be determined from measurements of Steam flow, Steam pressure, Feedwater flow and Feedwater temperature.

15 1-3. WHIH OF TH FOLLOWING STTMNTS IS INORRT: Self-powered in-core detectors are used to measure the neutron flux below about 1%FP. Ion chambers are located on the outside of the calandria shell and measure the neutron leakage flux. Platinum flux detectors have fast response to changes in neutron flux, and can be used to control both the spatial flux distribution and the total reactor power level of the Reactor. Vanadium flux detectors are sensitive to neutrons and not to gammas, but have a slow response. The thermal power produced by the Reactor can be determined from measurements of Steam flow, Steam pressure, Feedwater flow and Feedwater temperature. is incorrect: the in-core flux detectors cannot measure the neutron flux below 1% of full power.

16 1-4. WHIH OF TH FOLLOWING STTMNTS IS INORRT: The Flux Mapping program uses the vanadium detector signals to compute the spatial power distribution in the reactor The Power Measurement and alibration program computes corrections to the Platinum flux detector readings by using the results of the Flux Mapping Program and the measurements of Thermal Power. The emanded Power Routine program calculates the power level that the operator should demand. The Setback Routine will reduce reactor power at rates between 0.5 and 1%FP/sec to power levels in the range from 0 to 60%FP while the condition causing the Setback is in effect. The Stepback logic allows the control absorber rods to drop fully or partially into the reactor core, resulting in a step reduction of reactor power.

17 1-4. WHIH OF TH FOLLOWING STTMNTS IS INORRT: The Flux Mapping program uses the vanadium detector signals to compute the spatial power distribution in the reactor The Power Measurement and alibration program computes corrections to the Platinum flux detector readings by using the results of the Flux Mapping Program and the measurements of Thermal Power. The emanded Power Routine program calculates the power level that the operator should demand. The Setback Routine will reduce reactor power at rates between 0.5 to 1%FP/sec to power levels in the range from 0 to 60%FP while the condition causing the Setback is in effect. The Stepback logic allows the control absorber rods to drop fully or partially into the reactor core, resulting in a step reduction of reactor power. is incorrect: the emanded Power Routine (a) selects the unit s operating mode, (b) calculates the reactor power setpoint, and (c) calculates the effective power error.

18 1-5. WHIH OF TH FOLLOWING STTMNTS IS INORRT: The liquid zone control system provides the means of fine control of bulk reactor power, as well as control of the spatial flux distribution. NU reactors are made up of 14 regions with the flux controlled independently in each zone. Four Mechanical ontrol bsorber rods can be driven into the reactor when negative reactivity to supplement the liquid zones is required. Up to 21 neutron absorbing djuster Rods are normally located in the reactor for the purpose of flattening the neutron flux and to provide positive reactivity when needed. Hardware interlocks control the movement of the neutron absorbing control and shutdown rods to ensure that the maximum amount and rate of reactivity insertion never exceeds the design limits.

19 1-5. WHIH OF TH FOLLOWING STTMNTS IS INORRT: The liquid zone control system provides the means of fine control of bulk reactor power, as well as control of the spatial flux distribution. NU reactors are made up of 14 regions with the flux controlled independently in each zone. Four Mechanical ontrol bsorber rods can be driven into the reactor when negative reactivity to supplement the liquid zones is required. Up to 21 neutron absorbing djuster Rods are normally located in the reactor for the purpose of flattening the neutron flux and to provide positive reactivity when needed. Hardware interlocks control the movement of the neutron absorbing control and shutdown rods to ensure that the maximum amount and rate of reactivity insertion never exceeds the design limits. is incorrect: the bulk reactor flux is controlled in the zones by changing the zone levels in unison (i.e. the same amount); for the control of the spatial flux the individual zone levels may deviate from the average to correct flux tilts.

20 1-6. WHIH OF TH FOLLOWING STTMNTS IS INORRT: Typical refuelling operations require that eight fuel bundles be replaced in one or two channels per day. The fuel in a NU reactor is normally changed during full power operations, using two fuelling machines connected to the ends of a fuel channel. ll the transfer operations from the Fuelling Machine to the Irradiated Fuel Storage ay take place under water, ensuring that the fuel is cooled at all times during removal and transfer. Fuel bundles, typically eight at a time, can be loaded manually into the reactor during shutdown. Natural uranium fuel, new or irradiated, cannot achieve criticality in air or in ordinary (light) water, regardless of the storage configuration of the fuel.

21 1-6. WHIH OF TH FOLLOWING STTMNTS IS INORRT: Typical refueling operations require that eight fuel bundles are replaced in one or two channels per day. The fuel in a NU reactor is normally changed during full power operations, using two fuelling machines connected to the ends of a fuel channel. ll the transfer operations from the Fuelling Machine to the Irradiated Fuel Storage ay take place under water, ensuring that the fuel is cooled at all times during removal and transfer. Fuel bundles, typically eight at a time, can be loaded manually into the reactor during shutdown. Natural uranium fuel, new or irradiated, cannot achieve criticality in air or in ordinary (light) water, regardless of the storage configuration of the fuel. is incorrect: fuel bundles may be loaded manually into the fuelling machine, but not into the reactor once the reactor had been made critical, even if it has been shut down. (However, the very first load of fuel may be loaded manually directly into the fuel channels.)

22 1-7. WHIH OF TH FOLLOWING STTMNTS IS INORRT: ll NU reactors use heavy water as the moderator, in a system that is completely separate from the reactor coolant heavy water. The Moderator Main ircuit removes the heat generated in the moderator during reactor operation and maintains the moderator level in the alandria. The over Gas System above the free moderator surface prevents moisture in the air downgrading the heavy water concentration. The Liquid Poison System provides a means of reactivity control by adding poison to the moderator. The Moderator Purification System removes light water impurities from the heavy water moderator.

23 1-7. WHIH OF TH FOLLOWING STTMNTS IS INORRT: ll NU reactors use heavy water as the moderator, in a system that is completely separate from the reactor coolant heavy water. The Moderator Main ircuit removes the heat generated in the moderator during reactor operation and maintains the moderator level in the alandria. The over Gas System above the free moderator surface prevents moisture in the air down-grading the heavy water concentration. The Liquid Poison System provides a means of reactivity control by adding poison to the moderator. The Moderator Purification System removes light water impurities from the heavy water moderator. is incorrect: the main function of the Moderator Purification System is to remove liquid poison (oron and Gadolinium) from the Moderator heavy water.

24 1-8. WHIH OF TH FOLLOWING STTMNTS IS INORRT: The Heat Transport System must continuously cool the fuel and contain any fission products. The Main irculating pump/motor units have sufficient rotational inertia so that, on loss of motor power, the rate of coolant flow reduction matches the reactor power rundown following a reactor trip. The feeders that connect each fuel channel to the reactor inlet and outlet headers are sized such that the coolant flow to each channel is proportional to the power output of the channel. Liquid poison added to the Heat Transport system can provide additional reactivity control. The Pressurizer maintains the required Heat Transport System pressure under normal operating conditions by the addition or removal of heat from the heavy water in the Pressurizer.

25 1-8. WHIH OF TH FOLLOWING STTMNTS IS INORRT: The Heat Transport System must continuously cool the fuel and contain any fission products. The Main irculating pump/motor units have sufficient rotational inertia so that, on loss of motor power, the rate of coolant flow reduction matches the reactor power rundown following a reactor trip. The feeders that connect each fuel channel to the reactor inlet and outlet headers are sized such that the coolant flow to each channel is proportional to the power output of the channel. Liquid poison added to the Heat Transport system can provide additional reactivity control. The Pressurizer maintains the required Heat Transport System pressure under normal operating conditions by the addition or removal of heat from the heavy water in the Pressurizer. is incorrect: in NU reactors poison for reactivity control is never added to the Heat Transport oolant, it is always added to the Moderator System.

26 1-9. WHIH OF TH FOLLOWING STTMNTS IS INORRT: When the turbine cannot accept the full steam flow, the excess steam can be discharged to the atmosphere or bypass the turbine by flowing directly to the condenser. Three out of the four Main Steam Safety Valves provide 115% of the flow from each steam generator. The tmospheric Steam ischarge Valves have a capacity of 10% of the unit s full power steam flow. The ondenser Steam ischarge Valves can pass 100% steam flow to the ondenser indefinitely. The Turbine Stop Valves close rapidly when required to protect the turbine against over-speed.

27 1-9. WHIH OF TH FOLLOWING STTMNTS IS INORRT: When the turbine cannot accept the full steam flow, the excess steam can be discharged to the atmosphere or bypass the turbine by flowing directly to the condenser. Three out of the four Main Steam Safety Valves provide 115% of the flow from each steam generator. The tmospheric Steam ischarge Valves have a capacity of 10% of the unit s full power steam flow. The ondenser Steam ischarge Valves can pass 100% steam flow to the ondenser indefinitely. The Turbine Stop Valves close rapidly when required to protect the turbine against over-speed. is incorrect: the ondenser Steam ischarge Valves can only pass 100% steam flow to the condenser for a short period (a few minutes). Long term capacity is 60% full power steam flow. Note that in Section 11 of Module 1, item (5) should say 12 instead of 2 ondenser Steam ischarge Valves.

28 1-10. WHIH OF TH FOLLOWING STTMNTS IS INORRT: The feedwater system supplies demineralized and preheated light water to the steam generators. The flow of feedwater to each Steam Generator is controlled to a fixed value to maintain a constant water level in the Steam Generators. Feedwater flow measurements are used for the computation of Reactor Thermal Power. Steam Flow measurements are input to the Steam Generator Level ontrol program to control the opening of the Feedwater valves. The Steam Generator Level setpoint is varied as a function of reactor power to ensure a consistent inventory of water in the Steam Generators.

29 1-10. WHIH OF TH FOLLOWING STTMNTS IS INORRT: The feedwater system supplies demineralized and preheated light water to the steam generators. The flow of feedwater to each Steam Generator is controlled to a fixed value to maintain a constant water level in the Steam Generators. Feedwater flow measurements are used for the computation of Reactor Thermal Power. Steam Flow measurements are input to the Steam Generator Level ontrol program to control the opening of the Feedwater valves. The Steam Generator Level setpoint is varied as a function of reactor power to ensure a consistent inventory of water in the Steam Generators. is incorrect: the flow of feedwater is varied by the steam generator level control system to maintain a constant inventory of water in each of the steam generators.

30 1-1. WHIH OF TH FOLLOWING SYSTMS IS NOT PRT OF TH OP OR ONVNTIONL PRT OF NULR GNRTING UNIT: The low pressure turbine. The main condenser. The governor valves. The pressurizer. The feedwater heating system.

31 1-1. WHIH OF TH FOLLOWING SYSTMS IS NOT PRT OF TH OP OR ONVNTIONL PRT OF NULR GNRTING UNIT: The low pressure turbine. The main condenser. The governor valves. The pressurizer. The feedwater heating system. is incorrect: the Pressurizer is part of the Primary or Nuclear Steam Supply System.

32 1-2. WHIH OF TH FOLLOWING STTMNTS IS INORRT: The main output transformer raises the generator output voltage to that of the electrical power grid. The switchyard changes the source of electrical supply between the station and unit service transformers. Normally the unit s electrical loads are supplied by both the unit and the station service transformers. ither the unit or the station service transformer is capable of supplying all the electrical loads of a unit The nuclear reactor can continue to operate even if the unit is disconnected from the electrical grid.

33 1-2. WHIH OF TH FOLLOWING STTMNTS IS INORRT: The main output transformer raises the generator output voltage to that of the electrical power grid. The switchyard changes the source of electrical supply between the station and unit service transformers. Normally the unit s electrical loads are supplied by both the unit and the station service transformers. ither the unit or the station service transformer is capable of supplying all the electrical loads of a unit the nuclear reactor can continue to operate even if the unit is disconnected from the electrical grid. is incorrect: the primary purpose of the switchyard is to connect the unit to the grid.

34 1-3. WHIH OF TH FOLLOWING STTMNTS IS INORRT: lass IV power supplies the largest electrical loads of a unit but is the least reliable of the four classes. lass III power may be supplied from either lass IV or from Standby Generators. lass I power supplies uninterruptible current to triplicated reactor safety circuits. Uninterruptible power is supplied from lass II buses to critical motor and instrument loads. The reactor of a nuclear power plant can continue to operate at full power without lass IV power supplies.

35 1-3. WHIH OF TH FOLLOWING STTMNTS IS INORRT: lass IV power supplies the largest electrical loads of a unit but is the least reliable of the four classes. lass III power may be supplied from either lass IV or from Standby Generators. lass I power supplies uninterruptible current to triplicated reactor safety circuits. Uninterruptible power is supplied from lass II buses to critical motor and instrument loads. The reactor of a nuclear power plant can continue to operate at full power without lass IV power supplies. is incorrect: the reactor must be tripped and shut down if there is failure of lass IV power, since the full-power heat generated in the fuel cannot be removed if the Heat Transport Main Pumps, which are connected to lass IV power, are not operational.

36 1-4. WHIH OF TH FOLLOWING STTMNTS IS INORRT: Most major control systems in modern nuclear power plants (NPPs) use digital computer technology. NUs use dual redundant digital control computers (s) for station control and information display. If one fails, control of field devices will be automatic switched to the other. If both s fail, control of field devices will be automatic switched to the spare computer. Input signals are checked for rationality in absolute and relative values, and are rejected if irrational.

37 1-4. WHIH OF TH FOLLOWING STTMNTS IS INORRT: Most major control systems in modern nuclear power plants (NPPs) use digital computer technology. NUs use dual redundant digital control computers (s) for station control and information display. If one fails, control of field devices will be automatic switched to the other. If both s fail, control of field devices will be automatic switched to the spare computer. Input signals are checked for rationality in absolute and relative values, and are rejected if irrational. is incorrect: no field devices are connected to the spare computer. If both s fail, the reactor must be shut down.

38 1-5. WHIH OF TH FOLLOWING STTMNTS IS INORRT: ll nuclear power plants have one full capability and one back-up, slow acting reactor shutdown system. Solid neutron absorbing rods dropped into the core is the common way to shut down nuclear reactors. Injecting liquid poison into the heat transport system cannot produce fast reactor shutdown the two full capability NU reactor shutdown systems are functionally and physically independent. Injecting liquid poison into the moderator system of a NU reactor produces fast reactor shutdown.

39 1-5. WHIH OF TH FOLLOWING STTMNTS IS INORRT: ll nuclear power plants have one full capability and one back-up, slow acting reactor shutdown system. Solid neutron absorbing rods dropped into the core is the common way to shut down nuclear reactors. Injecting liquid poison into the heat transport system cannot produce fast reactor shutdown the two full capability NU reactor shutdown systems are functionally and physically independent. Injecting liquid poison into the moderator system of a NU reactor produces fast reactor shutdown. is incorrect: NU units have two full capability fast acting reactor shutdown systems.

40 1-6. WHIH OF TH FOLLOWING STTMNTS IS INORRT: The need for mergency ore oolant Injection is indicated when heat transport system pressure falls below a specified value, accompanied by either high reactor building pressure, or high moderator level, or sustained low reactor outlet header pressure. High pressure injection of emergency core coolant is provided by a combination of water tanks and high pressure gas tanks to all the reactor headers. There are four stages to emergency core coolant injection: (1) high pressure (2) medium pressure, (3) low pressure and (4) very low pressure. Medium pressure injection is provided by pumping water from the dousing tank to the reactor headers. Long term low pressure recirculation and cooling is provided by pumping the spilled water accumulating in the reactor basement back to the reactor headers via heat exchangers

41 1-6. WHIH OF TH FOLLOWING STTMNTS IS INORRT: The need for mergency ore oolant Injection is indicated when heat transport system pressure falls below a specified value, accompanied by either high reactor building pressure, or high moderator level, or sustained low reactor outlet header pressure. High pressure injection of emergency core coolant is provided by a combination of water tanks and high pressure gas tanks to all the reactor headers. There are four stages to emergency core coolant injection: (1) high pressure (2) medium pressure, (3) low pressure and (4) very low pressure. Medium pressure injection is provided by pumping water from the dousing tank to the reactor headers. Long term low pressure recirculation and cooling is provided by pumping the spilled water accumulating in the reactor basement back to the reactor headers via heat exchangers is incorrect: there is no very low pressure stage, only the other three.

42 1-7. WHIH OF TH FOLLOWING STTMNTS IS INORRT: The ontainment System forms one of the five barriers that protect the public from the accidental release of radioactivity from a nuclear reactor. The airlocks ensure that any radioactivity that escapes into the air from the reactor is locked inside the reactor building. The dousing system is designed to prevent an excessive buildup of pressure inside the reactor building. For a small LO the building coolers would maintain building pressure at atmospheric level. If radioactivity is detected in the reactor building ventilation system, the containment system is automatically isolated from the outside environment.

43 1-7. WHIH OF TH FOLLOWING STTMNTS IS INORRT: The ontainment System forms one of the five barriers that protect the public from the accidental release of radioactivity from a nuclear reactor. The airlocks ensure that any radioactivity that escapes into the air from the reactor is locked inside the reactor building. The dousing system is designed to prevent an excessive buildup of pressure inside the reactor building. For a small LO the building coolers would maintain building pressure at atmospheric level. If radioactivity is detected in the reactor building ventilation system, the containment system is automatically isolated from the outside environment. is incorrect: the irlocks are used to provide access for people and equipment into the reactor building while maintaining containment.

44 1-8. WHIH OF TH FOLLOWING STTMNTS IS INORRT: nuclear plant must be built on the shores of a large body of water to cool the reactor and condensers. nuclear plant may be built within less than 1 km of residential areas. Nuclear power plants can be designed to safely survive the maximum level of seismic activity predicted for the site. ccess to the site of a nuclear power plant is usually by a combination of road, rail and water. The power plant site is usually located so as to be easily connected to the electrical power system.

45 1-8. WHIH OF TH FOLLOWING STTMNTS IS INORRT: nuclear plant must be built on the shores of a large body of water to cool the reactor and condensers. nuclear plant may be built within less than 1 km of residential areas. Nuclear power plants can be designed to safely survive the maximum level of seismic activity predicted for the site. ccess to the site of a nuclear power plant is usually by a combination of road, rail and water. The power plant site is usually located so as to be easily connected to the electrical power system. is incorrect: by using cooling towers, the reactor and the condensers can be cooled without having to locate the power plant on the shores of a large body of water.

46 1-1. WHIH OF TH FOLLOWING STTMNTS IS INORRT: The purpose of overall unit control is to maintain operating conditions in the plant consistent with the power level specified by the plant operator. The overall unit control system must be able to maintain the frequency of the electrical generation within the specified limits, even when disconnected from electric power grid. hanges in the flow and temperature of the condenser cooling water can affect the power output of a nuclear generating unit. The purpose of overall unit control is to produce the electricity needs of the electrical power system. Fluctuations in heat transport temperature and pressure, and/or variations in the flow and pressure of the steam system may be indications of a lack of overall unit control.

47 1-1. WHIH OF TH FOLLOWING STTMNTS IS INORRT: The purpose of overall unit control is to maintain operating conditions in the plant consistent with the power level specified by the plant operator. The overall unit control system must be able to maintain the frequency of the electrical generation within the specified limits, even when disconnected from electric power grid. hanges in the flow and temperature of the condenser cooling water can affect the power output of a nuclear generating unit. The purpose of overall unit control is to produce the electricity needs of the electrical power system. Fluctuations in heat transport temperature and pressure, and/or variations in the flow and pressure of the steam system may be indications of a lack of overall unit control. is incorrect: the electric power system may need more electricity than the unit can provide, and overall unit control is designed not to exceed the power production capability of the unit.

48 1-2. WHIH OF TH FOLLOWING STTMNTS IS INORRT: The Reactor Regulating System maneuvers reactor power to the required level and keeps it there. The Heat Transport Pressure ontrol System maintains a constant level of high coolant pressure. Steam Generator Level ontrol is achieved by altering the openings of the Feedwater Flow ontrol Valves. hanging the openings of the Governor Valves will alter the power produced by the turbine-generator. If steam pressure exceeds the safety limit, the Safety Relief Valves bypass steam flow to the condenser.

49 1-2. WHIH OF TH FOLLOWING STTMNTS IS INORRT: The Reactor Regulating System maneuvers reactor power to the required level and keeps it there. The Heat Transport Pressure ontrol System maintains a constant level of high coolant pressure. Steam Generator Level ontrol is achieved by altering the openings of the Feedwater Flow ontrol Valves. hanging the openings of the Governor Valves will alter the power produced by the turbine-generator. If steam pressure exceeds the safety limit, the Safety Relief Valves bypass steam flow to the condenser. is incorrect: the flow through the Safety Relief Valves is to the atmosphere, and not to the ondenser.

50 1-3. WHIH OF TH FOLLOWING STTMNTS IS INORRT: The turbine-leading-reactor mode of operation means that the turbine comes ahead of the reactor when computing the power error. The difference between actual generator output and its setpoint is called the generator power error. hanging the openings of the governor valves will change the amount of steam flowing through them, and will change the pressure in the steam generators. In all NU 6 and 9 units the station control system works with the steam generator pressure setpoint kept constant during normal power operations. In the turbine-leading-reactor mode of operation the unit operator specifies generator electrical output and its rate of change as the setpoint for the overall unit control system.

51 1-3. WHIH OF TH FOLLOWING STTMNTS IS INORRT: The turbine-leading-reactor mode of operation means that the turbine comes ahead of the reactor when computing the power error. The difference between actual generator output its setpoint is called the generator power error. hanging the openings of the governor valves will change the amount of steam flowing through them, and will change the pressure in the steam generators. In all NU 6 and 9 units the station control system works with the steam generator pressure setpoint kept constant during normal power operations. In the turbine-leading-reactor mode of operation the unit operator specifies generator electrical output and its rate of change as the setpoint for the overall unit control system. is incorrect: in the turbine-leading-reactor mode of operations, requests for changes in power production are applied to the turbine-generator, which will change first, followed later by the reactor, in response to steam generator pressure changes

52 1-4. WHIH OF TH FOLLOWING STTMNTS IS INORRT: In the reactor-leading-turbine mode of operation, the unit operator specifies the demanded level of reactor power and its rate of change as the setpoint. reactor power error will result in changing the positions of the reactivity mechanisms so as to eliminate the error. t a given governor valve opening, increases in reactor power will increase steam generator pressure, and reductions in reactor power will result in a reduction of steam pressure. In the reactor-leading-turbine mode of operation the governor valve responds to changes in steam generator pressure. n increase in reactor power will raise heat transport system pressure, causing the relief valve to open.

53 1-4. WHIH OF TH FOLLOWING STTMNTS IS INORRT: In the reactor-leading-turbine mode of operation, the unit operator specifies the demanded level of reactor power and its rate of change as the setpoint. reactor power error will result in changing the positions of the reactivity mechanisms so as to eliminate the error. t a given governor valve opening, increases in reactor power will increase steam generator pressure, and reductions in reactor power will result in a reduction of steam pressure. In the reactor-leading-turbine mode of operation the governor valve responds to changes in steam generator pressure. n increase in reactor power will raise heat transport system pressure, causing the relief valve to open. is incorrect: Heat Transport Pressure is normally kept constant (unless there is some equipment failure), irrespective of changes in reactor power level.

54 1-5. WHIH OF TH FOLLOWING STTMNTS IS INORRT: In NORML mode the Unit Power Regulator sends a signal to the Turbine ontroller to adjust the Governor valves to eliminate any generator power error. In LTRNT mode the Reactor Regulating System uses the operator input target values to change the reactor power setpoint from its existing value to the new value at the specified rate. In MNUL mode the operator computes the power error and takes the necessary corrective action. In NORML mode the Steam Generator Pressure ontroller sends a new setpoint to the Reactor Regulating System whenever there is a steam generator pressure error. In all three modes of control if the steam pressure rises above predetermined levels, the Steam Generator Pressure ontroller will open the SVs, and if the pressure continues to rise, the SVs.

55 1-5. WHIH OF TH FOLLOWING STTMNTS IS INORRT: In NORML mode the Unit Power Regulator sends a signal to the Turbine ontroller to adjust the Governor valves to eliminate any generator power error. In LTRNT mode the Reactor Regulating System uses the operator input target values to change the reactor power setpoint from its existing value to the new value at the specified rate. In MNUL mode the operator computes the power error and takes the necessary corrective action. In NORML mode the Steam Generator Pressure ontroller sends a new setpoint to the Reactor Regulating System whenever there is a steam generator pressure error. In all three modes of control if the steam pressure rises above predetermined levels, the Steam Generator Pressure ontroller will open the SVs, and if the pressure continues to rise, the SVs. is incorrect: in MNUL mode the operator controls the Turbine via the speeder gear, can also control manually the SVs and SVs, but the Reactor Regulating System programs continue to operate in LTRNT Mode and compute the power error.

56 1-6. WHIH OF TH FOLLOWING STTMNTS IS INORRT: The Unit Power Regulator controls Generator Output by varying the opening of the Governor Valves. The Steam Generator Pressure ontrol program controls the pressure in the Steam Generators by varying the amount of heat they receive from the Heat Transport System. The Reactor Regulating System controls the total neutron flux and its spatial distribution in the Reactor. Under normal operating conditions the Pressure of the Heat Transport System is controlled by the pressure in the Pressurizer, and the oolant Inventory is controlled via the level of the Pressurizer. Steam Generator Level is controlled so that the inventory of light water they hold is constant

57 1-6. WHIH OF TH FOLLOWING STTMNTS IS INORRT: The Unit Power Regulator controls Generator Output by varying the opening of the Governor Valves. The Steam Generator Pressure ontrol program controls the pressure in the Steam Generators by varying the amount of heat they receive from the Heat Transport System. The Reactor Regulating System controls the total neutron flux and its spatial distribution in the Reactor. Under normal operating conditions the Pressure of the Heat Transport System is controlled by the pressure in the Pressurizer, and the oolant Inventory is controlled via the level of the Pressurizer. Steam Generator Level is controlled so that the inventory of light water they hold is constant. is incorrect: SGP controls SG pressure by either sending a setpoint change request to RRS (in NORML mode), or by altering the Governor Valve opening (in LTRNT mode), and by altering the openings of the SVs and/or SVs

58 Review Questions Module 2

59 2-1. WHIH OF TH FOLLOWING STTMNTS IS INORRT: The alandria of a NU Reactor is a large horizontal cylinder shaped vessel that contains the heavy water Moderator. The alandria and the nd Shields are the main structural components that hold the fuel channels. The flow of reactor coolant in adjacent pressure tubes is in opposite directions, resulting in bi-directional coolant flow through the core. NU 6 reactors have 360 fuel channels and NU 9 reactors have 490 fuel channels. With the exceptions of the alandria-shield Tank assembly, all major components of a NU reactor can be refurbished or replaced to achieve a 60 year operating life.

60 2-1. WHIH OF TH FOLLOWING STTMNTS IS INORRT: The alandria of a NU Reactor is a large horizontal cylinder shaped vessel that contains the heavy water Moderator. The alandria and the nd Shields are the main structural components that hold the fuel channels. The flow of reactor coolant in adjacent pressure tubes is in opposite directions, resulting in bi-directional coolant flow through the core. NU 6 reactors have 360 fuel channels and NU 9 reactors have 490 fuel channels. With the exceptions of the alandria-shield Tank assembly, all major components of a NU reactor can be refurbished or replaced to achieve a 60 year operating life. is incorrect: NU 6 reactors have 380 fuel channels and NU 9 reactors have 480 fuel channels.

61 2-2. WHIH OF TH FOLLOWING STTMNTS IS INORRT: NU 9 Reactor ssembly includes a Shield Tank and two nd Shields, but a NU 6 Reactor ssembly has nd Shields and the Vault holds the biological shield water. The nd Shields are filled with steel balls to allow access to the reactor face during power operations. The NU Reactor ore is the volume that holds the fuel, and corresponds to the pressure tubes and the moderator that surrounds them. The NU 9 Reactor ore is 6 metres long and has a diameter of 7 metres. The alandria and the Moderator extend about 70 centimetres beyond the core to act as a reflector of neutrons.

62 2-2. WHIH OF TH FOLLOWING STTMNTS IS INORRT: NU 9 Reactor ssembly includes a Shield Tank and two nd Shields, but a NU 6 Reactor ssembly has nd Shields and the Vault holds the biological shield water. The nd Shields are filled with steel balls to allow access to the reactor face during power operations. The NU Reactor ore is the volume that holds the fuel, and corresponds to the pressure tubes and the moderator that surrounds them. The NU 9 Reactor ore is 6 metres long and has a diameter of 7 metres. The alandria and the Moderator extend about 70 centimetres beyond the core to act as a reflector of neutrons. is incorrect: The nd Shields only provide sufficient shielding to allow personnel access to the reactor face when the reactor is shut down.

63 2-3. WHIH OF TH FOLLOWING STTMNTS IS INORRT: The Pressure Tubes hold the fuel bundles and the high pressure heavy water coolant. The ratio of length to diameter of a pressure tube is approximately 35:1. The Zirconium and 2.5% Niobium Pressure Tubes are connected to stainless steel nd Fittings. Feeder Pipes connect the nd Fittings to the reactor inlet and outlet headers. The Positioning ssembly holds both ends of the pressure tube firmly in place.

64 2-3. WHIH OF TH FOLLOWING STTMNTS IS INORRT: The Pressure Tubes hold the fuel bundles and the high pressure heavy water coolant. The ratio of length to diameter of a pressure tube is approximately 35:1. The Zirconium and 2.5% Niobium Pressure Tubes are connected to stainless steel nd Fittings. Feeder Pipes connect the nd Fittings to the reactor inlet and outlet headers. The Positioning ssembly holds both ends of the pressure tube firmly in place. is incorrect: Only one end of the pressure tube is locked in place by the positioning assembly, the other end can move to allow for pressure tube elongation resulting from normal operations.

65 2-4. WHIH OF TH FOLLOWING STTMNTS IS INORRT: The horizontal flux measuring devices are located parallel to the fuel channels. In NU 6 and 9 Reactors the fuel channels are arranged on a square lattice pitch of 286 mm. One reason for the spacing of the fuel channels is to allow sufficient access to the pressure tubes by the fuelling machines. The horizontal liquid poison injection assemblies of Reactor Shutdown System 2 are located perpendicular to the fuel channels. The relatively wide spacing of the fuel channels promotes thermalization of neutrons.

66 2-4. WHIH OF TH FOLLOWING STTMNTS IS INORRT: The horizontal flux measuring devices are located parallel to the fuel channels. In NU 6 and 9 Reactors the fuel channels are arranged on a square lattice pitch of 286 mm. One reason for the spacing of the fuel channels is to allow sufficient access to the pressure tubes by the fuelling machines. The horizontal liquid poison injection assemblies of Reactor Shutdown System 2 are located perpendicular to the fuel channels. The relatively wide spacing of the fuel channels promotes thermalization of neutrons. is incorrect: The horizontal flux measuring devices (used for Reactor Shutdown System 2 Trips) are located perpendicular to the fuel channels.

67 2-5. WHIH OF TH FOLLOWING STTMNTS IS INORRT: There are 12 fuel bundles in each NU 6 and NU 9 fuel channel. The fuel pellets for NU are made of natural uranium dioxide. 30 such pellets fit into a fuel pencil. The approximately 50-cm length of the NU fuel bundle is very suitable for manually loading it into the reactor. The 37-fuel-pencil design produces a large surface area for efficient heat transfer. The Shield Plug holds the fuel bundles in place against the force of the reactor coolant flow.

68 2-5. WHIH OF TH FOLLOWING STTMNTS IS INORRT: There are 12 fuel bundles in each NU 6 and NU 9 fuel channel. The fuel pellets for NU are made of natural uranium dioxide. 30 such pellets fit into a fuel pencil. The approximately 50 cm length of the NU fuel bundle is very suitable for manually loading it into the reactor. The 37 fuel pencil design produces a large surface area for efficient heat transfer. The Shield Plug holds the fuel bundles in place against the force of the reactor coolant flow. is incorrect: The NU fuel bundles are loaded manually into one of the two magazines of the new fuel port.

69 2-6. WHIH OF TH FOLLOWING STTMNTS IS INORRT: The Fuel hannel Spacer is designed to maintain the separation between the Pressure Tube and the alandria Tube. The Helium gas between the Pressure and alandria Tubes helps to provide thermal insulation. There is a high risk of deuterium ingress from the heat transport coolant into the Pressure Tube if the latter comes into direct contact with the much cooler alandria Tube. The coil structure of the Spacers does not impede the flow of annulus gas. ue to creep and thermal expansion, the Pressure Tube moves relative to the alandria Tube, so the Spacers cannot be fixed in place.

70 2-6. WHIH OF TH FOLLOWING STTMNTS IS INORRT: The Fuel hannel Spacer is designed to maintain the separation between the Pressure Tube and the alandria Tube. The Helium gas between the Pressure and alandria Tubes helps to provide thermal insulation. There is a high risk of deuterium ingress from the heat transport coolant into the Pressure Tube if the latter comes into direct contact with the much cooler alandria Tube. The coil structure of the Spacers does not impeded the flow of annulus gas. ue to creep and thermal expansion, the Pressure Tube moves relative to the alandria Tube, so the Spacers cannot be fixed in place. is incorrect: The O 2 gas between the Pressure and alandria Tubes helps to provide thermal insulation.

71 2-1. WHIH OF TH FOLLOWING STTMNTS IS INORRT: The NU 6 and 9 Reactor ores need to be controlled spatially in 14 regions or zones. In a heavy water moderated reactor light water acts as a neutron absorber, that can be used for reactivity control. ach of the 14 reactivity control zones in a NU core has a flux detector and a compartment in which the volume of water can be altered. The spatial distribution of the flux can only be altered by the liquid zone control system. The level and hence the volume of water in a zone control compartment are varied by altering the rate of water inflow and keeping the outflow constant.

72 2-1. WHIH OF TH FOLLOWING STTMNTS IS INORRT: The NU 6 and 9 Reactor ores need to be controlled spatially in 14 regions or zones. In a heavy water moderated reactor light water acts as a neutron absorber, that can be used for reactivity control. ach of the 14 reactivity control zones in a NU core has a flux detector and a compartment in which the volume of water can be altered. The spatial distribution of the flux can only be altered by the liquid zone control system. The level and hence the volume of water in a zone control compartment are varied by altering the rate of water inflow and keeping the outflow constant. is incorrect: The spatial distribution of the flux will be altered whenever there is an uneven change of reactivity in the core. This includes all reactivity device movements and refuelling.

73 2-2. WHIH OF TH FOLLOWING STTMNTS IS INORRT: ach axial half of a NU reactor core has seven zones. The 14 zones of a NU core are separated by zirconium dividers. The following zones are axial pairs, i.e. a given fuel channel goes through both zones: 2 & 14, 4 & 11. The Reactor Regulating System raises or lowers the levels in each zone by the same relative amount for the purpose of bulk reactor power control. The Reactor Regulating System can raise or lower the water level in each zone by different relative amounts for the purpose of spatial power control.

74 2-2. WHIH OF TH FOLLOWING STTMNTS IS INORRT: ach axial half of a NU reactor core has seven zones. The 14 zones of a NU core are separated by zirconium dividers. The following zones are axial pairs, i.e. a given fuel channel goes through both zones: 2 & 14, 4 & 11. The Reactor Regulating System raises or lowers the levels in each zone by the same relative amount for the purpose of bulk reactor power control. The Reactor Regulating System can raise or lower the water level in each zone by different relative amounts for the purpose of spatial power control. is incorrect: There are no physical barriers that separate the 14 zones of a NU core. The 14 zones these are regions of the core identified by calculations and confirmed by measurements, which behave with a significant level of independence as far as reactivity effects are concerned.

75 2-3. WHIH OF TH FOLLOWING STTMNTS IS INORRT: Since the control rods absorb neutrons, a reactor that has all of its control rods out of the core can operate at a higher power level than a reactor with control rods inserted. ecause the flux distribution in a reactor is not uniform, two control rods in one area of the core can have approximately the same reactivity worth as three or four control rods in another region of the core. ll djuster Rods are fully inserted into the core during normal operations, but can be withdrawn when positive reactivity change is required. The rate of movement of bsorber Rods can be altered by the Reactor Regulating System. If one or more djuster Rod banks are withdrawn from the core, the maximum reactor power must be reduced from 100%FP to prevent overrating of the fuel.

76 2-3. WHIH OF TH FOLLOWING STTMNTS IS INORRT: Since the control rods absorb neutrons, a reactor that has all of its control rods out of the core can operate at a higher power level than a reactor with control rods inserted. ecause the flux distribution in a reactor is not uniform, two control rods in one area of the core can have approximately the same reactivity worth as three or four control rods in another region of the core. ll djuster Rods are fully inserted into the core during normal operations, but can be withdrawn when positive reactivity change is required. The rate of movement of bsorber Rods can be altered by the Reactor Regulating System. If one or more djuster Rod banks are withdrawn from the core, the maximum reactor power must be reduced from 100%FP to prevent overrating of the fuel. is incorrect: Maximum reactor power can only be achieved, under normal operating conditions, when all the absorber rods are in the fully inserted position, so that the flux is flattened and every fuel channel is contributing its maximum power production without any being overrated.

77 2-4. WHIH OF TH FOLLOWING STTMNTS IS INORRT: Mechanical ontrol bsorbers are normally located outside the core, but can be inserted when negative reactivity beyond the range of control of the liquid zones is required. Reactor Stepback is a sudden, step-like reduction in reactor power, achieved by dropping all Ms into the core. epending on the length of time the clutches are de-energized, the Ms may be fully or only partially dropped into the core. The Ms are normally under the control of the Reactor Regulating System, but can also be operated manually from the control room. The only difference between djuster Rods and Mechanical ontrol bsorber rods is that former are normally in the core and the latter are normally out of the core.

78 2-4. WHIH OF TH FOLLOWING STTMNTS IS INORRT: Mechanical ontrol bsorbers are normally located outside the core, but can be inserted when negative reactivity beyond the range of control of the liquid zones is required. Reactor Stepback is a sudden, step-like reduction in reactor power, achieved by dropping all Ms into the core. epending on the length of time the clutches are de-energized, the Ms may be fully or only partially dropped into the core. The Ms are normally under the control of the Reactor Regulating System, but can also be operated manually from the control room. The only difference between djuster Rods and Mechanical ontrol bsorber rods is that former are normally in the core and the latter are normally out of the core. is incorrect: The other differences are in length, the Ms are longer than the djusters, the Ms can be dropped if their clutch is released, the bsorbers do not have clutches; the Ms are made of cadmium and stainless steel, while the bsorbers use stainless steel only.

79 2-5. WHIH OF TH FOLLOWING STTMNTS IS INORRT: The Shutdown Rods are dropped into the core with the assistance of an accelerating spring. The Shutdown Rods are used only for the purpose of safely shutting down the reactor, they are not used for reactor regulation. ll Shutdown Rods must drop into the core for the safe operation of Shutdown System #1. lthough the Shutdown Rods are withdrawn under the control of the Reactor regulating System, their reactor shutdown function remains independent of the Reactor Regulating System. Two actions are needed to withdraw the shutdown rods: the trip condition must have cleared and the trip must have been reset by the operator.

80 2-5. WHIH OF TH FOLLOWING STTMNTS IS INORRT: The Shutdown Rods are dropped into the core with the assistance of an accelerating spring. The Shutdown Rods are used only for the purpose of safely shutting down the reactor, they are not used for reactor regulation. ll Shutdown Rods must drop into the core for the safe operation of Shutdown System #1. lthough the Shutdown Rods are withdrawn under the control of the Reactor regulating System, their reactor shutdown function remains independent of the Reactor Regulating System. Two actions are needed to withdraw the shutdown rods: the trip condition must have cleared and the trip had been reset by the operator. is incorrect: ven if the two rods with the greatest reactivity worth fail to drop into the core, the reactor will still be safely shut down.

81 2-6. WHIH OF TH FOLLOWING STTMNTS IS INORRT: ll connections to the vertical reactivity control devices are made via the Reactivity Mechanism eck. uring the initial part of inserting a solid neutron absorber from its parked position, there will be no significant change in core reactivity. The only method to add positive reactivity to the core of a NU reactor is to replace some of the spent fuel with fresh fuel. Gadolinium is a strong neutron absorber, and can be either slowly added to the Moderator for the purpose of reactivity control, or rapidly injected into the Moderator to shut down the reactor. djuster rods provide some flux and power flattening.

82 2-6. WHIH OF TH FOLLOWING STTMNTS IS INORRT: ll connections to the vertical reactivity control devices are made via the Reactivity Mechanism eck. uring the initial part of inserting a solid neutron absorber from its parked position, there will be no significant change in core reactivity. The only method to add positive reactivity to the core of a NU reactor is to replace some of the spent fuel with fresh fuel. Gadolinium is a strong neutron absorber, and can be either slowly added to the Moderator for the purpose of reactivity control, or rapidly injected into the Moderator to shut down the reactor. djuster rods provide some flux and power flattening. is incorrect: Moving control rods (e.g., adjusters, water in liquid zone controllers) can add positive reactivity.

83 2-1. WHIH OF TH FOLLOWING STTMNTS IS INORRT: The heavy-water Moderator is used to thermalize the fast neutrons produced by fission. The heavy water of the Heat Transport System flowing through the Pressure Tubes contributes as much to the thermalization of fast neutrons as the Moderator heavy water does. The Moderator receives about 5% of the total heat generated by fission. lthough the nnulus Gap between the Pressure Tube and the alandria Tube minimizes the heat transfer from the oolant to the Moderator, about 5% of the heat that appears in the Moderator comes from the oolant. Gamma rays from fission product decay continue to add heat to the Moderator even after the fission reaction has stopped.

84 2-1. WHIH OF TH FOLLOWING STTMNTS IS INORRT: The heavy water Moderator is used to thermalize the fast neutrons produced by fission. The heavy water of the Heat Transport System flowing through the Pressure Tubes contributes as much to the thermalization of fast neutrons as the Moderator heavy water does. The Moderator receives about 5% of the total heat generated by fission. lthough the nnulus Gap between the Pressure Tube and the alandria Tube minimizes the heat transfer from the oolant to the Moderator, about 5% of the heat that appears in the Moderator comes from the oolant. Gamma rays from fission product decay continue to add heat to the Moderator even after the fission reaction has stopped. is incorrect: Most of the thermalization of fast neutrons takes place in the Moderator heavy water, since the neutrons spend much more of their time in the Moderator than in the Heat Transport heavy water. (For the same reason, the concentration of 2O does not have to be as high in the Heat Transport than in the Moderator.

85 2-2. WHIH OF TH FOLLOWING STTMNTS IS INORRT: If no cooling were provided to the Moderator, it would boil in a few minutes at 100%FP operation. The temperature of the Moderator is controlled to be about 60 o. The level of the Moderator is held constant under normal operating conditions by the Head Tank. For all NU Moderator systems the outflow is from the bottom of the alandria. The Moderator System can act as a back-up heat sink in case of a loss of coolant accident.

86 2-2. WHIH OF TH FOLLOWING STTMNTS IS INORRT: If no cooling were provided to the Moderator, it would boil in a few minutes at 100%FP operation. The temperature of the Moderator is controlled to be about 60. The level of the Moderator is held constant under normal operating conditions by the Head Tank. For all NU Moderator systems the outflow is from the bottom of the alandria. The Moderator System can act as a back-up heat sink in case of a loss of coolant accident. is incorrect: For NU 6 the outflow is from the bottom of the alandria, but for NU 9 the outflow is from the upper part of the alandria, above the inlet pipes.

87 2-3. WHIH OF TH FOLLOWING STTMNTS IS INORRT: Helium provides an inert gas cover for the Moderator heavy-water free surfaces to prevent corrosion and reduce radioactivity. atalytic Recombiners are used to form 2 O from any dissociated deuterium gas. ir is a suitable cover gas for the Moderator System. Under normal operating conditions one of the two gas compressors is used to circulate the over Gas. The free moderator surfaces in the Head Tank and the Over-pressure Relief Pipes are covered with Helium Gas.

88 2-3. WHIH OF TH FOLLOWING STTMNTS IS INORRT: Helium provides an inert gas cover for the Moderator heavy water free surfaces to prevent corrosion and reduce radioactivity. atalytic Recombiners are used to form 2O from any dissociated deuterium gas. ir is a suitable cover gas for the Moderator System. Under normal operating conditions one of the two gas compressors is used to circulate the over Gas. The free moderator surfaces in the Head Tank and the Over-pressure Relief Pipes are covered with Helium Gas. is incorrect: ir is a not suitable cover gas for the Moderator System, because it is not inert, the moisture from the air would downgrade the Moderator heavy water, and the oxygen and nitrogen would be activated.

89 2-4. WHIH OF TH FOLLOWING STTMNTS IS INORRT: Liquid poison in the Moderator is used to provide negative reactivity beyond the range of the liquid zones and solid neutron absorbing reactivity mechanisms. The excess reactivity in fresh fuel needs to be compensated by adding long lasting liquid poison to the Moderator. Reduction in Xenon-135 following a reactor shutdown must be compensated by the use of liquid poison. Liquid poison addition for reactor regulation is completely independent from liquid poison injection for reactor shutdown. oron and Gadolinium have essentially the same characteristics when used as reactor poisons.

90 2-4. WHIH OF TH FOLLOWING STTMNTS IS INORRT: Liquid poison in the Moderator is used to provide negative reactivity beyond the range of the liquid zones and solid neutron absorbing reactivity mechanisms. The excess reactivity in fresh fuel needs to be compensated by adding long lasting liquid poison to the Moderator. Reduction in Xenon-135 following a reactor shutdown must be compensated by the use of liquid poison. Liquid poison addition for reactor regulation is completely independent from liquid poison injection for reactor shutdown. oron and Gadolinium have essentially the same characteristics when used as reactor poisons. is incorrect: oron and Gadolinium have different neutron absorption crosssections and burn-up characteristics.

91 2-5. WHIH OF TH FOLLOWING STTMNTS IS INORRT: The Moderator Purification System removes any light water that may have contaminated the heavy water. It is possible to add negative reactivity to the moderator by the Purification System. The removal of corrosion products from the Moderator minimizes the build-up of activation products. The Moderator Purification System can remove both oron and Gadolinium poisons. Gadolinium, whether injected for reactor regulation or reactor shutdown purposes, is removed by the Moderator Purification System.

92 2-5. WHIH OF TH FOLLOWING STTMNTS IS INORRT: The Moderator Purification System removes any light water that may have contaminated the heavy water. It is possible to add negative reactivity to the moderator by the Purification System. The removal of corrosion products from the Moderator minimizes the build-up of activation products. The Moderator Purification System can remove both oron and Gadolinium poisons. Gadolinium, whether injected for reactor regulation or reactor shutdown purposes, is removed by the Moderator Purification System. is incorrect: The Moderator Purification System removes dissolved neutron poisons and impurities, but not the light water that may have contaminated the heavy water. (Light water is removed from the Moderator in a separate off-line 2 O Upgrading System.)

93 Review Questions Module 3

94 3-1. WHIH OF TH FOLLOWING STTMNTS IS INORRT: The leakage flux that reaches an ion chamber is not always proportional to the average flux inside the reactor, because local reactivity effects can distort the flux near the ion chamber. Neutrons passing through any ion chamber will cause ionization, giving a measure of the neutron flux in the reactor. Ion chambers used to measure the neutron flux in a reactor work on the principle that a thermal neutron reacting with oron-10 results in the emission of an alpha particle. The particle emitted as a result of the boron-10 neutron interaction ionizes the Hydrogen gas inside the ion chamber. lead housing around the ion chamber is used to shield it from radiation.

95 3-1. WHIH OF TH FOLLOWING STTMNTS IS INORRT: The leakage flux that reaches an ion chamber is not always proportional to the average flux inside the reactor, because local reactivity effects can distort the flux near the ion chamber. Neutrons passing through any ion chamber will cause ionization, giving a measure of the neutron flux in the reactor. Ion chambers used to measure the neutron flux in a reactor work on the principle that a thermal neutron reacting with oron-10 results in the emission of an alpha particle. The particle emitted as a result of the boron-10 neutron interaction ionizes the Hydrogen gas inside the ion chamber. lead housing around the ion chamber is used to shield it from radiation. is incorrect: Since neutrons do not cause ionization, regular ion chambers will not respond to neutron bombardment. material such as oron-10 which produces an ionizing radiation (in this case ) needs to be included in the ion chamber for it to measure neutron flux.

96 3-2. WHIH OF TH FOLLOWING STTMNTS IS INORRT: Three ion-chamber signals, from three different housings that are mounted at the side of the calandria, are fed to three separate ion-chamber amplifiers. ach ion-chamber amplifier produces a Lin N signal, a Log N signal and a Log N Rate signal. Since the ion-chamber signal is not an accurate measure of the absolute value of the flux inside the reactor, the Lin N signal cannot be used directly to control reactor power. The inaccuracy of the ion chamber Log N signal is relatively small, so it can be used directly for control of reactor power. The Log N Rate signal is not affected by the inaccuracies in the absolute value of the ion chamber signal, since it is only concerned with the rate of change of the signal.

97 3-2. WHIH OF TH FOLLOWING STTMNTS IS INORRT: Three ion chamber signals, from three different housings that are mounted at the side of the calandria, are fed to three separate ion chamber amplifiers. ach ion chamber amplifier produces a Lin N signal, a Log N signal and a Log N Rate signal. Since the ion chamber signal is not an accurate measure of the absolute value of the flux inside the reactor, the Lin N signal cannot be used directly to control reactor power. The inaccuracy of the ion chamber Log N signal is relatively small, so it can be used directly for control of reactor power. The Log N Rate signal is not affected by the inaccuracies in the absolute value of the ion chamber signal, since it is only concerned with the rate of change of the signal. is incorrect: the ion chamber signal can only be used directly for the control of reactor power at low levels (<15%FP), not above this power level.

98 3-3. WHIH OF TH FOLLOWING STTMNTS IS INORRT: Self-powered in-core flux detectors are used to measure the total neutron flux and its distribution in the reactor s core. Self-powered flux detectors work on the principle that neutron and gamma radiation eject from the emitter electrons which, after travelling across the insulator to the collector, create a voltage difference. The Inconel type flux detectors are called self-powered because their output is so powerful that it does not need any amplification. Most flux detectors under-respond to neutrons and over-respond to gamma rays, resulting in not enough prompt response, and too much delayed response as reactor power level changes. The main advantages of the in-core flux detectors are that they give an immediate and linear response to a change in flux and are the basis of reactor power control from 5%FP to 120%FP.

99 3-3. WHIH OF TH FOLLOWING STTMNTS IS INORRT: Self-powered in-core flux detectors are used to measure the total neutron flux and its distribution in the reactor s core. Self-powered flux detectors work on the principle that neutron and gamma radiation eject from the emitter electrons which, after traveling across the insulator to the collector, create a voltage difference. The Inconel type flux detectors are called self-powered because their output is so powerful that it does not need any amplification. Most flux detectors under-respond to neutrons and over-respond to gamma rays, resulting in not enough prompt response, and too much delayed response as reactor power level changes. The main advantages of the in-core flux detectors are that they give an immediate and linear response to a change in flux and are the basis of reactor power control from 5%FP to 120%FP. is incorrect: Inconel flux detectors are called self-powered because they generate an electric current without the application of an external power supply. However the signal is very weak, and needs to be amplified before it can be used for control.

100 3-4. WHIH OF TH FOLLOWING STTMNTS IS INORRT: ach of the 14 reactor zones has two flux detectors to provide redundancy, with the signal from both detectors in a given zone connected to the same amplifier. Inconel type in-core flux detectors are located in the 14 control zones of a NU core, so that they can measure both the spatial distribution and the total flux of the reactor. ecause the in-core flux detectors respond to gamma as well as neutron radiation, they need some corrections before they can be used for the control of reactor power. The Lin N signal from the in-core detector amplifiers is used by RRS to control power above 5%FP. The Lin N bulk power signal is calibrated by the thermal power measurements. RRS uses the output of the Flux Mapping routine to calibrate the flux detector readings for the purpose of controlling the spatial power distribution.

101 3-4. WHIH OF TH FOLLOWING STTMNTS IS INORRT: ach of the 14 reactor zones has two flux detectors to provide redundancy, with the signal from both detectors in a given zone connected to the same amplifier. Inconel type in-core flux detectors are located in the 14 control zones of a NU core, so that they can measure both the spatial distribution and the total flux of the reactor. ecause the in-core flux detectors respond to gamma as well as neutron radiation, they need some corrections before they can be used for the control of reactor power. The Lin N signal from the in-core detector amplifiers is used by RRS to control power above 5%FP. The Lin N bulk power signal is calibrated by the thermal power measurements. RRS uses the output of the Flux Mapping routine to calibrate the flux detector readings for the purpose of controlling the spatial power distribution. is incorrect: The two flux detector signals are connected to two different amplifiers.

102 3-5. WHIH OF TH FOLLOWING STTMNTS IS INORRT: The thermal output of the reactor can be measured as (1) the heat transferred to the coolant as if flows through the reactor, and (2) as the heat transferred to the feedwater in the steam generator. The heat transferred from the fuel to the coolant can be determined by measuring the flow rate and the temperature difference between fuel channel inlet and outlet. ue to transport lag and the time constant of the sensor itself, there is a delay from the time the fuel temperature changes until this change registers as a correct reading at the RTs. y measuring steam flow and temperature, as well as feedwater flow and temperature, the amount of heat transferred across the steam generators can be determined. Measuring the coolant flow and temperature change across the NU reactor core will give an accurate, if delayed value for the heat transferred from the fuel to the coolant, at all power levels.

103 3-5. WHIH OF TH FOLLOWING STTMNTS IS INORRT: The thermal output of the reactor can be measured as (1) the heat transferred to the coolant as if flows through the reactor, and (2) as the heat transferred to the feedwater in the steam generator. The heat transferred from the fuel to the coolant can be determined by measuring the flow rate and the temperature difference between fuel channel inlet and outlet. ue to transport lag and the time constant of the sensor itself, there is a delay from the time the fuel temperature changes until this change registers as a correct reading at the RTs. y measuring steam flow and temperature, as well as feedwater flow and temperature, the amount of heat transferred across the steam generators can be determined. Measuring the coolant flow and temperature change across the NU reactor core will give an accurate, if delayed value for the heat transferred from the fuel to the coolant, at all power levels. is incorrect: Measuring the coolant flow and temperature change across a NU reactor will only give an accurate value for the heat transferred below the power level at which boiling begins to take place, typically below 70%FP.

104 3-6. WHIH OF TH FOLLOWING STTMNTS IS INORRT: Measurements of neutron flux by the out-of-core ion chambers and the in-core self-powered flux detectors are prompt, but not sufficiently accurate to be used directly for reactor control above 5%FP. Thermal power measurements across the reactor and across the steam generators are accurate, but are delayed by several seconds relative to any changes in neutron flux. Reactor Neutron and Thermal Power are proportional. When positive reactivity is added to the core, both increase by the same amount. etermination of actual bulk power is from the ion chambers below 5%FP, and from the in-core detector readings above 15%, with a linear changeover between them. Spatial power is controlled on the basis of the in-core flux detector readings, corrected every two minutes by the Flux Mapping routine.

105 3-6. WHIH OF TH FOLLOWING STTMNTS IS INORRT: Measurements of neutron flux by the out of core ion chambers and the in-core self-powered flux detectors are prompt, but not sufficiently accurate to be used directly for reactor control above 5%FP. Thermal power measurements across the reactor and across the steam generators are accurate, but are delayed by several seconds relative to any changes in neutron flux. Reactor Neutron and Thermal Power are proportional. When positive reactivity is added to the core, both increase by the same amount. etermination of actual bulk power is from the ion chambers below 5%FP, and from the in-core detector readings above 15%, with a linear changeover between them. Spatial power is controlled on the basis of the in-core flux detector readings, corrected every two minutes by the Flux Mapping routine. is incorrect: ecause ecay heat changes much more slowly than heat from fission after a reactivity change, the two will not change in the same proportion.

106 3-1. WHIH OF TH FOLLOWING STTMNTS IS INORRT: In Normal mode Target Reactor Power Setpoint is specified by the oiler Pressure ontrol (P) program. In lternate mode Target Reactor Power Setpoint is specified by the operator typing the value and the rate on the keyboard. t the completion of the hold power mode or of the setback mode the Reactor Regulating System is placed into alternate mode. The Mode Select program selects the Overall Unit ontrol Mode, between normal and alternate. The two basic modes of reactor control are normal and alternate, since both of the other modes result in reactor control being placed into alternate mode.

107 3-1. WHIH OF TH FOLLOWING STTMNTS IS INORRT: In normal mode Target Reactor Power Setpoint is specified by the oiler Pressure ontrol (P) program. In alternate mode Target Reactor Power Setpoint is specified by the operator typing the value and the rate on the keyboard. t the completion of the hold power mode or of the setback mode the Reactor Regulating System is placed into alternate mode. The Mode Select program selects the Overall Unit ontrol Mode, between normal and alternate. The two basic modes of reactor control are normal and alternate, since both of the other modes result in reactor control being placed into alternate mode. is incorrect: The Mode Select program selects the appropriate mode from lternate, Normal, setback or hold power.

108 3-2. WHIH OF TH FOLLOWING STTMNTS IS INORRT: The emanded Power level change is achieved by ramping the instantaneous reactor power setpoint up to the target value at the specified rate. On each iteration of the emanded Power program the nominal rate is the maximum amount that can be added to the previous value of emanded Power. uring large differences between Target Setpoint and emanded Power, the specified rate of setpoint change is used as an upper limit on the step size per iteration. On a HOL POWR operation the Reactor Power Setpoint Target is set equal to the value emanded Power has on that iteration, and the change in demanded power is stopped. The deviation limiter in the emanded Power Routine prevents the power setpoint from being more than 5% above the actual power.

109 3-2. WHIH OF TH FOLLOWING STTMNTS IS INORRT: The emanded Power level change is achieved by ramping the instantaneous reactor power setpoint up to the target value at the specified rate. On each iteration of the emanded Power program the nominal rate is the maximum amount that can be added to the previous value of emanded Power. uring large differences between Target Setpoint and emanded Power, the specified rate of setpoint change is used as an upper limit on the step size per iteration. On a HOL POWR operation the Reactor Power Setpoint Target is set equal to the value emanded Power has on that iteration, and the change in demanded power is stopped. The deviation limiter in the emanded Power Routine prevents the power setpoint from being more than 5% above the actual power. is incorrect: Since the emanded Power computer program executes once every 0.5 second, half of the nominal rate is the maximum amount that can be added to the emanded Power on each iteration.

110 3-3. WHIH OF TH FOLLOWING STTMNTS IS INORRT: The zone power error is a measure of the difference between the measured power in the zone and the demanded power of the reactor, plus a rate of change of power error term. The bulk power error is a measure of the difference between the measured power and the demanded power of the reactor, plus a rate of change of power error term. ffective power error = k1(actual power - demanded power) + k2(actual rate - demanded rate) The sign of the power error determines whether to increase or decrease the levels of the zones, remove or insert adjuster rods, remove or insert the mechanical control absorbers. When the power error is zero, no movement of reactivity devices will be ordered, although device movements ordered before the error became zero will be completed.

111 3-3. WHIH OF TH FOLLOWING STTMNTS IS INORRT: The zone power error is a measure of the difference between the measured power in the zone and the demanded power of the reactor, plus a rate of change of power error term. The bulk power error is a measure of the difference between the measured power and the demanded power of the reactor, plus a rate of change of power error term. ffective power error = k1(actual power - demanded power) + k2(actual rate - demanded rate) The sign of the power error determines whether to increase or decrease the levels of the zones, remove or insert adjuster rods, remove or insert the mechanical control absorbers. When the power error is zero, no movement of reactivity devices will be ordered, although device movements ordered before the error became zero will be completed. is incorrect: The zone power error is the difference between the flux in the zone and the average flux.

112 3-4. WHIH OF TH FOLLOWING STTMNTS IS INORRT: The Reactor Setback function is a part of the Reactor Regulating System, and works via the emanded Power Routine. Reactor Setback is the forced reduction of the reactor power setpoint at specified rates and endpoints, the values of which depend on the condition that initiated the setback. Reactor Setback is designed principally to protect the turbine in case of excessive reactor power, and in case of a loss of condenser cooling water. Setback is activated to ensure that the reactor is controlled to safe power levels and that the fuel is cooled at all times. Reactor Setback overrides other reactor power demands and is accompanied by alarm window annunciation.

113 3-4. WHIH OF TH FOLLOWING STTMNTS IS INORRT: The Reactor Setback function is a part of the Reactor Regulating System, and works via the emanded Power Routine. Reactor Setback is the forced reduction of the reactor power setpoint at specified rates and endpoints, the values of which depend on the condition that initiated the setback. Reactor Setback is designed principally to protect the turbine in case of excessive reactor power, and in case of a loss of condenser cooling water. Setback is activated to ensure that the reactor is controlled to safe power levels and that the fuel is cooled at all times. Reactor Setback overrides other reactor power demands and is accompanied by alarm window annunciation. is incorrect: Reactor Setback is designed to protect the fuel from overheating, to protect the various reactor structures, to protect the turbine, and to protect against any loss of heat sink.

114 3-5. WHIH OF TH FOLLOWING STTMNTS IS INORRT: The Stepback Routine monitors a number of plant parameters and reduces reactor power in a STP fashion by dropping the mechanical control absorbers either fully or partly into the reactor. The Stepback Routine is independent of RRS and executes in both control computers: both s must identify the requirement for Stepback to occur. In case of a reactor trip the Stepback function is activated, so that all the control absorbers will be dropped into the core, thereby aiding the rapid shutdown of the reactor. Unit control mode will be placed in LTRNT mode whenever STPK is activated. Once the initiating conditions for a Stepback are present, the control absorbers will be dropped into the core, and the clutches cannot be re-energized until the rods have dropped all the way into the core.

115 3-5. WHIH OF TH FOLLOWING STTMNTS IS INORRT: The Stepback Routine monitors a number of plant parameters and reduces reactor power in a STP fashion by dropping the mechanical control absorbers either fully or partly into the reactor. The Stepback Routine is independent of RRS and executes in both control computers: both s must identify the requirement for Stepback to occur. In case of a reactor trip the Stepback function is activated, so that all the control absorbers will be dropped into the core, thereby aiding the rapid shutdown of the reactor. Unit control mode will be placed in LTRNT mode whenever STPK is activated. Once the initiating conditions for a Stepback are present, the control absorbers will be dropped into the core, and the clutches cannot be reenergized until the rods have dropped all the way into the core. is incorrect: s soon as the conditions which initiated the Reactor Stepback clear the clutches are re-energized, and this can prevent the control absorbers from being fully dropped into the core.

116 3-6. WHIH OF TH FOLLOWING STTMNTS IS INORRT: The zones are the primary means of reactivity control in NU. They are used to ensure bulk and spatial reactor power control, in response to setpoint changes as well as other reactivity variations. positive power error will require a negative reactivity change, so the zone levels will be raised, while for a negative power error the zone levels will be lowered. The signal to each zone control valve is based on the difference between the flux in the zone and the average flux, and also the deviation of individual zone water levels from the average liquid zone level. In the steady state, that is at zero power error, the zone compartment water level is kept constant by adjusting the opening of the outlet valve to match that of the inlet valve. Logic in the control program overrides the signal before it reaches the valve if the level in a given zone falls below 5% or rises above 95%.

117 3-6. WHIH OF TH FOLLOWING STTMNTS IS INORRT: The zones are the primary means of reactivity control in NU. They are used to ensure bulk and spatial reactor power control, in response to setpoint changes as well as other reactivity variations. positive power error will require a negative reactivity change, so the zone levels will be raised, while for a negative power error the zone levels will be lowered. The signal to each zone control valve is based on the difference between the flux in the zone and the average flux, and also the deviation of individual zone water levels from the average liquid zone level. In the steady state, that is at zero power error, the zone compartment water level is kept constant by adjusting the opening of the outlet valve to match that of the inlet valve. Logic in the control program overrides the signal before it reaches the valve if the level in a given zone falls below 5% or rises above 95%. is incorrect: The zone control outlet valves have a constant opening, it is the inlet valves that have their openings varied.

118 3-7. WHIH OF TH FOLLOWING STTMNTS IS INORRT: The method of reactivity device control in NUs is based on the values of the parameter pair: average liquid zone level and power error, on each iteration of the Reactor Regulating System program. shortage of positive reactivity will be indicated by either a high zone controller level, that is the average zone level is above 80% full, or a positive power error, or both. For power errors between 4 and +3%FP and average liquid zone levels between 15 and 80%, reactor power control is achieved by the actions of the liquid zone control system. Outside the range of power errors between 4 and +3%FP and average liquid zone levels between 15 and 80%, the actions of the liquid zones are supplemented by the adjuster and control absorber rods. shortage of positive reactivity will be indicated by either a low zone controller level, that is the average zone level is below 15%, or a negative power error, or both.

119 3-7. WHIH OF TH FOLLOWING STTMNTS IS INORRT: The method of reactivity device control in NUs is based on the values of the parameter pair: average liquid zone level and power error, on each iteration of the Reactor Regulating System program. shortage of positive reactivity will be indicated by either a high zone controller level, that is the average zone level is above 80% full, or a positive power error, or both. For power errors between 4 and +3%FP and average liquid zone levels between 15 and 80%, reactor power control is achieved by the actions of the liquid zone control system. Outside the range of power errors between 4 and +3%FP and average liquid zone levels between 15 and 80%, the actions of the liquid zones are supplemented by the adjuster and control absorber rods. shortage of positive reactivity will be indicated by either a low zone controller level, that is the average zone level is below 15%, or a negative power error, or both. is incorrect: The conditions described here are for a shortage of negative reactivity; gives the correct conditions for a shortage of positive reactivity.

120 3-8. WHIH OF TH FOLLOWING STTMNTS IS INORRT: The djuster rods provide a reserve of positive reactivity to override any xenon transient that may develop following a reactor trip or other large power level reductions. The djuster rods are normally fully inserted into the core, so as to flatten the neutron flux and to provide a reserve of positive reactivity when the range of control of the liquid zones has been used up. djuster rod auto out-drive is initiated by RRS for average zone levels below 15% N for power errors less than 4%FP; also for all liquid zone levels when the power error is less than 4%FP. djuster rod auto in-drive is initiated by RRS for average zone levels above 75% N for power errors that are more than 4%FP; also for all liquid zone levels when the power error is greater than 4%FP. If the operating point is within the normal range of control for the liquid zones, RRS will not initiate adjuster drive movement, but it is good operating practice not to leave rods partially in the core.

121 3-8. WHIH OF TH FOLLOWING STTMNTS IS INORRT: The djuster rods provide a reserve of positive reactivity to override any xenon transient that may develop following a reactor trip or other large power level reductions. The djuster rods are normally fully inserted into the core, so as to flatten the neutron flux and to provide a reserve of positive reactivity when the range of control of the liquid zones has been used up. djuster rod auto out-drive is initiated by RRS for average zone levels below 15% N for power errors less than 4%FP; also for all liquid zone levels when the power error is less than 4%FP. djuster rod auto in-drive is initiated by RRS for average zone levels above 75% N for power errors that are more than 4%FP; also for all liquid zone levels when the power error is greater than 4%FP. If the operating point is within the normal range of control for the liquid zones, RRS will not initiate adjuster drive movement, but it is good operating practice not to leave rods partially in the core. is incorrect: The djuster rods are limited in their ability to override Xenon transients, for example to about 40 minutes following a reactor trip.

122 3-9. WHIH OF TH FOLLOWING STTMNTS IS INORRT: ontrol bsorber rods are normally located completely outside the core. They are driven into the core to provide negative reactivity when the liquid zones have used up their range of control. The control absorbers can also be dropped into the core fully or part way by the Stepback program. ontrol bsorber drive is stopped if the average zone level is between 20% and 80% N the power error is between 4%FP and 3%FP. ontrol bsorber auto in-drive is initiated by RRS for average zone levels above 80% N the power error greater than 4%FP; also for all liquid zone levels when the power error is greater than 3%FP. ontrol bsorber auto out-drive is initiated by RRS for average zone levels below 75% N the power error is less than 3%FP; also for all liquid zone levels when the power error is less than 4%FP.

123 3-9. WHIH OF TH FOLLOWING STTMNTS IS INORRT: ontrol bsorber rods are normally located completely outside the core. They are driven into the core to provide negative reactivity when the liquid zones have used up their range of control. The control absorbers can also be dropped into the core fully or part way by the Stepback program. ontrol bsorber drive is stopped if the average zone level is between 20% and 80% N the power error is between 4%FP and 3%FP. ontrol bsorber auto in-drive is initiated by RRS for average zone levels above 80% N the power error greater than 4%FP; also for all liquid zone levels when the power error is greater than 3%FP. ontrol bsorber auto out-drive is initiated by RRS for average zone levels below 75% N the power error is less than 3%FP; also for all liquid zone levels when the power error is less than 4%FP. is incorrect: ontrol bsorber drive is stopped if the average zone level is between 75% and 80% N the power error is between -4%FP and 3%FP

124 3-10. WHIH OF TH FOLLOWING STTMNTS IS INORRT: Variable speed control is provided to drive the adjuster and mechanical control absorber rods into or out of the core. For power errors between -4%FP and +4%FP, the rods are moved at 40% of their maximum speed. For large power errors, that is outside the +5%FP range, a faster change of reactivity is required, so the adjuster and mechanical control absorber rods are moved at their full design speeds. The automatic control of the reactivity mechanisms is subject to a number of interlocks to limit the consequences of a gross loss of regulation. To prevent the reactor from being started up with the safety systems unavailable, the adjusters and the control absorber rods are inhibited from being withdrawn unless both shutdown systems are poised. n interlock is provided which prevents the shutdown rods from being withdrawn unless the mechanical control absorbers are completely out of the core.

125 3-10. WHIH OF TH FOLLOWING STTMNTS IS INORRT: Variable speed control is provided to drive the adjuster and mechanical control absorber rods into or out of the core. For power errors between -4%FP and +4%FP, the rods are moved at 40% of their maximum speed. For large power errors, that is outside the +5%FP range, a faster change of reactivity is required, so the adjuster and mechanical control absorber rods are moved at their full design speeds. The automatic control of the reactivity mechanisms are subject to a number of interlocks to limit the consequences of a gross loss of regulation. To prevent the reactor from being started up with the safety systems unavailable, the adjuster and the control absorber rods are inhibited from being withdrawn unless both shutdown systems are poised. n interlock is provided which prevents the shutdown rods from being withdrawn unless the mechanical control absorbers are completely out of the core. is incorrect: n interlock prevents the shutdown rods from being withdrawn unless the mechanical control absorbers are completely in the core. (Note that this interlock does not work on the Simulator.)

126 Review Questions Module 4

127 True or False 4.1-a The NU 9 Primary Heat Transport System Main ircuit layout resembles two interconnected figure of 8 loops. From the ROH the coolant flow is to the Steam Generator, from there the coolant flows to the inlet of the circulating pumps, and after the pumps it flows to the Reactor Inlet Header, and finally via a feeder pipe to a pressure tube.

128 True 4.1-a The NU 9 Primary Heat Transport System Main ircuit layout resembles two interconnected figure of 8 loops. From the ROH the coolant flow is to the Steam Generator, from there the coolant flows to the inlet of the circulating pumps, and after the pumps it flows to the Reactor Inlet Header, and finally via a feeder pipe to a pressure tube.

129 True or False 4.1-b The NU 9 Primary Heat Transport System Main ircuit layout resembles two interconnected figure of 8 loops. From the ROH the coolant flow is to the Steam Generator where it mixes with the light water of the feedwater system, from there the coolant flows to the inlet of the circulating pumps, and after the pumps it flows to the Reactor Inlet Header, and finally via a feeder pipe to a pressure tube.

130 False; a correct form of the statement is: 4.1-b The heavy water coolant of the primary circuit and the light water of the feedwater system are kept separate in the steam generators, they do not mix, only the heat is transferred across the walls of the tubes in the steam generator.

131 True or False 4.2-a The assignment of pressure tubes to either of the two loops is random to ensure maximum redundancy and to allow flexibility in making the connections between pressure tubes and feeder pipes.

132 False; a correct form of the statement is: 4.2-a The assignment of the pressure tubes to one of the two loops is not random, the fuel channels are connected to the respective outlet headers so that each loop serves half of the reactor, on either side of a vertical center-plane.

133 True or False 4.2-b The PHT Main ircuit is designed to provide cooling of the reactor fuel at all times during reactor operation and provide for the heavy water coolant to remove decay heat when the reactor is shut down.

134 True 4.2-b The PHT Main ircuit is designed to provide cooling of the reactor fuel at all times during reactor operation and provide for the heavy water coolant to remove decay heat when the reactor is shut down.

135 True or False 4.3-a When the Main ircuit pumps are turned ON, the flow of heavy water is from the Reactor Outlet Headers through the Shutdown ooling Pumps and Heat xchangers, to the Reactor Inlet Headers.

136 False; a correct form of the statement is: 4.3-a When the Main ircuit Pumps are turned ON, the flow of heavy water is from the Reactor Inlet Headers through the Shutdown ooling Pumps and Heat xchangers, to the reactor Outlet Headers. This is why the Shutdown ooling Pumps must not be ON as long as the Main ircuit Pumps are ON: the much stronger Main ircuit Pumps would burn out the Shutdown ooling Pump motors.

137 True or False 4.3-b The Shutdown ooling System is used to cool the heat transport heavy water below the 177 limit possible with the steam generators, and has the capability to indefinitely remove reactor decay heat following the shutdown of the reactor.

138 True 4.3-b The Shutdown ooling System is used to cool the heat transport heavy water below the 177 limit possible with the steam generators, and has the capability to indefinitely remove reactor decay heat following the shutdown of the reactor.

139 True or False 4.4-a The main circuit pressure must be maintained so that there is adequate saturation margin in the reactor outlet headers, and that the required net positive suction head for the circulation pumps is provided.

140 True 4.4-a The main circuit pressure must be maintained so that there is adequate saturation margin in the reactor outlet headers, and that the required net positive suction head for the circulation pumps is provided.

141 True or False 4.4-b The Pressurizer is designed to compensate for the pressure drop that occurs as the heavy water coolant flows around the Main ircuit.

142 False; a correct form of the statement is: 4.4-b The Pressurizer is designed to maintain the pressure in the Main ircuit. The Main irculating Pumps are designed to compensate for the pressure drop that occurs as the heavy water coolant flows around the Main circuit.

143 True or False 4.1-a Under normal operating conditions the NU 9 Heat Transport System Main ircuit ROH Pressure setpoint is varied as a linear function of Reactor Power.

144 False; a correct form of the statement is: 4.1-a Under normal operating conditions the NU 9 Heat Transport System Main ircuit ROH Pressure setpoint is held constant at 10 MPa.

145 True or False 4.1-b The pressure of the Main ircuit must be maintained at a sufficiently high value so that only the design amount of boiling takes place in the fuel channels, and except under specific shutdown conditions, the inventory of heavy water must keep the Main ircuit completely filled.

146 True 4.1-b The pressure of the Main ircuit must be maintained at a sufficiently high value so that only the design amount of boiling takes place in the fuel channels, and except under specific shutdown conditions, the inventory of heavy water must keep the Main ircuit completely filled.

147 True or False 4.2-a Under normal operating conditions there is a free flow of heavy water between the Pressurizer and the Main ircuit that equalizes any pressure differences between the Main ircuit and the Pressurizer.

148 True 4.2-a Under normal operating conditions there is a free flow of heavy water between the Pressurizer and the Main ircuit that equalizes any pressure differences between the Main ircuit and the Pressurizer.

149 True or False 4.2-b Normal Mode of unit control requires Normal Mode of Heat Transport Pressure ontrol.

150 False; a correct form of the statement is: 4.2-b Under at power normal operating conditions OPG s NU units are operated in lternate mode Overall Unit ontrol, while the Heat Transport Pressure ontrol system is in Normal Mode.

151 True or False 4.3-a Under normal operating conditions the pressure in the Pressurizer is controlled to the Pressurizer Pressure Setpoint.

152 False; a correct form of the statement is: 4.3-a Under normal operating conditions the pressure in the Pressurizer is controlled to the Reactor Outlet Header Pressure setpoint. Only under certain shutdown conditions, typically during Heat Transport System warm-up or cool-down, when the Pressurizer is isolated from the Main ircuit (i.e. Main ircuit in Solid Mode Pressure ontrol) is the pressure of the Pressurizer controlled to the Pressurizer Pressure Setpoint.

153 True or False 4.3-b The Pressurizer s liquid and steam are kept at saturation, and at a pressure that keeps the reactor outlet header at its setpoint (10 MPa for NU-9 operating in the at power range).

154 True 4.3-b The Pressurizer s liquid and steam are kept at saturation, and at a pressure that keeps the reactor outlet header at its setpoint (10 MPa for NU-9 operating in the at power range).

155 True or False 4.4-a The level setpoint in the Pressurizer is increased automatically so that all the swell resulting from power increases is stored in the Pressurizer. The opposite takes place on a reactor power decrease. The level in the Pressurizer, and therefore the Heat Transport System inventory, is normally controlled via the main circuit feed and bleed valves.

156 True 4.4-a The level setpoint in the Pressurizer is increased automatically so that all the swell resulting from power increases is stored in the Pressurizer. The opposite takes place on a reactor power decrease. The level in the Pressurizer, and therefore the Heat Transport System inventory, is normally controlled via the main circuit feed and bleed valves.

157 True or False 4.4-b Pressurizer level above the setpoint is an indication of excess heavy water inventory in the Main Loop, so some bleed is taken into the Main ircuit from the 2 O Storage Tank.

158 False; a correct form of the statement is: 4.4-b Pressurizer level above the setpoint is an indication of excess heavy water inventory in the Main Loop, so some bleed is taken from the Main ircuit into the 2 O Storage Tank.

159 True or False 4.5-a The Overpressure relief valve of the leed ondenser is used to release non-condensable gases, mostly Nitrogen, which can accumulate in the Heat Transport coolant.

160 False; a correct form of the statement is: 4.5-a The Overpressure relief Valve of the leed ondenser is used to relieve 2 O steam and liquid from the leed ondenser in case of excessive pressure build-up. Non-condensable gases are released by opening the degassing valve.

161 True or False 4.5-b The principal purpose of the leed ondenser is to reduce the pressure and temperature of the heavy water coolant from the Main ircuit and of the heavy water steam from the Pressurizer. The leed ondenser at normal operating conditions contains heavy water and steam at saturation conditions, at a pressure around 1.7 MPa.

162 True 4.5-b The principal purpose of the leed ondenser is to reduce the pressure and temperature of the heavy water coolant from the Main ircuit and of the heavy water steam from the Pressurizer. The leed ondenser at normal operating conditions contains heavy water and steam at saturation conditions, at a pressure around 1.7 MPa.

163 True or False 4.6-a ll NU nuclear steam supply systems use a Pressurizer to control the pressure of the Main Heat Transport ircuit.

164 False; a correct form of the statement is: 4.6-a While all the operating NU 6 and 9 units use a Pressurizer to control the pressure in the Main Heat Transport ircuit under normal operating conditions, some of the early NU units, such as the ones at Pickering, use a feed and bleed system for pressure control (known as SOLI mode for NU 6 and 9 units).

165 True or False 4.6-b The term Solid Mode is used for a feed and bleed heat transport pressure control system, as it relies on the tensile strength of the vessels and piping of the main circuit, and on the very small compressibility of water.

166 True 4.6-b The term Solid Mode is used for a feed and bleed heat transport pressure control system, as it relies on the tensile strength of the vessels and piping of the main circuit, and on the very small compressibility of water.

167 True or False 4.7-a In normal mode the Inventory of heavy water in the Heat Transport Main ircuit is controlled by the Pressurizer Level ontrol system. It is achieved by feed and bleed, and is designed to compensate for volume changes as a function of coolant temperature.

168 True 4.7-a In normal mode the Inventory of heavy water in the Heat Transport Main ircuit is controlled by the Pressurizer Level ontrol system. It is achieved by feed and bleed, and is designed to compensate for volume changes as a function of coolant temperature.

169 True or False 4.7-b The Feed and leed system is used for inventory control of the Primary Heat Transport System in both normal mode and solid mode, and in both modes inventory control is independent of pressure control.

170 False; a correct form of the statement is: 4.7-b In solid mode both inventory and pressure control of the Primary Heat Transport System are achieved using the feed and bleed system.

171 True or False 4.8-a ll vessels that are under pressure must, by law, have the appropriate form of over-pressure protection. To prevent main heat transport high pressure transients exceeding system limits the Liquid Relief valves will open to the bleed condenser.

172 True 4.8-a ll vessels that are under pressure must, by law, have the appropriate form of over-pressure protection. To prevent main heat transport high pressure transients exceeding system limits the Liquid Relief valves will open to the bleed condenser.

173 True or False 4.8-b The purification circuit is protected against high pressures and temperatures by closing the by-pass valve and opening the inlet and outlet valves to the Purification System.

174 False; a correct form of the statement is: 4.8-b The Purification ircuit is protected against high pressure and temperature by opening the bypass valve and closing the inlet and outlet valves to the Purification System.