Chemical Engineering 693R

Transcription

1 Chemical Engineering 693R Reactor Design and Analysis Lecture 11 Nuclear Safety

2 Spiritual Thought 2 2 Kings 6:16 And he answered, Fear not: for they that be with us are more than they that be with them.

3 Fission Energy Distribution 3 Type Process Percent Location

4 Shutdown Fission Heat 4 Even after shutdown heat is produced Fissions Solve kinetic eqns. large negative reactivity ϕ tt = ϕ oo ββ ββ ρρ ee γγ1tt ρρ ββ ρρ tt ββ ρρ ee ll l = neutron life, t = time after transient initiation, ββ = total delayed neutron faction, ρρ = reactivity change, γγ 1 = decay constant for longest-lived delayed neutron precursor For 235 U, water moderated with ρ = -0.09, (in seconds) Q.rat ( t) Q.rat ( t) t t 0100

5 Decay Heat 5 Decay of Fission Products Simple approximations: β-energy release rate = 1.4t -1.2 MeV/fission s γ-energy release rate = 1.26t -1.2 MeV/fission s Assuming 200 MeV, 3.1x10 10 fissions/w PP ββ = 2.18EEEEqq 0 ττ ττ.2 ss ττ.2 MeV/cm 3 s PPγ = 1.95EEEEqq 0 ττ ττ ss.2 ττ.2 MeV/cm 3 s ττ = time since reactor startup ττ ss = time after shutdown Total Power is thus: PP = ττ ττ.2 PP ss ττ.2 oo

6 ANS Standard 6 Concentrated effort to provide uniform, trustworthy decay heat curve Experiments run in 1961 From 1 to 1E9 seconds 235U, 238U, and 239Pu Revised experiments (1985) are more accurate Given in chapter 3 of the text

7 PWR Normal Operation 7

8 PWR Parameters 8 CORE REACTOR VESSEL Total heat output MWT Overall vessel length of assembled vessel ft Heat generated in fuel 97.40% Inside diameter of shell 173 in Nominal system pressure 2250 psia Radius from center of vessel to nozzle face Total coolant flow rate 138.4x10 6 Inlet ft Equivalent core diamtere ft Outlet ft Core length, between fuel ends 120 ft Nominal cladding thickness in Fuel weight, uranium (first core) kg Minimum cladding thickness in Number of fuel assemblies 193 Coolant volume with core and internals in place 4740 ft 3 FUEL ROD PARAMETERS Operating pressure 2250 psia Outer diameter in Design pressure 2500 psia Cladding thickness in Design temperature 650 F Fuel/clad gap in Vessel material Carbon Steel Pellet diameter in Pitch in Rods array in assembly 15x15 or 17x17 Rods in assembly 204 Total number of fuel rods in core 39,372

9 BWR Normal Operation 9

10 BWR Operating Parameters 10 OVERALL Total heat output 3293 MW Electrical output 1100 MW Vessel dome pressure 71.7 kg/cm2 Main steam flow 6410 tons/hr Feedwater temperature C Turbine TC6F-41in Reheat No reheat Overall efficiency 33.40% NUCLEAR BOILER Reactor vessel Inner diameter 6.4 m Outer diameter 22.1 m Power density 50.0 kw/l Fuel assemblies 764 Assembly configuration 8x8 Control rod materials 185-B 4 C

11 Accidents and Safety 11 steady state or operational margins Designed to prevent failures during operation Based upon best estimate calcs Technical Specifications operation to avoid transient margins Accident based margins Defined in licensing Transient calculations to test limits Prevent public exposure during accidents

12 Design Basis Accidents (DBA) 12 Design-basis criticality: A criticality accident that is the most severe design basis accident of that type applicable to the area under consideration. design-basis earthquake (DBE): That earthquake for which the safety systems are designed to remain functional both during and after the event, thus assuring the ability to shut down and maintain a safe configuration. Design-basis event (DBE): A postulated event used in the design to establish the acceptable performance requirements of the structures, systems, and components. Design-basis explosion: An explosion that is the most severe design basis accident of that type applicable to the area under consideration. Design-basis fire: A fire that is the most severe design basis accident of this type. In postulating such a fire, failure of automatic and manual fire suppression provisions shall be assumed except for those safety class items or systems that are specifically designed to remain available (structurally or functionally) through the event. Design-basis flood: A flood that is the most severe design basis accident of that type applicable to the area under consideration. Design-basis tornado (DBT): A tornado that is the most severe design basis accident of that type applicable to the area under consideration. Most Common: LOCA, LOFA, Overpower

13 Beyond Design Basis Accidents (BDBA) 13 Beyond scope of design Unlikely events Extreme conditions Extremely severe Station Blackout Fukushima Significant focus

14 Margins 14

15 PRA Margins 15

16 PWR Operational Margins 16 MDNBR Code evaluation ~2.17 Pressure Drop 29 psia Fuel Temp 2800 C PT 1440 C Avg Axial Flow Velocity 7 m/s

17 PWR Transient Margins 17 PCT of 1200 C Maximum clad oxidation of less than 17% of the clad thickness Hydrogen generation of less than that required for the deflagration limits for containment integrity Less than 1% clad strain or a MDNBR of % overpower limit (16.03 kg/ft)

18 BWR Transient Margins 18 Linear Heat Generation Rate 25 kw/ft Critical Power Ratio 1.06 Average Planer Linear Heat Generation Rate Less than 1% clad strain or a MDNBR of % overpower limit (16.03 kg/ft)

19 Safety Systems 19 Required for licensing Prevent Public Dose Designed to protect in DBAs For BDBAs Provide some credit Inadequate Fukushima 7 typical safety systems in PWRs and BWRs

20 The Reactor Protection System (RPS) 20 Control rods Safety Injection/Standby liquid control

21 Essential Service Water System (ESWS) 21

22 Emergency Core Cooling System (ECCS) 22 High Pressure Safety Injection System (HPSI) Initiated by: Low pressurizer pressure High containment pressure Steam line pressure/flow anomalies Automatic Depressurization System 7 SRVs in vessel head Rapidly decrease system pressure Initiated by low level + time delay

23 ECCS (continued) 23 Low Pressure Safety System (HPSI) Only functions after blowdown Larger supply Later in accident Containment cooling system Spray system Actuated by high containment pressure/temperuture Core Spray System (BWR only)

24 Emergency Electrical Systems (EES) 24 Diesel Generators Flywheels Batteries

25 Containment Systems 25 Clad Containment Secondary Containment

26 Standby Gas Treatment Systems (SBGT) 26 Secondary Containment Maintain negative pressures (pull air in, rather than release radioactivity) Primarily for BWRs

27 Ventilation and Radiation Protection Systems 27 Prevention of radiation gas release Auxiliary Building Shield Building Reactor Building Turbine Building Radwaste Building Control Room Screenhouse Vent, Filter, Blowers

28 What s the Worry? 28 Frequency? Immediate Deaths? Radiation? Land Impact? Cost? Cancer? Public Stress? Annual Death rates per TW*hr Source Deaths US Electricity Percentage Coal % Oil 36 1% Natural Gas 4 27% Biofuel/Biomass % Solar % Wind % Hydro 1.4 6% Nuclear % Nuclear power has infrequent, but SEVERE social impact due to accidents!

29 Original Safety Strategy 29 Deterministic Approach Lacked theory & computational input Empirical approach to safety Multiple Layers of protection Clad RPV primary system containment etc. Significant effort to mitigate possibilities with additional layers of defense Airplane crash additional containment thickness

30 Challenges with Deterministic 30 More accident possibilities = more layers Expensive, complex and boundless Possible to bypass all layers (Fukushima) Competing effects for systems Core cooling systems LOHS Loss of heat sink LOCA Loss of coolant accident Therefore a more scientifically, data driven approach was desired.

31 Probability Risk Assessment 31

32 Sample = LOPA 32

33 Passive Safety Systems 33 4 levels of passivity (IAEA) A. no moving working fluid B. no moving mechanical part C. no signal inputs of intelligence D. no external power input or forces Do not yet have fully passive systems Increase in degree of passivity Many systems move to level C passivity Last 3-7 days depending on system

34 AP

35 ESBWR 35

36 Westinghouse SMR 36