Nuclear power plants observe a strict safety culture. Boat shed in Pyhäjoki, 2008.

Transcription

1 Nuclear power plants observe a strict safety culture. Boat shed in Pyhäjoki, The safety requirements related to the use of nuclear energy are based on the Nuclear Energy Act (990/1987), according to which nuclear power plants must be safe and must not cause any danger to people, the environment or property. 114 Nuclear safety

2 6 Nuclear safety Safety is the primary objective in the design, construction and operation of a nuclear power plant. Nuclear safety covers all of the measures with which the safety of employees, residents and the environment is ensured when using nuclear energy with regard to radioactive radiation. The safety and reliability of nuclear power plants is continuously being developed. In order to ensure the safe operation of nuclear energy, strict safety culture, special safety principles and regulations, as well as developed quality assurance methods, are followed in the design and operation of a nuclear power plant. The use of nuclear energy comprises operations subject to a license regulated by legislation. Safety requirements are taken into account in the design of the plant. The applicant of the permit related to the use of nuclear energy (holder later) is solely responsible for the safety of the operations. 6.1 Nuclear safety requirements The safety requirements related to the use of nuclear energy are based on the Nuclear Energy Act (990/1987), according to which nuclear power plants shall be safe and shall not cause any danger to people, the environment or property. The regulations of the Nuclear Energy Act are detailed in the Nuclear Energy Decree (161/1988). The general principles of the safety requirements set for nuclear power plants are issued in Decisions of Council of State /1991 and 478/1999. Their scope of application covers different sections of the safe use of nuclear energy. Detailed regulations for the safety, emer- Nuclear safety 115

3 The application for a decision-in-principle regarding the nuclear power plant is submitted to the Government. Stairs of the parliament house in gency and safety planning and nuclear material control of the use of nuclear energy are issued in the nuclear power plant guide series (YVL Guides) issued by the Radiation and Nuclear Safety Authority (STUK). Legislation concerning nuclear energy is currently being revised. On May 7, 2008, Parliament approved the Government bill for revising the Nuclear Energy Act (Government Bill 117/2007) and the revised act entered into force on June 1, In addition, the revision of the Government Decrees concerning nuclear safety (to replace Decisions of Council of State /1991, 478/1999) has advanced far. At the same time, the Radiation and Nuclear Safety Authority has launched preparations aimed at the long-term revision of the YVL Guide. The objective is to update the guide system s structure and revise them so that the number of individual guides can be reduced. According to the act for revising the Nuclear Energy Act as approved by Parliament, the leading principle of nuclear safety is that the safe use of nuclear energy is maintained at such a high level as possible through practical measures. Safety must be developed further on the basis of experience and safety studies, taking into account scientific and technological development. According to the defense in depth principle, the safety of nuclear facilities must be ensured using successive and independent protective measures. This safety principle must be extended to the plant s operational and structural safety. The possibility of operational failures and accidents must also be taken into account in the design of nuclear power plants. The legislation and instructions concerning nuclear safety have taken into account international agreements and safety requirements, such as the International Atomic Energy Agency s (IAEA) guidelines (STUK 2008, IAEA 2008). The fulfillment of the safety requirements will be assessed for each plant unit in great detail. On the basis of their consideration, STUK and the license holder can set design objectives that are tighter than the valid safety requirements. The safety requirements followed in Finland are considered to be tight internationally. 6.2 Nuclear safety principles and their implementation The safety of nuclear power plants is based on the defense in depth principle. Several independent and supplementary protection levels are to be applied to the design and use of the power plant (Figure 6-1) (IAEA 2000): 116 Nuclear safety

4 NORMAL OPERATION Individual device failure DISTURBANCE OR FAULT MANAGEMENT Failure of entire system ACCIDENT MANAGEMENT Total failure of safety systems SEVERE ACCIDENT MANAGEMENT Containment failure PREPAREDNESS ACTIONS, CIVIL DEFENCE Figure 6-1. Several levels of protection are applied to the design and operation of nuclear power plants in accordance with the defense in depth principle. Prevention of operational defects and failures through high-quality design and construction, as well as appropriate maintenance procedures and operations Observation of operational defects and failures and returning the situation to normal using protection, control and safety systems Management of design basis accidents using existing and planned safety features Observation and management of severe accidents using the accident management system Reduction in the consequences of releasing radioactive substances through emergency response operations. Nuclear power plants are to be designed so that the failure of operations at any single protection level does not result in danger to people, the environment or property. In order to guarantee reliability, each of the levels is to be built on several supplementing technical systems, as well as limitations and regulations related to the use of the power plant. Proven technology will be applied to the design of the nuclear power plant and all processes are designed to be naturally stable. For example, reactors will be designed to be naturally stable with regard to power control. This means that the reactor s inherent feedbacks will limit any uncontrolled increases in power. Safety in light water reactors is increased because a temperature increase in the coolant controls the power increase and a coolant leakage in the reactor shuts down the chain reaction. All safety-related equipment and functions will be designed on the basis of special safety inspections, taking into account improbable failures and applying sufficient safety margins. In addition, high quality requirements will be applied to the manufacture of safety-related equipment. As part of efficient quality control, the nuclear power plant s systems, devices and structures will be divided into categories on the basis of their significance to safety. The more important the category, the higher quality is required. Despite all of this, safety planning always starts from the assumption that there may be equipment failure or plant operators may make mistakes. The design of the plant will take into account internal incidents, such as device failures and mistakes made by plant operators, and external factors, including exceptional weather and environmental conditions, risks related to the operation of cooling water routes, such as clogging, and plane crashes, etc. The nuclear power Nuclear safety 117

5 Redundancy principle N+2 IN OPERATION UNDERGOING MAINTENANCE FAULT or IN OPERATION IN OPERATION UNDERGOING MAINTENANCE FAULT 100 % 100 % 100 % 50 % 50 % 50 % 50 % Diversity principle Separation principle E.g. Centrifugal pump Piston pump Figure 6-2. Design principles of safety systems. plant will be equipped with safety systems, using which the progress and impact of failures and accidents can be prevented or limited. The capacity of the safety systems will be designed to be redundant in relation to the need so that they can be divided into several parallel subsystems (Figure 6-2). The system entity consisting of redundant subsystems is able to perform its safety functions, even if any single device in the system fails and, simultaneously, any devices affecting the safety function is disabled for maintenance purposes. Because of redundancy, the safety systems will operate reliably and the reliability can be improved by using several devices of different types to perform one task. For severe accidents (melting of the fuel core), the plant will be equipped with special protection equipment and structures. Due to the improbability of such accidents, it is sufficient for the systems designed for the purpose that each system can perform their safety function, even if any single device in the system fails (STUK 2004). The nuclear power plant s safety planning ensures that radioactive substances contained by the plant, fuel in particular, can be prevented from spreading as reliably as possible in all situations. Radioactive fuel is prevented from spreading to the environment using several technical spreading barriers inside one another (Figure 6-3). Each of these barriers must be sufficient to independently prevent the spreading of radioactive substances into the environment. The first barrier is formed by the gas-tight and mechanically durable, metallic cladding of fuel rods. The second barrier consists of the reactor s pressure-resistant and tight cooling circuit. A pressure-resistant and gas-tight containment building surrounding the reactor forms the outermost barrier. According to Finnish safety requirements, the primary containment building is to be surrounding by another containment building cast of concrete (i.e. reactor building). This double protection forms the most important barrier for preventing radioactivity from spreading in the event of a severe accident. The space between the containment buildings can be maintained in a state of under-pressure in relation to outdoor air in all expected accident situations. As a result, any gases leaking from the inner containment building can be recovered and filtered in order to minimize gaseous emissions. The solid outer containment building also protects the inner containment building and reactor against external threats. The nuclear power plant will be designed so that the containment building also endures severe accidents, including melting of the fuel core. The containment building will prevent the molten core and the majority 118 Nuclear safety

6 Robust outer containment building Leak-proof inner containment Leak-proof reactor circuit made of steel Gas-tight fuel rod cladding Figure 6-3. Spreading barriers for fuel radioactivity. of gaseous radioactive substances from spreading into the environment. Outside the containment building, the radiation level will be at a safe and low level, even if radioactivity has been released inside the building. The nuclear power plant will follow a high safety culture and developed quality assurance measures. The procedures required for inspecting the use and failures of the nuclear power plant and any accidents will have thorough instructions and they will be practiced regularly. Special attention will be paid to attentiveness and precision in connection with maintenance and repair work. The objective is to protect the plant from failures and employees from radiation. The Radiation and Nuclear Safety Authority will supervise personnel training and review the instructions. STUK will also supervise the nuclear power plant s safety management. 6.3 Implementation of nuclear safety requirements and principles in the design, construction and operation of a nuclear power plant s nuclear power plant will be designed and constructed so that it meets the requirements set by legislation and the authorities. The design of the nuclear power plant will closely follow the safety principles and one of its central design criteria is preparation for different failure and accident situations, including the possibility of severe accidents caused by melting of the fuel core, as well as any threats caused by external factors. The nuclear power plant s containment building and the surrounding outer containment building, or reactor building, will protect the reactor and safety systems from external threats, such as extreme weather conditions, different flying objects, explosions, burning and poisonous gases and intentional damaging. The nuclear power plant will be constructed so that it will endure a collision from an airliner without releasing any significant emissions into the environment. The design of buildings important for safety will take into account the collision force caused by the plane and the fire generated by its fuel. The nuclear power plant s combined use in electricity and heat production does not have an impact on the plant s nuclear safety properties because the changes do not need to be targeted at the power plant s nuclear parts. If the need for adjusting the electrical or thermal power of a nuclear power plant operating in combined use has a significant impact on any malfunctions at the plant, e.g. by causing malfunction or failure types that do not occure in condenser use, they will be taken into account in the plant s safety design as dimensioning incidents similarly to other dimensioning incidents. Nuclear safety 119

7 Table 6-1. Observed natural phenomena, extreme value predictions for the latter half of the present century. Simo Pyhäjoki Ruotsinpyhtää Sea level (cm) (the figures for 2008 given in parentheses) Temperature, momentary Temperature, 24 hrs min (-189) -219 (-192) -122 (-132) max (+231) +161 (+188) +214 (+204) min max min max Rainfall (mm) 24 h days Snow load (kg/m 2 ) Wind velocity (m/sec.) gust, 3 sec average, 10 min The access of radioactive substances into the district heating network will be prevented by transferring heat from the turbine circuit to the district heating network using a closed and pure intermediate circuit so that the operating pressure in the intermediate circuit is greater than in the heat exchanger on the turbine side and the heat exchanger in the district heating network. has available long-term operating experience in the transfer of heat from the nuclear power plant to a nearby industrial plant s needs using such an intermediate circuit arrangement obtained at the Stade plant in Germany. In the event of a leak in the heat exchangers on the turbine side, it would be directed from the pure intermediate circuit towards the turbine. According to assessments, combined production will not have other nuclear or radiation safety impacts. The nuclear power plant s safety planning will take into account any natural phenomena occurring at the plant site, such as extreme weather conditions that may become more frequent or extreme due to the climate change. The design will also prepare for the increasing sea water temperature and rising sea levels caused by the climate change. The location areas to be assessed have isostatic uplift and its consequences will be assessed when designing the nuclear power plant. However, the uplift is steady and it is not expected to cause any special requirements for the design of the nuclear power plant. The design of the nuclear power plant will take into account extreme nature conditions and their estimates of occurrence. If required, the ground level of the nuclear power plant s location site will be raised sufficiently. The Finnish Institute of Marine Research has conducted a water level study, covering all of the alternative location municipalities for the nuclear power plant. The study is based on a comprehensive international literary review concerning changes in the ocean level during the next hundred years. In addition to the ocean level, the level of the Baltic Sea depends on changes in the total water volume in the Baltic Sea, isostatic uplift following the ice age and other weather and climate factors. (Finnish Institute of Marine Research 2008a). The Finnish Meteorological Institute has conducted a similar prognosis concerning the probability of occurrence of extreme temperatures, rainfall and wind speeds in these municipalities. The study is based on the best prognoses concerning global climate development, on the basis of which the occurrence of local weather conditions has been modeled. The confidence intervals of the prognosis for certain weather phenomena are rather great, because the observation material is obtained from a short period compared with the length of the prognosis horizon. (Finnish Meteorological Institute 2008a). The table (Table 6-1) presents prognoses concerning the studied natural phenomena at the end of the current century. The recurrence of the weather conditions is 1,000 years, which means that the reported value occurs once in a thousand years on average. The maximum and minimum values of water level are prognoses for The design of the plant and its cooling systems will take into account any oil accidents in the sea area and the occurrence and impact of frazil ice and packed ice. The structures will be designed so that the formation of frazil ice or packed ice cannot cause an obstacle to the nuclear power plant s cooling. The nuclear power plant and the nuclear materials used will be protected from illegal actions, such as vandalism and sabotage. Threats caused by terrorism or other illegal actions will be mitigated by thorough and continuous security arrangements. They will supplement the protection provided by the sturdy structure and protection of sensitive sections required by the plant s basic safety planning. The backgrounds of the personnel working continuously at the nuclear power plant or in outages will be checked and the employees movement in the plant area will be restricted to areas necessary for working using 120 Nuclear safety

8 The impacts on landscape have been extensively studied. Boats in Ruotsinpyhtää, access permits of different levels. When preparing for external threats, a situation where a threat is formed by a person or a group of people who work regularly or temporarily at the plant and have access permits will also be taken into account. At this stage, will analyze three different nuclear power plant options. When applying for a decision-in-principle, STUK will prepare a preliminary safety assessment for the application, assessing how these options meet the nuclear safety requirements valid in Finland. At the decision-in-principle application stage, safety will be assessed at the principle level. will select the plant location and the type to be built after a favorable decision-in-principle. The detailed implementation of the safety solutions for the plant option selected will be described in great detail when applies for a construction permit for the nuclear power plant in accordance with the Nuclear Energy Act. The license holder and STUK will assess the implementation of the safety solutions throughout the project s construction period. The structures implemented and the results received from test operations will be assessed as a whole when applies for the operating permit in accordance with the Nuclear Energy Act. Supervision of the use and safety of nuclear energy is the responsibility of STUK and the safety of the nuclear power plant will be controlled using different authority inspections. Inspections to be defined and recorded in the plant-specific operational inspection program by STUK will be performed at the nuclear power plant at regular intervals. In addition, the inspections required by the YVL Guide will be performed at the plant. In support of the supervision procedures, STUK is to be provided with regular reports and and reports on possible disturbance situations. Radiation caused by the nuclear power plant to the residents of the surrounding areas, the health effects of radiation and the emergency and rescue operations related to the operation of the nuclear power plant are described in more detail in Section 8. Radiation control is described in Section 11. Nuclear safety 121