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85 4 Nuclear safety

Safety is the first priority in the design, construction, and operation of the Fennovoima nuclear power plant. Nuclear safety covers all the measures utilized to ensure the safety of employees, the people, and the environment with regard to radioactive radiation when using nuclear energy.

The safety and operational reliability of nuclear power plants are under constant improvement. In order to ensure the safe use of nuclear energy, a strict safety culture, special safety principles and regulations, as well as advanced qual-ity assurance methods, will be followed in the design and operation of the nuclear power plant. The use of nuclear energy is subject to a license and is regulated by legislation.

Statutory safety requirements shall be taken into account in the design of the plant. The licensee relating to the use of nuclear energy carries the sole responsibility for the safety of the operations.

4.1 Nuclear safety requirements

The safety requirements relating to the use of nuclear energy are based on the Finnish Nuclear Energy Act (990/1987), according to which nuclear power plants shall be safe and shall not cause any danger to people, the envi-ronment, or property.

The regulations of the Nuclear Energy Act are specified further in the Nuclear Energy Decree (161/1988). The general principles of the safety requirements set for nuclear power plants are laid down in Government Decrees 734/2008, 736/2008, 716/2013, and 717/2013. Their scope of application covers the different areas of the safety of nuclear energy use. Detailed regulations for the safety of nuclear energy use, safety and emergency preparedness arrangements, and nuclear material safeguards are given in the regulatory guides on nuclear safety (YVL Guides) issued by the Radia-tion and Nuclear Safety Authority (STUK). Various naRadia-tional and international regulations and standards also control the use of nuclear energy. Figure 4-1 shows the hierarchy of Finnish nuclear safety legislation and the corresponding requirements.

The legislation concerning nuclear energy is currently being revised. Government Decrees 716/2013 and 717/2013

Nuclear

have entered into force on October 2013. At the time of writing, STUK is in the process of revising the YVL Guides.

The objective of the revision process is to update the guide’s structure and edit the whole guides so that the number of guidelines is reduced. Most of the new YVL Guides entered into force on December 1, 2013.

According to the Nuclear Energy Act, the leading prin-ciple concerning nuclear safety is to maintain the level of safety of the use of nuclear energy as high as reasonably achievable. Safety shall be developed further on the basis of operation 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 pro-tective measures. This safety principle must be extended to include the plant’s operational and structural safety. The possibility of operational transients and accidents must also be taken into account in the design of nuclear power plants.

International agreements and other safety requirements, such as the International Atomic Energy Agency’s (IAEA) guidelines, have been taken into account in the preparation of the legislation and instructions concerning nuclear safety (STUK 2013l, IAEA 2013).

The fulfillment of the safety requirements will be assessed individually for each plant unit. STUK and the licensee may, at their discretion, set design objectives that are more stringent than existing safety requirements. The safety requirements observed in Finland are internationally considered to be stringent.

4.2 Nuclear safety principles and their implementation

The safety of nuclear power plants is based on the defense-in-depth principle, which Fennovoima will implement in the project. Several independent and supplementary levels of protection will be applied in the design and use of the plant (Figure 4-2) (IAEA 2000):

• Prevention of operational transients and failures through high-quality design and construction, as well as appropriate maintenance procedures and operation

• Observation of operational transients and failures and returning the situation to normal using protection, con-trol, 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

• Mitigation of the consequences of releasing radioactive substances through emergency and rescue operations.

Nuclear power plants are designed so that the failure of operations at any single level of protection will not endan-ger people, the environment, or property. In order to ensure reliability, each of the levels is built on several mutually complementary technical systems and based on the limita-tions and regulalimita-tions relating to the use of the plant.

Figure 4-1. The hierarchy of Finnish nuclear safety legislation and the corresponding requirements.

86 4 Nuclear safety Proven technology will be applied in the design of the

nuclear power plant, and all processes will be designed for natural stability. For example, light water reactors are designed to be naturally stable with regard to power con-trol. This means that the reactor’s inherent feedbacks will automatically limit any uncontrolled power increases. The safety of light water reactors is further improved by the fact that an increase in the temperature of the coolant restrains increases in power, and coolant leakage from the reactor shuts down the chain reaction.

The design of all safety-related equipment and functions will be based on special safety analyses, for which even improbable failures are taken into account and sufficient safety margins are applied. In addition, high-quality require-ments will be applied in the production of safety-related equipment. As part of efficient quality management, the nuclear power plant’s systems, equipment, and struc-tures will be divided into categories on the basis of their significance to safety. The higher the category, the higher the required quality will be. Despite all this, safety design always starts from the assumption that equipment failures or plant operator errors are possible. Internal incidents, such as equipment failure and mistakes made by the oper-ating personnel, and external factors, including exceptional weather and environmental conditions, risks related to the operation of cooling water routes (such as clogging), and airplane crash will be taken into account in the design of the plant. The nuclear power plant will be equipped with safety systems enabling the prevention, or, at least, limi-tation, of the progression and impacts of transients and accidents.

The safety systems will be divided into multiple par-allel subsystems, the combined capacity of which will be designed to exceed the requirement several times over (the redundancy principle, Figure 4-3). The overall system consist-ing of multiple redundant subsystems will be able to

per-form its safety functions even in the case of failure of any single piece of the system equipment and the simultane-ous unavailability of any piece of equipment contributing to the safety function due to maintenance or some other reason. Redundancy ensures the operational reliability of the safety systems. Reliability can be further improved by utilizing several pieces of equipment of different types to perform the same function. This eliminates the chance of type-specific defects preventing the performance of the safety function (the diversity principle, Figure 4-3). The redun-dant subsystems will be separated from each other so that a fire or a similar incident cannot prevent the performance of the safety function. One alternative for implementing the separation is to place the subsystems in separate rooms (the separation principle, Figure 4-3).

To cope with a severe accident (core meltdown), 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 to cope with them to per-form their safety function even in the case of inoperability of any single piece of the system equipment (STUK 2004).

During the normal operation of the nuclear power plant, radioactive substances will be released in a controlled man-ner into the environment. The emission amounts will remain very small and below the limits set by the authorities. These emissions will have no significance for the safety of the local population or the environment. Uncontrolled emissions into the environment can be reliably prevented under all condi-tions. The uncontrolled emissions of radioactivity into the environment will be prevented with several successive tech-nical barriers (Figure 4-4). Each of these barriers alone will be sufficient to contain the radioactive substances.

The first barrier is the gas-tight and mechanically resist-ant, metallic protective cladding of the fuel rods. The second barrier consists of the reactor’s pressure-resistant and leak-tight cooling circuit. The pressure-resistant and gas-leak-tight,

Figure 4-2. Several protection levels will be applied in the design and operation of the nuclear power plant in accordance with the defense in depth principle.

87 4 Nuclear safety

Figure 4-3. The design principles of safety systems.

double-shell containment building surrounding the reactor forms the outermost barrier. The inner containment shell will be made of pre-stressed reinforced concrete that is capable of withstanding the tensile stresses caused by overpressure under accident conditions. The carbon steel lining installed at the inner surface of the shell will ensure its leaktight-ness. The outer containment shell will be a thicker structure

made of ordinary reinforced concrete. It will protect the reactor and the inner containment shell from external haz-ards and the associated collision loads. It will be designed to withstand loads including a large passenger airplane crash.

Furthermore, the outer shell will function as an additional barrier against the dispersion of radioactivity and limit the releases into the environment in accident situations.

Figure 4-4. The barriers preventing the dispersion of the radioactivity contained in the fuel.

88 4 Nuclear safety The nuclear power plant will be designed so that the

containment building will also withstand a severe core meltdown accident. The containment building will pre-vent the spreading of the core melt, and the dispersion of the majority of the gaseous radioactive substances con-tained therein, into the environment. Outside the contain-ment building, the radiation levels will remain low and safety will be maintained even in the case of a release of radioactivity inside.

A high standard of safety culture will be followed, and advanced quality assurance procedures utilized in the operations of the nuclear power plant. Comprehensive instructions will be prepared for the performance of meas-ures required in the various operating conditions of the nuclear power plant, as well as for clearing any transient and accident conditions. Furthermore, these measures will be drilled regularly. During maintenance and repair work, particular attention will be paid to attentiveness and precision. The objective is to protect the plant from distur-bances, and the employees from radiation. The Radiation and Nuclear Safety Authority (STUK) will audit personnel training and revise the safety-related instructions. STUK will also audit the whole safety management system of the nuclear power plant.

4.3 Management of external hazards

The nuclear power plant will be designed to withstand the loads resulting from various external hazards. These include extreme weather conditions, sea and ice-related phenom-ena, earthquakes, various missiles, explosions, flammable and toxic gases, as well as intentional damage. The nuclear power plant will be constructed so that it will withstand a large commercial airplane crash without significant emis-sions into the environment. Both the collision force caused by the airplane itself and the eventual fire caused by its fuel will be taken into account in the design of the buildings that are important to safety.

Other factors that will be taken into account in the design include the eventual impacts of the climate change, such as the increasing frequency of extreme weather phe-nomena, the warming of the sea water, and the rising of the average sea water level.

The impacts of the land uplift occurring at the Pyhäjoki area will be assessed in conjunction with the designing of the nuclear power plant. However, the uplift is steady and is not expected to place any particular requirements on the design of the nuclear power plant.

Various weather-related phenomena, such as low and high temperatures, rain and snowfall, snow loads, strong wind, whirlwinds, downbursts, air humidity, and lightning, will be taken into account in the design of the nuclear power plant. As regards extreme natural phenomena, the safety system design values will be determined in accord-ance with the requirements of the YVL Guide B.7 so that they are expected to be exceeded with a frequency lower

than once in 100,000 years. Conditions occurring at an even lower frequency will be prepared for by ensuring that the most important safety functions can be performed even in the case that the design values are exceeded.

The high temperature, as well as low and high levels of sea water will also be taken into account in the design. In accordance with the YVL Guide B.7, the design value for the nuclear power plant with regard to the sea water level (i.e. the construction elevation) shall be determined by adding a wave margin plus an additional two meters to the highest sea water level occurring at the location once in a hundred years. The construction elevation determined for the Fennovoima plant (approximately +4.9 meters accord-ing to the N2000 system) fulfills the YVL Guide require-ment by a good margin. This means that the plant will not be endangered even under highly exceptional flooding conditions. Flooding has also been taken into account in the design of the roads leading to the plant site; the plant site can be accessed via two separate roads, of which at least one will remain available even when the sea water level is exceptionally high.

Factors with a potential impact on sea water intake, such as oil spills in the surrounding sea, formation of pack ice, frazil ice (formation of ice crystals in subcooled water), and extensive occurrences of algae and fish, will be taken into account in the design of the plant and its cooling sys-tems. The various impurities will be removed from the sea water by successive screens and filters. The sea water intake structures will be located in the harbor area which will be protected with breakwaters. Breakwaters will prevent ice from entering the intake harbor. In addition to the main channel, cooling water can be taken through the auxiliary cooling water intake channel or from the outlet side, if required. Despite these design solutions, provision will also be made for situations in which sea water cooling is totally lost.

As Finland is located in the central part of the Eura-sian continental platform, intense earthquakes are very rare and highly improbable. Earthquakes will nevertheless be taken into account in the design of the nuclear power plant. The design basis earthquake was determined, in accordance with the requirement of the YVL Guide B.7, so that greater earthquakes are estimated to occur with a frequency lower than once in 100,000 years. The perfor-mance of the most important safety functions will be possible even in the case that the design basis earthquake is exceeded.

Experience gained from the Fukushima accident has also been utilized in the design of nuclear power plants.

The reliability of the power supply during various extreme natural phenomena will be taken even more into account than before in the design of new reactors. Passive systems enable reactor cooling even in possible loss-of-power sit-uations. Provision will be made for the loss of sea water cooling by equipping the reactor so that heat can be transferred into the atmosphere. Sufficient cooling must be ensured for both the nuclear fuel contained inside the reactor and the spent fuel placed in separate cooling pools outside the reactor.

89 4 Nuclear safety

The nuclear power plant and the nuclear materials used will be protected from illegal activities, such as vandalism and sabotage. Threats caused by terrorism or other illegal activities will be addressed through continuous implemen-tation of comprehensive security arrangements. They will supplement the protection provided by the sturdy structure and protection of sensitive components required by the plant’s basic safety planning.

The backgrounds of the personnel working at the nuclear power plant either permanently or during annual maintenance outages will be checked, and employee access in the plant area will be restricted, with the help of access permits of different levels, to areas necessary for the perfor-mance of their work tasks. Preparation for external hazards will also cover situations in which the threat is presented by a person or a group of people who work regularly or tem-porarily at the plant and have access permits.

4.4 Verification of nuclear safety and authority supervision

The Ministry of Employment and the Economy requires that the Radiation and Nuclear Safety Authority (STUK) assess the safety of the AES-2006 pressurized water reactor plant. The implementation of the safety solutions will be described in detail in conjunction with Fennovoima’s sub-mittal of a construction license application for the nuclear power plant in accordance with the Nuclear Energy Act.

The licensee and STUK will continue the supervision of the implementation of safety solutions throughout the project’s construction period. The solutions implemented, and the results gained from pre-operational testing, will be assessed as a whole when Fennovoima applies for an operating license 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 by means of various author-ity inspections. STUK will determine, and record in the plant-specific operational inspection program, the inspec-tions to be periodically performed at the nuclear power plant. Additional inspections performed at the plant will include those required in the YVL Guides. To support the supervision operations, STUK is to be provided with peri-odic reports as well as reports on any transient situations.

The radiation exposure of the local population caused by the nuclear power plant, the health impacts of radiation, and the emergency and rescue operations relating to the operation of the nuclear power plant are discussed in more detail in Sec-tion 4.5.3. RadiaSec-tion monitoring is discussed in Chapter 10.

4.5 Management of abnormal and accident situations

For nuclear power plant design and safety assessments, potential nuclear power plant conditions are classified as follows:

1. normal operating conditions 2. anticipated operational transients 3. postulated accidents

4. severe accidents.

This chapter deals with the last three categories, i.e. condi-tions which constitute a deviation from normal operation.

4.5.1 Abnormal situations at nuclear power plants and the related requirements According to the Nuclear Energy Act, provision shall be made in the design of nuclear power plants for the possibil-ity of operational transients and accidents. A nuclear power plant accident does not necessarily constitute a situation in

4.5.1 Abnormal situations at nuclear power plants and the related requirements According to the Nuclear Energy Act, provision shall be made in the design of nuclear power plants for the possibil-ity of operational transients and accidents. A nuclear power plant accident does not necessarily constitute a situation in