EJERCICIO LIII

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.

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