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NUCLEAR SAFETY

What are the three most important factors for nuclear safety at the nuclear power plant?
1. Reactor management and controlled reactor shutdown in all conditions.
2. Removal of residual heat, when necessary.
3. Isolation of radioactive substances from the environment.

For how long does spent fuel have to be cooled?
In Olkiluoto, water cooling is used to cool spent fuel removed from the reactor until it is placed in the final disposal facility. Because of the generation of residual heat, the fuel assembly needs to be cooled in water for about one year. Due to radioactivity, the assemblies are stored underwater also after this.

What arrangements are in place at the Olkiluoto boiling water plants for the eventuality of the reactor core melting?
Preparations for core melting consist at the Olkiluoto 1 and 2 plant units of severe accident management systems. The systems are used to ensure the integrity of the containment and to minimise any releases into the environment. The reactor pressure is reduced automatically, if water level in the reactor decreases. This prevents the rupture of the reactor pressure vessel caused by high pressure and the resulting damage to the containment building.

Hydrogen fires and explosions are prevented by making the containment atmosphere inert with nitrogen. If required, the lower drywell of the containment can be flooded from the suppression pool. This cools the core melt in case it is discharged into the containment following the rupture of the reactor pressure vessel. Water will also protect the penetrations in the containment lower drywell against high temperatures. The penetrations are further protected by mechanical shields. The gas space of the containment is cleaned by filling it with water, which also slows down the rising of pressure inside the containment due to residual output.

If the cooling of the containment cannot be restored, the rupture disks in the top of the containment drywell open at 6 bar pressure to control containment pressure and temperature by allowing steam and gas to escape through a filter and the ventilation stack. The filter will minimise the environmental releases. The filter line is designed to withstand the pressure and temperature prevailing during severe accidents.

Has the probability of a severe reactor accident in Olkiluoto been assessed?
The safety and the potential risk factors of the Olkiluoto plant units have been assessed in many ways. One of the methods used is the probabilistic risk analysis, which can be carried out to identify and assess accident conditions leading to severe reactor core damage, and their probabilities.

The most recent analyses carried out for the Olkiluoto 1 and 2 plant units give an accident leading to severe reactor core damage a probability of ca. 1.2*10^-5 per one reactor year, i.e. per one year of operation at one plant unit. This result means that the probability of such an accident in one year is less than one out of 80,000 at one unit.

The graph above presents the relative reduction in the frequency of severe core damage at the OL1 and OL2 plant units during the years 1990–2010. Safety upgrades that have contributed to the reduction of the core damage frequency include e.g. the improvement of seismic resistance, the improvement of fire protection, improvements in the pressure venting systems of the reactors, and the building of a gas turbine plant in Olkiluoto to supply power to the safety systems of the Olkiluoto plant units, if necessary.

Several modifications have been implemented at the plant units that are not reflected in reduced core damage frequency, but have been designed to improve the management of a severe reactor accident and to minimise environmental consequences. These modifications include e.g. the reinforcement of the reactor containment against steam explosions and the filtered containment pressure relief system, which can be used in accident conditions to restrict any radioactive releases into the environment caused by the venting of the containment.

How has safety been ensured in the fuel pools of the reactor building?
If required, fire water can be used to cool the fuel in the fuel pools. The capacity of the fuel pools allows any one pool to be emptied at any time with the fuel transferred to the other pools.

And how is the safety of the interim storage for spent fuel ensured?
The interim storage facility for spent fuel also has multiple and redundant safety systems just like the power plant units. The cooling pools of the storage facility have been excavated underground in the bedrock. The pools feature a duplex cooling system (cooling chains) with only one chain needed to cool the pools. In an extreme situation, the cooling of the pools can be ensured by flooding them with seawater by means of special arrangements.

Over the years, TVO has made many improvements to the plant units. We are currently in the process of expanding the interim fuel storage facility. The storage facility will be equipped with new safety systems. The safety features of the pool will be improved by providing concrete covers for the storage pools and by reinforcing external structures.

Can a hydrogen explosion occur at Olkiluoto?
The reactor containment of the OL1 and OL2 plant units currently in operation in Olkiluoto is filled with nitrogen during power operation, which prevents hydrogen explosions. The leak-tightness of the containment is ensured by the steel liner embedded in concrete and the steel dome that forms the roof of the containment.

The containment has permanent systems for the controlled combustion of hydrogen released in a potential accident. This prevents the accumulation of combustible gas mixtures in the containment in case of a loss of coolant accident. Hydrogen fires and explosions can thereby be prevented in the reactor building. Pressure and hydrogen can be vented from the containment in an accident through a pressure relief line fitted with a SAM filter (Severe Accident Mitigation).

The generation of hydrogen due to the overheating of spent fuel in the reactor hall pools, as well as any hydrogen fires, are prevented by securing the cooling of fuel.

What are the safety features of OL3?
The systems of OL3 meet all current regulatory requirements and the plant unit has many new safety features. The plant unit has been designed from the start to prevent any significant environmental releases even in case of a severe reactor accident.

OL3 is designed to withstand an earthquake. There are many mutually supporting response arrangements in place for the loss of AC power or cooling water. There are several different backup systems for electrical power supply and several different systems are also available for the cooling of the reactor and for the removal of residual heat from the reactor and the containment.

Particular attention has been paid to the separation and independence of different safety systems. Extreme phenomena have been considered in the design: high seawater level, low seawater level, high outdoor temperature, low outdoor temperature and prolonged snowstorm. The safety of the plant is secured even if seawater cooling is lost.

OL3 is the first plant unit in Europe in which severe reactor accidents have been taken into account in the design from the very beginning. Severe accident management systems are used to ensure the integrity of the containment and to minimise releases into the environment.

During an accident, pressure is reduced in the reactor using the severe accident pressure relief system. This prevents the rupture of the reactor pressure vessel at a high pressure and the resulting damage to the containment building. The plant unit will be equipped with a core melt cooling pool. This so-called core catcher will ensure that even if the reactor core were to melt, it can be cooled down in the containment, thus securing the integrity of the containment.

The containment of OL3 is air-filled. The hydrogen released in a potential severe accident is tackled by hydrogen management systems, which will prevent hydrogen explosions. The containment can withstand hydrogen fires. Hydrogen can be removed from the containment using passive autocatalytic recombinators. Pressure and temperature inside the containment are controlled with a containment cooling system built specifically with severe accidents in mind. In addition, OL3 also features a filtered containment pressure relief system.

What size earthquakes and tsunamis is Olkiluoto prepared for?
Our location on the shore of the Bothnian Sea is quite safe from earthquakes and tsunamis. Finland is located in a seismically stable area and the Baltic Sea is too shallow for a tsunami to develop. Regardless, the earthquake resistance and the safety features of the plant have been improved in many ways over the years. The worst threats that could hit Olkiluoto have been taken into account in engineering as well as in the development and construction of safety systems. Such threats include e.g. storms, floods, freezing conditions, fires and earthquakes.

The Olkiluoto 1 and 2 plant units were not originally designed to withstand earthquakes. Analyses indicate that the modifications carried out ensure that the plant units in Olkiluoto will withstand an earthquake of such a magnitude that the probability of it occurring in Olkiluoto is in the order of one in 100,000 years. The probability of earthquakes has been estimated on the basis of the seismic history of the area, the location of the area in relation to continental shelves and the available geological knowledge. The actual seismic analysis for the plant units was carried out about 15 years ago.

The safety systems of the plant have been constructed to allow for a sea level increase of about four metres from its current level in Olkiluoto. On the plant site, a sea level increase of up to ground level is prepared for, which is approximately four metres above the sea level. Sea level measurements have been carried out in Rauma since 1933. The highest sea level with an increase of 123 centimetres was measured in January 2007.

What size floods or other natural disasters can Posiva's final disposal facility withstand?
The height of all the buildings and structures is +10 metres above sea level. Even a 10-metre rise in sea level would not constitute a major safety risk, as Posiva only handles cooled fuel and only in small amounts for short periods at a time. Once the fuel is placed in the transport cask or in the final disposal canister, it is safe from natural disasters.

How is Olkiluoto prepared for an oil spill accident occurring at sea?
The Government Decree on the safety of a nuclear power plant requires that power companies are prepared for any conditions caused by the environment or human activities that could endanger the operation of the power plant, and an oil spill occurring at sea is one of those potential conditions.

TVO has estimated the possibility of oil spills in the sea areas surrounding Olkiluoto and their impact on plant safety. The impact of an oil spill accident on plant safety is minor. In spite of that, TVO has acquired oil spill response equipment to be placed not only in the immediate vicinity of the plant but also on nearby islands, because the oil spill equipment of the municipal rescue services will probably be used elsewhere in case of an oil disaster.

The equipment containers of TVO's response equipment will be placed on the Kuusisenmaa and Lippo islands. The oil spill equipment will prevent any oil driven by the wind from spreading to the water area south of Olkiluoto and from being carried from there to the plants in the cooling water. According to plans, oil booms will be laid between the following islands: Kuusisenmaa - Lippo - Nousiainen - Kovakynsi.

TVO took part in EU-wide stress test of the safety of nuclear power plants. What was this stress test all about?
Following the accident at the Fukushima Dai-ichi nuclear power plant in Japan on 11 March 2011 the European Council decided on 25 March 2011 to carry out Europe-wide safety assessments of nuclear power plants.

TVO naturally takes part in this assessment and as a Finnish nuclear power plant licensee gives its full support to this process. The report that TVO has now submitted to the Radiation and Nuclear Safety Authority (STUK) is at reviewed first in Finland and then next spring as part of the assessment that covers the whole of Europe.

What is TVO's estimate of the costs of EU's stress test of nuclear power plants and what kind of investments does TVO expect to become necessary as a result of them?
In TVO's view, the required safety improvements will not cause any significant costs over the existing investment plans for the Olkiluoto plant. TVO follows a long-term plant improvement strategy.

Can we be sure that TVO has sufficient safety systems to prevent a severe accident from occurring?
Preparedness for different disturbances and the prevention of accidents form the basis for our operation. To this end, each of our plant units is equipped with multiple safety systems that are independent of each other, separated and operate according to different principles. The design basis is the elimination of the possibility of all safety systems being lost for the same reason.

We follow events occurring elsewhere in the world, and if necessary, analyse our own safety systems together with Finnish and international experts and authorities. Should it prove necessary, we develop our own plant units and practices and procedures to ensure an even higher safety level.

How has power supply to safety systems been secured at Olkiluoto?
The power supply of the emergency cooling systems at our plant has been secured in many ways by means of physically separated electrical systems.

In normal operating conditions, electricity is supplied from the plant unit's own main generator.
  • If the main generator of the plant unit is not available, electricity is supplied either from the other plant unit or from the national 400 kV or 110 kV grids.
  • Both plant units have four diesel generators that start automatically if power supply is lost.
  • The diesel generators can also be used to supply the other plant unit via the connecting line between the OL1 and OL2 plant units.
  • The emergency power station (gas turbine plant) in Olkiluoto can supply electricity to both plant units either via land cable connections or via the 110 kV switching station.
  • Certain systems have battery backup.
  • By special arrangement, electrical power is also available from the 20 kV network of power company Paneliankosken Voima
  • By special arrangement, electrical power is also available from the Harjavalta hydroelectric plant.
  • OL3 will have six diesel generators of its own, and it will be connected to the same power supply pool with the existing plant units.

Has the extra safety assessment been necessary due to safety deficiencies at Olkiluoto power plant?
There are no safety deficiencies at the Olkiluoto power plant; these safety assessments have been carried out at all European nuclear power plants. TVO follows a proactive philosophy as regards the development of the safety of the Olkiluoto nuclear power plant and as a result of this takes actively part in all processes designed to improve safety.

What action is TVO now planning to implement to improve the safety of the plant units?
The security of water supply to the reactor buildings of OL1 and OL2 will be further improved for the eventuality of loss of water supply through permanent safety systems. TVO will utilise the fire water system to realise non-electricity dependent water supply to the fuel pools of the reactor building and to the reactor pressure vessel.

TVO is also in the process of evaluating, designing or already realising the following plant upgrades for OL1 and OL2:
  • improving charging possibilities in prolonged power cuts for plant systems provided with battery backup.
  • improving independence of plant units from seawater cooling.
  • extensive renovation of stormwater drains in plant outdoor areas to improve their functioning in heavy and prolonged rainfall.
  • safety upgrades and extension of the interim storage of spent fuel.
  • adding more supports for I&C cabinets to improve the resistance of the systems to earthquakes.
  • installing on and off shore oil spill response equipment in Olkiluoto.
  • increasing the number of mobile safety equipment, such as diesel-driven pumps and aggregates on the plant site.

That is a long list of improvements – why haven't you thought of them before?
The safety of the plant units is mainly being improved by TVO of its own accord and the improvements that TVO has brought up in the safety report represent such self-initiated development efforts. As far as individual plant improvements are concerned, it should be borne in mind that TVO analyses these matters over the long term and in compliance with the philosophy of continuous improvement. The impact that several factors together have on the whole also has to be weighed thoroughly and no improvements are implemented without comprehensive investigations. The general principle is that safety and safety culture constitute the key factor for the Company and must be considered in everything that is done. But matters must be prepared and implemented in a sustained manner.

OL1 and OL2 are already old plants. Can they still be safe?
TVO has successfully fostered the culture of continuous improvement for decades. And as long as we continue to do so, we can vouch for the good, pristine condition of our plants. Our starting point is that, from a technical point of view, the plant units always have a further 40 years of service life left. Continuous improvement of safety features is also part of TVO's corporate culture. Examples of this include the previous improvements realised at the plant in terms of severe accident management, and the 160-million euro upgrade project designed to increase the lifespan of OL1 and OL2, which was completed this year.

Is TVO having problems with fuel pools inside the reactor buildings?
The majority of spent fuel is stored in the interim storage for spent fuel, where no safety deficiencies have been identified. The water reserves of the storage pools are so abundant that the loss of power supply or cooling due to an accident would not cause an emergency situation for weeks.

The spent fuel storage is at present undergoing a scheduled safety upgrade and extension project. Moreover, a comprehensive probabilistic safety analysis will be carried out on the spent fuel storage by the end of the year 2013. This type of a safety analysis has not previously been commonly required of spent fuel storages in the world, but TVO has decided to make the analysis proactively in preparation for future regulatory requirements.

OL3 is boasted to be airplane crash resistant. How about OL1 and OL2?
An airplane crash resistance analysis, among others, has been carried out for the OL1 and OL2 plant units. The buildings were concluded to be extremely strong, but due to security reasons, no details can be given about the analysis, such as the size of the analysed crashing object.

Airplane crash resistance was not one of the original design bases of the plant units, however. Because of this, the crux of the analyses was to assess e.g. the strength of the existing reinforced concrete structures.

Will TVO use seawater for cooling of the plant units in case of an accident?
The reserves of freshwater stored in Olkiluoto are sufficient to cool the plants and spent fuel for several weeks. In other words, the use of seawater is not necessary by definition.