READY FOR THE UNEXPECTED
The EPR™ reactor design follows the “defense in depth” principle, which entails imagining all possible threats to ensure the integrity of the plant’s protective barriers and to confine the consequences of an unlikely accident inside the fence of the nuclear plant.
List of credible events ranked according to their probability of occurrence in the Design Basis conditions
- normal operation (e.g. Plant start up, Plant shut down, Plant house load…),
- anticipated operational occurrences (e.g. Loss of offsite power, Loss of main feedwater…),
- infrequent accidents (e.g. Steam generator tube rupture, Small break loss of coolant accident…),
- limiting accidents (e.g. Large break loss of coolant accident, Main steam line break…),
The Design extension conditions extend the range of postulated accidents showing the robustness of the design.
EPR reactor: safety robustness (3D animation)
AREVA's EPR reactor satisfies the most stringent safety standards. This means: An EPR unit would remain intact even in the event of an extreme impact. This animation demonstrates how the robustness of the EPR safety concept is ensured.
In the unlikely event of a core meltdown, the molten core would be passively collected, retained and cooled in a specially designed area inside the reactor containment building, with water coming from an in-containment storage tank after spreading.
Even in an extreme situation, the accident would be confined within the reactor containment building.
The EPR™ reactor is one of the few reactors in the world to include a corium spreading area, capable of containing and cooling molten core before it reaches the building’s foundations.
Hydrogen risk mitigation
Several EPR™ reactors features help mitigate risks incurred in the case of hydrogen production, resulting from a severe accident and fuel damage: the specific design of the containment and the large volume of the reactor building would bring about some passive dilution of the hydrogen and prevents any hydrogen detonation in the reactor building.
The containment also includes passive catalytic hydrogen recombiners, which prevent the accumulation of hydrogen gas and the associated risk of its combustion.
The pre-stressed concrete inner-shell of the containment is designed to withstand pressures that could result from the combustion of hydrogen releases.
Radiological external releases
The confinement of the EPR™ reactor has been designed to reduce radiological consequences to neighbouring populations thanks to:
- a robust design avoiding radioactive releases in the most likely normal and accidental situations
- a metal liner which prevents any radioactive material from escaping,
- an annular space between the double concrete shell of the reactor where all gaseous waste are kept under control.
Protection against intentionally malevolent acts, including the possibility of a large commercial airplane crashing into the site is reinforced by use of a strong double-walled concrete shell implemented to shelter the EPR™ reactor.
The EPR™ reactor building, as well as two of the four safeguards buildings, the spent fuel pool within the fuel building, is protected by:
- The outer shell provides enough strength to absorb the impact of such a crash. It is made of reinforced concrete. It protects all sensitive areas, including the reactor building, the fuel building and two out of four safeguard buildings.The external wall is 1.30 meters thick.
- The inner shell made of pre-stressed concrete which is covered on its inside side with a thick metallic liner. It is no less than 1.3 meters thick
In addition, to prevent radiological releases, the EPR reactor building is equipped with:
- An annulus space between the walls which is 1.8 meters wide.
Inside the plant, full separation of each safety train into a specific building improves protection against forced intrusion, as well as against the risks of fire or flood.
- A thick outer shell made of reinforced concrete. The external wall is 1.8 meters thick
- A thick inner shell made of pre-stressed concrete. The inner shell is internally covered with a leaktight metallic liner. And the inner wall is no less than 1.3 meters thick
- The annulus space between the walls is 1.8 meters wide.
The EPR™ reactor plant stands on a single 6-meter thick reinforced concrete basemat, helping it to withstand severe earthquakes.
Buildings are also more resistant to earth tremors, thanks to their minimized height. Finally, the heaviest components of the plant, such as the water tanks, are installed at the lowest possible level for safety.