In the case of an accident the safety systems prevent uncontrolled release of radioactive substances into the environment.
Great attention is paid to nuclear safety, especially even during the phases of plant design and construction. Safety systems are designed to ensure safety in all operating conditions, even in the case of a system component failure.
A nuclear power plant is in a safe condition if the following three basic safety conditions are met at all times:
- effective control of the reactor power,
- cooling of the nuclear fuel in the reactor, and
- prevention of the release of radioactivity.
A series of multiple physical barriers are used to prevent the escape of radioactive material (defence-in-depth):
- The first barrier is the nuclear fuel itself (solid ceramic fuel pellets), in which radioactive byproducts of the fission reaction are tightly bound.
- The second barrier is the cladding that surrounds the fuel pellets and prevents the escape of radioactive gases from the fuel.
- The third barrier is the primary system boundary (the pipe walls, the reactor vessel and other primary components). This serves to contain the radioactive water used to cool the reactor.
- The fourth barrier is the hermetically sealed, massive, reinforced steel and concrete structure – the containment, which seals off the primary cooling system from the environment.
The basic aim of the first three barriers is to prevent radioactive substances from reaching the next barrier. The fourth barrier prevents the direct release of radioactive substances into the environment of the nuclear power plant.
Since the functioning of the safety systems in the case of an error, malfunction or even the unlikely case of an accident, is of paramount importance, all components of the safety systems are duplicated. The two safety systems perform key functions independently, such that, if one fails, there is always another to back it up, thus providing continuous protection. Furthermore, all the safety systems or their individual components are systemically tested during plant operation and scheduled outages.