3. Defense in depth

For the environment of a nuclear power plant, radiological risks are essentially associated with the possibility of disseminating radioactive substances present in the reactor core and in spent fuel storage (in pools in the case of French facilities), as well as, to a lesser extent, in process fluids; these are essentially fission or activation products.

Containment of these radioactive substances is achieved by interposing "barriers" between them and the environment, to prevent them from spreading. Schematically, for a pressurized water reactor (PWR) of the type operated in France, there are three main "barriers" between the fuel and the fission products created in the reactor core, on the one hand, and the environment, on the other: these are the fuel rod cladding, inside which the fuel pellets (UO ₂ or UO _...