Les cahiers de l’ASN #02 - Conditions for the continued operation of EDF’s 900MWe reactors

ACCIDENTS WITH CORE MELTDOWN What is a core meltdown accident? C ore meltdown is the most serious accident that can occur in a nuclear reactor. It manifests itself by melting of the nuclear fuel which can then breach the reactor vessel, leading to a very substantial release of radioactivity (in the form of aerosols, gas and radioactive water) into the concrete containment. Releases of radioactivity into the environment are inevitable in such situations. These releases would be particularly significant if the air pressure within the containment was high enough to necessitate depressurisation to avoid damaging the reactor building . Releases of radioactivity into the ground are also possible if the “corium” – that is to say the mixture of molten nuclear fuel, steel and concrete – melts through the lower slab of the reactor building. 16 • Les cahiers de l’ASN • February 2021 ASN RESOLUTION AND REQUIREMENTS