ASNR’S DECISION AND REQUIREMENTS Core 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 increase in internal air pressure makes it necessary to depressurise the containment in order to prevent damage to the reactor building. Radioactivity could also be released into the ground if the mixture of molten nuclear fuel, steel and concrete (known as «corium») were to melt through the lower slab of the reactor building. Severe accidents involving core melt How can the effects be limited? 18 • Les cahiers de l’ASNR • July 2025
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