Requirements on future containments to avoid evacuation and relocation outside the plant

Contributions to the risk of nuclear reactors result essentially from severe accidents leading to core melting and subsequent phenomena that may result in the end in the failure of the reactor pressure vessel and the outer containment. The radiological impact of a severe accident on the area adjacent to a nuclear power plant can be assessed if the radioactive source term and the weather conditions are known using computer programs such as COSYMA for evaluating the off-site consequences of accidental releases of radioactive material to the atmosphere. For severe accidents with subsequent failure of the containment very large areas are calculated as a function of weather conditions, where the population must be evacuated. Although this can only occur with very small probabilities it is a basic safety problem of nuclear reactors. The solution to this is to design in such a way that containment failure cannot occur as a consequence of severe accidents. As a second condition, the containment must be double-walled and the radioactivity leaking into the space between the two walls must be filtered. With these design measures the reactor containment can also resist severe accidents. (author)