PWR severe accident behaviour and containment response for some sequences initiated at shutdown

Some shutdown accident sequences considered in the probabilistic safety assessment for the Sizewell B reactor are described. These are analysed using the MAPP3.OB severe accident analysis code and some of the results discussed. The core melt progression, containment response and fission product behaviour for these sequences are contrasted with those at full power. The phenomenon of air ingression following pressure vessel failure and its implications for core degradation and fission product release are examined. (8 figures; 1 table; 4 references). (UK)