Analysis of in-vessel natural circulation and lower head rupture of IRIS under severe accidents

The progression and mitigation measures of the severe accident induced by turbine trip and partial loss of forced coolant flow of IRIS reactor were analyzed with RELAP5/SCDAP computer code. The analytic results show that with the large inventory of water in the vessel, the core will under submerged for a long time, nearly 7 hours later the core begin uncovered, and the core became damaged 10 hours later. For the case with no injection and no depression, the vessel will fully dry out, there will be nature circulation between the core and steam generator, the core temperature will rise slowly, the low melting point control rod metal firstly meltdown, and fall into the lower plenum. The molten control rod material will heat the lower head, under high temperature and high pressure difference, the lower head will suffer creep rupture at the lowest region. For the case with no depression, a charging pump injection will mitigate the severe accident. (authors)