Thermal and structural analysis of calandria vessel of a PHWR during a severe core damage accident

In a hypothetical severe core damage accident in a PHWR, multiple failure of core cooling systems may lead to collapse of pressure tubes and calandria tubes, which may ultimately relocate inside the calandria vessel forming a debris bed. The debris bed which is at high temperature and still generating the decay heat, is cooled by the overlaying heavy water in the debris. However with time this water gets evaporated and what remains after sometime, is the hot dry debris bed generating a lot of heat. The debris ultimately melts down and forms a corium molten pool in the lower head of the calandria vessel. The molten corium pool is surrounded by calandria vault water which works like the ultimate heat sink. However, a few unresolved issues on this phenomenon are; i. How long the vault water can act as heat sink and after that how the calandria vessel fails? ii. If cold water is continuously required to replenish the evaporating water in the calandria vault as a part of severe accident management strategy, then what is the rate at which the decay heat is removed from corium molten pool and nature of the thermal stress pattern built-up in the calandria vessel? In the present study, a numerical analysis is performed to evaluate the thermal loads and the stresses due to that in the calandria vessel following the above accident scenario. The heat transfer from the corium molten pool to the surrounding is assumed to be by a combination of radiation, conduction through the wall of calandria vessel and convection from calandria vessel wall to vault water. From the temperature distribution in the vessel wall, the transient thermal loads have been evaluated. The strain rate and the vessel failure in the calandria has been evaluated for the above two scenario.