Study on heat removal capability of calandria vault water from molten corium in calandria vessel during severe accident of a PHWR

Abstract

Recent Fukushima nuclear accident has triggered further awareness amongst reactor designers regarding enhancing the safety measures in a nuclear reactor. It has become important to analyse the capability of decay heat removal in a reactor to avoid radioactivity releases to the environment. Such a study has been carried out for an Indian PHWR. In a hypothetical severe core damage accident in PHWR, multiple failure of the core cooling system may lead to collapse of pressure tubes and calandria tubes, which may ultimately relocate inside the calandria vessel forming a debris bed. Due to decay heat generation, the debris ultimately melts down forming a molten pool inside calandria vessel. Calandria vessel is surrounded by calandria vault water that acts as heat sink. In order to study the extent of heat transfer from molten pool to surrounding water under severe accident condition, an experiment was carried out wherein a simulant material was poured inside a simulated calandria vessel immersed in the simulated calandria vault water. The amount of melt and water present in calandria vault scaled proportionately with regard to an Indian 700MWe PHWR. Results show that as soon as the melt was poured in the vessel, a thick crust was formed on the inner calandria vessel, which reduced the heat transfer from the melt pool to vault water resulting high temperature gradient in melt. Even though the cylindrical vessel inner temperature was found to be very high, the water outside the vessel never boiled. When the cylindrical vessel was opened after the experiment, there was no gap observed between the vessel and crust. Numerical analysis was carried out to predict the temperature profile of the molten pool and the vessel which were in good agreement with experimental results. Results were compared for the crust growth rate and temperature profiles in the melt pool considering the decay heat and without decay heat. Results show that with no decay heat consideration, the crust thickness continuously increases with time and in case of decay heat generation, crust thickness is found to be a function of decay heat. The melt temperature is found to increase above a decay heat of 1%.