In Pressurized Heavy Water Reactors (PHWRs), multiple failures of engineered safety features may cause failure of core cooling eventually leading to core collapse. The failed channels relocate to the bottom of the calandria vessel and form a terminal debris bed which generates decay heat. With time, the moderator evaporates and terminal debris bed ultimately melts down and forms a molten pool of corium. If corium breaches the calandria vessel and enters the calandria vault, large amount of hydrogen and other fission gases may be generated due to molten core concrete interaction (MCCI), which may pressurise the containment leading to containment failure. In addition, the passive catalytic re-combiner devices(PCRDs) may be incapable to manage such large amount of hydrogen. Hence, in-vessel retention of corium is the only option to avert progression of accident. The heat removal capability of calandria vessel to contain the corium during severe accidents needs to be demonstrated in order to attain the goal of in-vessel retention. A lot of efforts have been done for numerical analysis of heat removal capability of calandria vessel. However, experimental demonstration of in-vessel retention has been rarely presented in literature, especially for PHWRs. In the present paper, in-vessel retention at prototypic temperatures has been presented. Experiments have been carried out in scaled calandria vessels. Different corium simulants have been used at elevated temperature and corium coolability has been demonstrated.

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