Romania is now a UE member since January first 2007. New challenges are for our country that includes, also, their nuclear power reactors. Romania operates since 1996 a CANDU nuclear power reactor and soon will start up a second unit. In EU are operated PWR reactors, so, ours have to meet UE standards. Safety analysis guidelines require to model severe accidents for these types of reactors. Starting from previous studies a CANDU degraded core thermalhydraulic model was developed. The initiating event is a LOCA with simultaneous loss of moderator cooling and the loss of emergency core cooling system (ECCS). This type of accident is likely to modify the reactor geometry and will lead to a severe accident development. When the coolant temperatures inside a pressure tube reaches 1000 C, a contact between pressure tube and calandria tube occurs and the decay heat is transferred to the moderator. Due to the lack of cooling, the moderator eventually begins to boil and is expelled, through the calandria vessel relief ducts, into the containment. Therefore the calandria tubes (fuel channels) uncover, then disintegrate and fall down to the calandria vessel bottom. All the quantity of calandria moderator is vaporized and expelled, the debris will heat up and eventually boil. The heat accumulated in the molten debris will be transferred through the calandria vessel wall to the shield tank water, which surrounds the calandria vessel. The thermal hydraulics phenomena described above are modeled, analyzed and compared with the existing data.

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