In the accident at the Fukushima Daiichi Nuclear Power Plant, fuel rods and reactor internals were overheated due to the radioactive decay heat and the oxidization of fuel cladding, and relocated to the lower region of the pedestal. To retrieve the fuel debris, location, distribution and characteristics of the fuel debris are quite important. However, this information still has not been clarified. To estimate the distribution of fuel debris, since experiments cannot provide sufficient information, numerical simulation will be a useful tool. There are some numerical codes for calculating conditions inside a reactor at severe accidents, however, those codes include assumptions and/or related to melt relocation, and cannot simulate relocation process directly.

In order to simulate the relocation and corium spreading behavior in PCVs without assumptions and scenario, a numerical simulation code that can phenomenologically evaluate the melting phenomena is required. Then, a phenomenologically-based numerical simulation code for predicting the melting core behavior, including solidification and relocation based on the computational fluid dynamics, JUPITER, has been developed in JAEA.

In this paper, by using JUPITER, we simulated the corium spreading process inside a pedestal. As a result, we confirmed that JUPITER has potential to estimate the debris distribution inside the pedestal region.

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