In case of a severe accident, the containment is the ultimate barrier to the environment. Therefore, reliable simulations tools for containment thermal hydraulics, including hydrogen distribution are indispensable. We simulated the behavior of the containment atmosphere under severe accident conditions with a postulated source term of water, steam and hydrogen. We used a detailed 3D CFD code (GASFLOW) and a lumped parameter code (MELCOR) in order to compare and assess their modeling capabilities. A simplified generic containment including all important components was used as a test bed. We analyzed the calculated pressure histories, mass and energy balances, convective flow as well as steam and hydrogen distributions. Integral values were modeled in good agreement by both codes. The overall flow was reasonably predicted. However we observed discrepancies in the calculated steam and hydrogen concentrations.

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