Improvement of core fusion modeling in ICARE/CATHARE. Application to the calculation of a six inches Loca leading to severely degraded situation

Titre du congrès :NURETH-12 (Nuclear Reactor Thermal Hydraulics) Ville du congrès :Pittsburgh Date du congrès :30/09/2007

In order to analyze the course of a hypothetical severe accident, the French “Institut de Radioprotection et de Sûreté Nucléaire” (IRSN) has developed in the last decade computer codes which have been extensively used to support the level 2 Probabilistic Safety Assessment (PSA2), and, in general, for the safety analysis of French PWRs. Particularly, the computer code ICARE/CATHARE V1 is a tool which has been widely validated and intensively used within the framework of the PSA2 of 900 MWe French PWR. This code has been tested on many accidental scenarii and the results obtained have been considered to be satisfactory and reliable up to the end of the early degradation phase. But, severe accidents in PWRs are characterized by a continuous evolution of the core geometry due to chemical reactions, melting and mechanical failure of the rods and other structures. These local variations of the porosity and other parameters lead to multi-dimensional flows and heat transfers. So the lack of multi-dimensional two-phase thermalhydraulics model appeared to be prejudicial to achieve best-estimate reactor studies with ICARE/CATHARE V1 in case of large core blockages and/or in case of large cavity appearance. In accordance, a full multi-dimensional modeling (covering both the fluid flow and the corium behavior) was developed and introduced in a new ICARE/CATHARE version referenced as V2, which includes two options for the thermalhydraulics modeling (either 1D or 2D). The first part of this paper demonstrates that, without activating the new V2 models, the ICARE/CATHARE V2(1D) is able to reproduce the results obtained with ICARE/CATHARE V1 on the basis of a six inches LOCA. Then, in order to illustrate some of the new V2 modeling improvements, the last part is focused on the results obtained with ICARE/CATHARE V2(2D) and a preliminary comparison is made with ICARE/CATHARE V2(1D). This 1D-2D comparison points out in particular the important role which could be played in the course of a severe accident by the multidimensional flow pattern.