Preliminary Analysis of Phebus FPT3 Experiment with the Severe Accident ICARE2 Code

The last test of the international Phebus Fission Product (FP) program, named FPT-3, has been performed in November 2004 at Cadarache. It used boron carbide as absorber material, instead of Ag-In-Cd in the previous tests. Such absorber material is used in power plants like BWR, VVER, some western type PWR and EPR, and the importance of its effects on the main degradation phenomena is still to be assessed. This paper relates the main experimental events issued from the test together with the interpretation work coming out from code calculations. Several results were unexpected and some are of importance for safety analyses, particularly concerning cladding oxidation and fuel degradation. The test interpretation has been undertaken with the ICARE2 code, developed by IRSN “Institut de Radioprotection et de Suˆrete´ Nucle´aire”. ICARE2 is the stand alone part of the mechanistic ICARE/CATHARE code, built for safety analyses purposes. In the last code versions, models deal with the degradation and the oxidation of the B₄C control rod. Already validated against QUENCH experiments, these models have been applied to the Phebus FPT-3 conditions. Calculations show that, although general trends of the test are very well reproduced, specific effects concerning the interaction of B₄C with the neighbouring rods cannot be calculated. Better detailed physical analysis and prediction capabilities of calculation tools are expected to come out from the International Source Term Program, in particular from the results of the BECARRE separate-effect-tests performed at IRSN. Though the detailed modelling of such interactions has still to be improved, the Phe´bus FP series of integral experiments have proved the capability of the program to significantly improve our understanding of the source term issue and important aspects of core degradation.