Estimation of the Grain Boundary Gas Inventory in MIMAS/AUC MOX Fuel and Consistency with REP-Na Test Results

Abstract

In the frame of its research activities on fuel safety, the French ''Institut de Radioprotection et de Sûreté Nucléaire'' performed the REP-Na program in the CABRI reactor devoted to the study of Reactivity Initiated Accidents. Focused on high burn-up UO 2 and MOX fuel behaviour, twelve tests (8 UO 2 and 4 MOX) were realized from 1993 to 2000. In all these tests, the influence of grain boundary gas was evidenced and it appeared necessary to perform some estimation of its inventory in irradiated fuel. Such evaluations are presented for the MOX MIMAS/AUC fuel, based on two different approaches: ''experimental'' and ''theoretical.'' The fission gas amount located at the grain boundaries increases with burn-up in correlation with the production, but also with the initial Pu enrichment, as soon as the agglomerates have reached the full restructuring threshold for the High Burn-up Structure. The consistency with the REP-Na test results is checked, showing that a significant cladding deformation is needed, clearly higher than for UO 2 fuel in order to release all the grain boundary gas in RIA. Furthermore, to the fission gas effect, adds the helium's occluded in the irradiated fuel whose amount increases with burn-up, Pu enrichment and 241 Pu and 241 Am initial content.