Extension to severe accident investigations: Investigation of MOX/steel interaction

Abstract

International audienceIn the frame of severe accident studies, the overheated nuclear MOX fuel may interact with the steel alloy cladding leading to the formation of a liquid phase at temperature well below the melting temperature of the fuel itself. Moreover, the presence of a miscibility gap in the liquid phase in the U-O, Pu-O and Fe-O binary sub-systems that extends in the U-Fe-O, Pu-Fe-O and U-Pu-O ternary systems may induce an accumulation of fissile elements in a specific region of the damaged reactor, with the risk of re-criticality. This work reports the experimental results obtained at JRC on the PuO2-Fe3O4 system (liquidus and solidus) and at CEA on the Fe-U-O system (miscibility gap in the liquid phase). The JRC laser heating setup was improved to account for oxygen potential effects on liquidus/solidus transitions. At CEA, a novel experimental setup was developed to obtain new data in the Fe-U-O liquid miscibility gap.These experimental results have been used to check on the quality of the existing thermodynamic description of the Fe-U-O system and to assess the thermodynamic model of the Pu-Fe-O system using the CALPHAD method. Thermodynamic calculation performed using the U-Pu-Fe-O thermodynamic database will allow to predict the formation of the liquid phases during a severe accident