Speciation and release kinetics of the fission products Mo, Cs, Ba and I from nuclear fuels in severe accident conditions

Abstract

International audienceA speciation and release kinetics study of the gaseous fission products freed fromnuclear fuels in conditions of severe accident (temperature range 1300°C –2500°C) as a function of the applied thermodynamic conditions is carried out usingthe experimental feedback collected at CEA within the VERCORS and VERDONprograms. The nature of the major species involved are deduced from the analysis of thespecific features of the Cs-Mo-Ba-I-O chemical system in the composition range ofirradiated nuclear fuels. Stable gaseous Cs and Ba molybdate species, i.e. $Cs_2Mo_2O_7$ (g)and $BaMoO_4 (g)$, clearly predominates in oxidising conditions, whichcontributes to increase significantly the released fraction of molybdenum ascompared to reducing conditions.An analytical model accounting for thermo-chemistry in the release rate ofchemically reactive elements is applied to estimate the release of Cs, I, Mo and Bain severe accident conditions. Good agreement is achieved with on-linemeasurement