Speciation and release kinetics of the fission products Mo, Cs, Ba and I from nuclear fuels in severe accident conditions
Abstract
A speciation and release kinetics study of the gaseous fission products freed from
nuclear fuels in conditions of severe accident (temperature range 1300°C –
2500°C) as a function of the applied thermodynamic conditions is carried out using
the experimental feedback collected at CEA within the VERCORS and VERDON
programs. The nature of the major species involved are deduced from the analysis of the
specific features of the Cs-Mo-Ba-I-O chemical system in the composition range of
irradiated 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, which
contributes to increase significantly the released fraction of molybdenum as
compared to reducing conditions.
An analytical model accounting for thermo-chemistry in the release rate of
chemically reactive elements is applied to estimate the release of Cs, I, Mo and Ba
in severe accident conditions. Good agreement is achieved with on-line
measurements
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