THE MODELLING OF FUEL VOLATILISATION IN ACCIDENT CONDITIONS

H. MANENC, P. MASON and M. KISSANE Journal of Nuclear Materials (ICCT-2000 Conference, Halifax, August 2000)

For oxidising conditions, at high temperatures, the pressure of uranium vapour species at the fuel surface is predicted to be high. These vapour species can be transported away from the fuel surface, giving rise to significant amounts of volatilised fuel, as has been observed during small-scale experiments. Hence, fuel volatilisation must be taken into account in the conduct of a simulated severe accident such as the Phébus FPT-4 experiment, a large-scale in-pile test designed to investigate the release of fission prod-ucts and actinides from irradiated UO2 fuel in a debris bed and molten pool configuration. Best estimate predictions for fuel volatilisation were performed before the test. This analysis was used to assess the maximum possible loading of filters collecting emissions and the consequences for the filter-change schedule. Following the successful comple-tion of the experiment, blind post-test analysis is being performed; the boundary condi-tions for the calculations are based on preliminary post-test analysis with the core degra-dation code ICARE2. The general modelling approach is presented here and then illustrated by the analy-sis of fuel volatilisation in Phébus FPT4 (for which results are not yet available). Effort was made to reduce uncertainties in the calculations by improving the understanding of the controlling physical processes and by using critically assessed thermodynamic data to determine uranium vapour pressures. The analysis presented here constitutes a pre-liminary, blind, post-test estimate of fuel volatilised during the test.