Radioactive Aerosols Emission in Fires

The inner walls of glove-boxes used in nuclear facilities may be contaminated by radioactive aerosols. It is therefore necessary to be able to predict the release rate of these aerosols in the case of a fire. This phenomenon has been studied in a small-scale test-chamber (volume 0.085 m3 ), using 10 em x 10 em Plexiglas samples, with cerium oxide (CeOz) as a substitute for plutonium oxide. Supply air flow-rate, heat flux density applied to the sample, mass of cerium oxide deposited on the surface of the sample and mass median aerody· namic diameter of the CeOz particles were made to vary in the experiments. Depending on supply air flow-rate and heat flux density, pyrolysis, or combustion were obtained. The highest release (up to 13% of the deposited mass) was obtained during combustion; during pyrolysis, the released mass fraction of contaminant ranged from 0.5 to 5%. In all cases, 80% of the release occurs during the first thirty minutes with a typical peak at the beginning of the experiment. The granulometric distributions of both pyrolysis / combustion aerosol and emitted Ce02 aerosol were also measured with an Andersen impactor. The results suggest that there is little interaction between the two aerosol populations. CeOz release rates correlate well with combustion flow-rate. Large-scale experiments involving several kilograms of Plexiglas seem to ʹconfirm this observation.