Assessment of medium-term radioactive releases in case of a severe nuclear accident on a Pressurized Water Reactor. Experimental study of fission products re-vaporisation from deposits (Cs, I)

The vaporisation of CsI aerosol deposits from stainless steel substrate simulating the surface of a RCS in severe accident conditions was studied. Tests were performed with CsI aerosols deposited on oxidized stainless steel surfaces, simulating material deposited in cold zone of the RCS. The nature of both vaporized species and the surface residue was investigated. It was observed that the atmosphere during the vaporisation process in the 450-650°C temperature range plays a major role on the reactivity of CsI: in presence of steam, CsI is entirely vaporized, and remains stable in the vapour phase so that only this species is transported in colder zone. No interaction with the oxidized surface was evidenced. In presence of oxygen (air atmosphere), Cs partly reacts with the surface leading to the formation of caesium-chromates inducing the release of molecular iodine. Nevertheless, the amount of gaseous iodine detected during these experiments cannot be accounted by this mechanism only; a second reaction pathway owed to the presence of steam or impurities in trace amount may explain the observed high gaseous molecular iodine release. These results will contribute to assess potential iodine delayed releases due to the revaporisation of CsI species deposited in the RCS, during a nuclear severe accident. (author)