Experimental studies on the revaporisation behaviour and aerosol formation of caesium under severe accident conditions

The uncontrolled release of radioactive fission products from the primary circuit of a nuclear power reactor, described as "release source term", is one of the major criteria for assessing a severe nuclear accident. One factor with the possible influences on the source term is the chemical revaporisation of fission products (especially 137Cs) after their initial deposition inside the primary circuit or containment. To study this behaviour using realistic samples of fission product deposits a lead shielded revaporisation device was designed and set-up inside a glove box. In the most recent test sequence, consisting of three experiments, samples from the Phébus FPT3 experiment was heated with 2 K/min up to 1273 K under steam, hydrogen or air. The tests showed similar results for the two experiments under oxidising conditions (steam and air) were used as process gas. The experiment using reducing conditions (pure hydrogen) showed diverse results. The results indicate that revaporisation is possible and that a switch in the atmospheric conditions (reducing to oxidising) lowers the threshold temperature which is necessary to trigger revaporisation. For the last experiment a sampling method was introduced to determine the size and chemical composition of the aerosols that form. Analysis showed a large number of particles in the diameter range 8-20 nm containing Cs, Re and Si