A review of revaporisation behaviour of radioactive Cs deposits and its impact on the source term in severe nuclear accidents from Phébus FP results and single effect testing

Revaporisation of the fission products deposited in the primary circuit of a reactor was identified as a possible late source of fission product release during a severe accident: e.g. loss of coolant accident (LOCA). Subsequent testing has shown that revaporisation is very likely to occur given a breach of the reactor and is an important contributor for the source term release to the containment and biosphere. The first part reviews the revaporisation mechanisms of Cs and other volatile or semi-volatile fission products transported in the primary circuit that were derived from the Phebus FP and associated programmes. The second part examines the separate effects testing to determine the high temperature chemistry of volatile and semi-volatile fission products (I, Mo, Ru) and structural materials (Ag, B), as well as atmospheric effects that substantially affect the source term. Finally, it examines Cs data from reactor accident sites that is providing additional knowledge of longer-term fission product chemistry. The results have been summarised in the form of a table and schematic diagram. This accumulated knowledge and experience has important applications in minimising contamination during decommissioning and site remediation techniques, as well as improving SA simulation codes and raising nuclear safety.