Revaporisation of fission product deposits in the primary circuit and its impact on accident source term

Chemical revaporisation or physical resuspension of fission product deposits from the primary circuit is now recognised to be a major source term in the late phase of severe fuel degradation in a nuclear accident. These results come from tests carried out under different experimental projects in the European Commission (EC) Framework Programmes. These include the revaporisation tests carried out at the Transuranium Institute (ITU), Karlsruhe under the Fourth Framework Programme (FP4), the Phébus FP post-test analysis (PTA) programme that examined FPT1, FPT3 and FPT4 deposits in separate-effect tests as well as EXSI-PC tests carried out at VTT, Espoo. The first tests at ITU and VTT concentrated on the behaviour of caesium as a very important fission product; this has helped detailed interpretation of the integral Phébus FP tests and has clarified some puzzling observations. Testing with Phébus FPT1 and FPT4 deposits at ITU demonstrated that revaporisation is a likely, rather than a possible, phenomenon with a severely degrading bundle. They have also shown that any changes in temperature (substrate or gas), flow rate or atmosphere composition or pressure can lead to the volatilisation or removal of the deposited caesium. Cs was particularly easy to follow given the high activity levels of Cs in the deposit. However further analysis of the deposits shows that other fission products are also subject to revaporisation. In the most recent FPT3 test chemical analysis of the filters has enabled examination of other fission products and demonstrated that these can be equally active in such conditions. Further separate-effect tests in the EXSI-PC facility have also given further insight as to the chemical reactions that major fission products (e.g. Cs, I) undergo under steam flows. A significant fraction of iodine was observed to be released and transported in gaseous form at rather low circuit temperatures. Pure theoretical approaches are also used at IRSN to exhibit the mechanisms of interaction of iodine and caesium vapours with substrates that are representative of the primary circuit under severe accident conditions. These approaches are expected to help in interpreting the above-mentioned experimental evidence of vaporisation and the forthcoming results of the ISTP/VERDON 2 experiment conducted by CEA in the eponymous facility under mixed air and steam conditions. These studies will enable a much improved understanding of the important chemical interactions in the primary circuit and so permit a more accurate simulation (with an improved SOPHAEROS databank) of the primary circuit chemistry in a severe nuclear accident, and hence help to reduce the uncertainties in estimating the possible source term to the environment