Changing perspectives on severe accident source terms

Subsequent to the TMI-2 accident, knowledge of severe accident processes which influence source terms has expanded dramatically. The methods of analysis that have been developed or are under development are more mechanistic and are better supported by experimental data. The uncertainties in source terms are large, however. Considering the magnitude of these uncertainties, it is unlikely that in the near future it can be demonstrated with a high degree of confidence that source terms have been reduced to the degree of confidence that source terms have been reduced to the degree that early fatalities or early injuries cannot occur in a severe accident. Although it is appropriate at this time to review source term based regulations, it is not clear that substantial relaxation of regulatory criteria will result. Point estimates of source terms should not be used without full consideration of the associated uncertainties. Furthermore, the magnitude of the source terms is affected not only by the processes of radionuclide release and transport, but by the timing and mode of containment failure and by the performance of containment safety systems such as sprays, suppression pools, icebeds, and air coolers. Thus there is now increased awareness of the potential benefits of accident management to reduce the consequence of severe accidents