Regulatory implications of source term studies

A possible set of criteria for the reassessment of severe accidents in LWRs is discussed as well as the definition of new upper bound generalized source terms for regulatory purposes, as a consequence of the indications given by recent source term studies. Also described is the outcome of recent work carried out at the Comitato Nazionale per la Ricerca e per lo Sviluppo dell'Energia Nucleare e delle Energie Alternative (ENEA/DISP), Italy, by an internal task force and by external supporting organizations. Probabilistic evaluations and qualitative-good judgement criteria are used to define an optimum accident reference level and the corresponding source terms. The approach suggests that only dominant sequences should be considered, neglecting the most unlikely lesser known phenomena, and that the possibility of pre-existing openings in the containment and of operator recovery and repair actions, besides human errors, should be included. Probabilistic evaluations are, in the first instance, used to discriminate between those sequences to be included and those to be neglected. To this purpose a relative cut-off probability, conditional upon core melt, is suggested. A relative threshold of 0.05 on each sequence is used. The exercise is limited to some typical accident sequences. As a result, external releases of volatile fission products (except noble gases) are conservatively found to be lower than 10-3 times the core inventory for both typical PWRs and BWRs. Only a summary of the ENEA/DISP studies and results is provided. (author)