Comparative risk evaluation of various design options for the divertor of a NET-type tokamak machine

Some results of a safety study being performed by ENEA-DISP as a contribution to the overall plant accident scenario identification and risk assessment for the design of a tokamak machine like the NET (Next European Torus) are presented. The authors focus on the divertor system, providing a comparative risk evaluation of different design options based on a systematic functional analysis. A double-null magnetic configuration and a structural layout with upper and lower targets have been assumed as common features for the different options, which mainly refer to the target-protection and heat-transfer implementation, and to the coolant-circuit distribution. Each version has been assessed by means of functional event-tree quantification and accident-consequence evaluation. Results are given in terms of doses versus expected frequencies of occurrence. As compared with the design safety targets, the appear to be acceptable as far as public exposure is concerned. However, the probability of plant unavailability subsequent to some typical failures is too high, and therefore more R&D effort is necessary. Suggestions regarding further research are provided