Risk evaluation of two design options for ITER first wall system

The authors present the result of a safety study aimed at identifying the accident scenario and assessing the risk connected with the first wall system of a tokamak machine like NET/ITER (Next European torus/International Thermonuclear Experimental Reactor). They compare risk evaluations of different design solutions for the first wall, based on a systematic functional analysis. These options differ mainly as to the coolant loop lay-out: one single cooling loop for all the inboard and outboard segments in the first option, and four independent cooling loops (one for each quarter of system) in the second option. The occurrence rates of the initiating events and the unavailability of the event tree different headings (i.e., the protective systems, both active and passive) have been assessed by means of the fault tree technique. Event tree quantification and accident consequence evaluation have been carried out for each option. Results are given in terms of doses to the public (early exposure) versus expected frequencies of occurrence. As compared with the safety design targets assumed for ITER, the results appear to be acceptable, but it is emphasized that, in the extreme case of failure of all the active safety systems and radioactivity confinement doses are higher than 100 mSv