Fusion reactor blanket and first wall risk analysis

One of the important safety issues of a fusion reactor is the behaviour of the blanket and the first wall on accidental conditions. The first choice of blanket for NET (Next European Torus) is an integrated first wall-blanket segment design with Li17Pb83 as breeder, cooled by pressurized water. Starting from this option, a methodology of safety analysis is defined, in order to be able to point out weak points of this design and to emphasize recommendations capable to increase safety. Possible initiating events are identified and the most dangerous is studied (loss of coolant inside blanket module or inside first wall). The data bases of failure rate from French facilities statistical studies (PWR and LMFBR) are adapted to specific fusion reactor problems (LiPb corrosion, cycling working of fusion reactor and irradiation induced damage influence). Event trees for blanket, first wall and overall segment are built up and calculated. The risks, i.e. occurrence probability time detriments, are analyzed on three points of view: machine damages, occupational hazards and environmental impact. On the machine consequences four levels are identified. From last level, machine damages may be considered very important: possible ingress of torus chamber by LiPb and water, etc... (overall probability occurrence of about 5 E-4 a year). Finally, the event tree after segment failure is developed and it points out that hall contamination may be very large (up to 1.5Kg of activated erosion dust, about 10g of tritium and volatile activated products coming from the breeder). Probability of such contamination is of the order of E-4 a year. Nevertheless, if occupational impact may be severe, it has been found that environment impact can be maintained to a level rather low (2 mRem for the M.E.I.) as far as the last barrier (wall of the reactor building) withstands. (author). 6 refs, 6 figs, 3 tabs