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This paper presents the results of a safety study aimed to identify the accident scenario and to assess the risk connected with the first wall system of a tokamak machine like NET/ITER. It provides a compared risk evaluation 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, four independent cooling loops (one for each quarter of system) in the second option. Common features of both the options are: structural lay-out with long poloidal panels integrated with the segment boxes; poloidal U-tubes coming back through the back plates; graphite or tungsten tiles for the physics and the technology phase of operation, respectively; water as primary coolant (1.5 MPa, 60-100 "C for normal operations and 3.5 MPa, 150 'C for wall conditioning). The occurrence rates ot 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 has to be emphasized that, in the extreme case of failure of all the active safety systems and radioactivity confinement, doses are higher than 100 mSv. Introduction The accident scenarios associated with failures of the first wall system (FW) of a tokamak machine like NET [l] or ITER [2] have been assessed referring to two different FW design options, in order to compare them in terms of risk to the maximum exposed individual (MEI) . Physics and technology phase of operation have been considered, both under conditions of normal running and thermal conditioning of the plasma facing components. The FW system includes the FW panels, theblanket segment box sidewalls andbackplates, and the corresponding primary cooling loops. The study is composed of two parts: Infunctional safety analysis'l and Inrisk evaluation and assessmentnn. The functional safety analysis, which is based on a systematic methodology [ 3 , 4 J , developed for fusion plants, leads to the identification of the initiating events (IE) of safety relevant accident sequences, and to the analysis of their propagation through the plant by means of function event trees. The risk evaluation consists ofthe quantification ofthe system event trees and of the containment event trees associated with the aforesaid function eventtrees [5]. Thisquantification is performed by evaluating, for each sequence, the expected 608 91CH3035-3$03.00 0 1992 IEEE frequency of occurrence and the dose to the MEI. That requires, first, the evaluation of the expected frequency of occurrence of the IE from which the event tree takes its origi...

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