Selection of Accident Sequences for the New Design of ITER

The design of the International Thermonuclear Experimental Reactor (ITER) is now evolving towards new solutions devoted to reduce costs, and consequently objectives, with respect to the ones characterizing the first ITER Engineering Design Activities (EDA) phase. Because the revision process will lead to new dimensionsystems configuration, compliance with the safety requirements fixed for the reactor has to be demonstrated again for the new design, since deterministic accident analyses, generally speaking, will not be applicable any more without detailed verification. An exhaustive new set of reference accident sequences has then to be defined and, transient analyses performed for such sequences have to assure that the overall range of possible plant damages has been assessed against targets for releases and other criteria. As in the prior phase, it has been decided to define the list of possible accident initiators through a new failure mode and effect analysis (FMEA) and then to develop these initiators in an event Tree (ET) model to define the relevant accident sequences. The extensive set of accident sequences that will come out has then to be treated to select the most representative ones. Not relevant accident sequences will be screened out taking also into account the results of the uncertainty analysis performed by a Monte Carlo method, as allowed by the RISKSPECTRUM software. This paper describes the work performed along these guidelines for the first wall (FW) cooling system: analogous analysis is in progress for the other ITER systems, as modified by the new design.