Risk evaluation of the in-vessel plant area of a NET type tokamak machine: direct, induced and common cause failures

An approach to the quantitative risk assessment of the plant area composed of in-vessel systems (first wall, divertor, driver blanket and torus vessel) and their primary cooling circuits) of a Next European Torus (NET)-type tokamak is described. Contribution to the overall risk from both direct failures (developing inside each single system) and induced failures (propagating from one system to another through their interfaces) have been taken into account. Common-cause failures associated with plant auxiliary systems (such as machine power supply and control) have been also considered. The functional analysis developed for the NET plant has been utilized to identify the functional interfaces between different systems. The key tools of the analysis are functional fault trees and event trees, each of them having a loss of process function as top event or initiating event, respectively. Contributions from induced and common-cause failures have been taken into account by modifying, for each event tree, the probability evaluation of the initiating event based on the direct failures only. The effect of the initiating events on safety functions has been considered, assigning conditional probabilities to the related safety systems. The results of the analysis provide for a probabilities versus consequences risk evaluation; they are to be compared with the assumed safety design targets and with the results obtained by the direct failure safety assessment concerning the same systems