A Risk Perspective for the Replacement Research Reactor in Australia

In July 2000, the Australian Nuclear Science and Technology Organisation (ANSTO) signed a contract with the company INVAP S.E. of Argentina for the design, construction and commissioning of a replacement research reactor (RRR). INVAP contracted CEDIAC to prepare the probabilistic safety assessment (PSA) for the RRR in support of the ANSTO application for the construction licence. The PSA is complementary to the safety analysis, in the sense that it asks questions such as ''What if the postulated initiating events were to occur and more than one piece of equipment were to fail? What if several things were to go wrong?''The PSA attempts to determine all the possible combinations of how the plant could respond to an initiating event, group all the possible outcomes, obtain conservative estimates of the frequency, and bounding estimates of the consequences (i.e. doses to the worst exposed individual of the public). The frequency and consequence constitute the risk, and when evaluated for all possible events, can be compared against the safety objectives set out in the regulatory principles. Besides the basic objective of the PSA, which is the quantitative evaluation of the risks associated with the RRR, and its comparison to the regulatory objectives, the PSA studies have been performed in parallel with the basic engineering phase of the project. Therefore, preliminary results from the 'risk point of view' were used as input to the design process, thus permitting improvements to be made to the design, and resulting in an effective reduction of the residual risk. To perform the PSA studies, several methodological developments were made in order to obtain a representative list of internal and external initiating events, to treat component and human related failures, to consider common-cause failures, and to consider some specific aspects of the design (i.e. fail safe components, passive systems, and lack of need for support systems). The PSA studies were performed to obtain not only quantitative estimations of the risk, but also quantitative estimations of the uncertainty associated with them. The overall results of the PSA indicate a very low residual risk for the RRR, and provide a valuable tool to analyse detailed engineering alternatives. From the regulatory point of view, the safety objectives have been fulfilled. Several design characteristics of the RRR contribute to the very low risk estimations (e.g. two completely independent shutdown systems, the lack of need for support systems for the safety functions, the absence of in-core experiments, the redundancy on the cooling modes, etc.). The PSA proved to be a valuable tool to increase the safety level of the RRR, and this was possible because the interaction between the PSA performers and the designers was straightforward. (author)