Role of Fast Running TH Codes and Their Coupling with PSA Tools

The ISA methodology is a useful tool for probabilistic assessment that is currently being incorporated into licensing activities at CSN. At the gist of this methodology lies the coupling between fast-running, integral codes typical of TH assessment with probability calculation codes typical of PSA development. The aim of the coupling is to dynamically generate the event tree stemming from an initiating event, possibly taking into account degraded equipment, by following the dynamic evolution of the plant, as well as to obtain the probability of each sequence. This may yield a large number of sequences to be simulated with enough detail as to allow an exploratory assessment. The thermal hydraulic code Tizona, developed heeding this need for flexible tools that allow fast yet accurate simulations, provides a suitable tool to attain this coupling. Its structure has been standardised in such a way that it can be considered a generic proposal for the integration of simulation codes. (authors)