Nuclear Reactor Pressure Vessel (RPV) integrity is a major issue concerning plant safety and this component is one of the few within a Pressurized Water Reactor (PWR) whose replacement is not considered as feasible. To ensure that adequate margins against failure are maintained throughout the vessel service life, research engineers have developed and applied computational tools to study and assess the probability of pressure vessel failure during operating and postulated loads. The Materials Ageing Institute (MAI) sponsored a benchmark study to compare the results from software developed in France, Japan and the United States to compute the probability of flaw initiation in reactor pressure vessels. This benchmark study was performed to assess the similarities and differences in the software and to identify the sources of any differences that were found. Participants in this work included researchers from EDF in France, CRIEPI in Japan and EPRI in the United States, with each organization using the probabilistic software tool that had been developed in their country. An incremental approach, beginning with deterministic comparisons and ending by assessing Conditional Probability of crack Initiation (CPI), provided confirmation of the good agreement between the results obtained from the software used in this benchmark study. This conclusion strengthens the confidence in these probabilistic fracture mechanics tools and improves understanding of the fundamental computational procedures and algorithms.

In France, nuclear reactor pressure vessels (RPV) integrity assessment follows a deterministic methodology and is performed according to the 1999 French regulation requirements with the use of safety coefficients. In the deterministic methodology, the input data are penalized in order to address the uncertainties. EDF and AREVA developed some couplings between integrity assessment models and uncertainty treatment software in order to address explicitly the uncertainties on the input data. In such analysis, the accumulation of conservatisms is replaced by a rational combination of parameters taking into account their uncertainty and variability. This paper presents a benchmark campaign conducted jointly by EDF and AREVA to validate the couplings between the codes and to strengthen the confidence in the calculations. The benchmark campaign was performed with several physical thermo-mechanical codes and uncertainty treatments software tools to evaluate the sensitivity of the analyses results as a function of code effects. The benchmark results are in good accordance between the several codes used. This conclusion gives confidence in the code couplings used for performing the uncertainty treatment analyses.