A benchmark study on risk-informed in-service inspection methodologies (RISMET)

Between 2005 and 2009 JRC and OECD/NEA coordinated a project for benchmarking various risk-informed in-service inspection (RI-ISI) methodologies. In the project, called RISMET, various RI-ISI methodologies were applied to the same case, consisting of 4 selected piping systems at the Swedish nuclear power plant Ringhals unit 4, a 3-loop Westinghouse pressure water reactor. The RI-ISI applications were among each other and to the deterministic ASME section XI ISI selection procedure. More than 20 organisations from Europe, U.S., Canada and Japan took part. The scope of the benchmark was limited to 4 systems, but the variety regarding safety class, potential degradation mechanisms and pipe break consequences ensured a good coverage of issues for a comparative study. The risk-informed methodologies showed some significant differences and resulted in slightly different risk ranking and selection of inspection sites. However, the results of the benchmark indicated that the risk impact of these differences is small, and the RI-ISI approaches identify safety important piping segments that are ignored by approaches not using the probabilistic safety assessment (PSA). The results of the benchmark exercise RISMET improve the knowledge on differences in approaches and their impact on plant safety, and promote the use of risk-informed ISI. This paper summarises the RISMET benchmark project and its main results.