Sensitivity studies on failure frequency of Japanese RPV subjected to a PTS event regarding uncertainty of weld residual stress

Mano, Akihiro ; Takamizawa, Hisashi 

For aged reactor pressure vessels (RPVs), there is a concern about their failures resulting from the degradation of fracture toughness due to neutron irradiation embrittlement. The structural integrity assessment plays an important role in ensuring low possibility of failure. Probabilistic fracture mechanics (PFM) analysis is one of the methods of assessment. In the analysis, uncertainties related to structural integrity are considered as probability distributions, and failure frequency of a RPV is quantified as the analysis result. In addition, since influence on failure frequency can be quantified regarding various assumptions such as inspection, the result is expected to be used in decision-making. To examine robustness of the result, sensitivity studies on uncertainties and assumptions are necessary. Weld residual stress (WRS) distribution is a representative influential factor of structural integrity of RPV and is well known to include large uncertainty. On the other hand, influence of the uncertainty in distribution on the failure frequency is still unclear. In the study, we perform sensitivity studies on the uncertainty of WRS distribution. The target is a typical RPV of Japanese pressurized water reactor subjected to a pressurized thermal shock event. The PFM analysis code PASCAL5 is used. In the code, the function to consider uncertainty of WRS distribution is incorporated. The uncertainties are estimated based on the WRS distributions obtained from the other research on experimental measurement and finite element analysis for fabricated RPV plates. Failure frequency of the RPV is quantified by PFM analysis with considering the uncertainty or not, changing number of samples for the uncertainty, magnitude of the uncertainty, and types of the uncertainty model, as well as the degradation level of fracture toughness. As a result, it is found that the influence of uncertainty is larger for CPF than CPI. In addition, the influence is relatively large in transients with severe pressurization, but small in transients with severe cooling. The quantitative variations of CPI and CPF for a transient with severe pressurization by considering the uncertainty are less than one order of magnitude and almost equivalent to those by considering additional membrane stress of 10 MPa. On the other hand, the influence can be large when large uncertainty is assumed in WRS distributions especially transients with severe pressurization.