Comparison of pressure vessel integrity analyses and approaches for VVER 1000 and PWR vessels for PTS conditions

Currently, license renewals and plant-life extension are important issues for nuclear industry. Pressure vessel integrity is one of the main concerns related to these issues. Pressure vessel integrity is of prime concern for pressurized reactors, since they operate at higher pressures and neutron fluxes when compared to boiling water reactors. Pressure vessel integrity analyses for two commercial pressurized water reactors are performed in this study; a Westinghouse 4 loop 1000MW PWR and a VVER 1000/320. Two most limiting loss of coolant accidents (LOCA) for pressurized thermal shock (PTS) are considered and deterministic and probabilistic failure analyses are performed. Differences in eastern and western regulatory approaches are also taken into account. Among the vessels simulated, the maximum nil ductility transition temperatures are found to be below the relevant regulatory limit. However, the results of probabilistic analyses are observed to be above the prescribed national limits. This is attributed to the use of rather conservative assumptions used in this study. Findings of this study may help the re-evaluation efforts of PTS screening criteria.