The Nuclear Regulatory Commission (NRC) on Monday issued a final rule to provide alternate requirements for protection against pressurized thermal shock events in nuclear power plant reactor vessels.
The rule seeks to increase the realism of calculations used to examine a pressurized water reactor’s (PWR’s) susceptibility to pressurized thermal shock (PTS). PTS can occur under some scenarios that rapidly cool the internal surface of the reactor vessel while the vessel is pressurized. This would subject the steel to substantial thermal stresses, which could lead to cracking and potential failure of the vessel. Boiling water reactors are not susceptible to PTS.
The rule allows licensees of operating PWRs to voluntarily adopt a more realistic technical approach for determining the probability of vessel failure during a PTS event. This revised approach was derived using data from research on currently operating PWRs, the NRC said. The data indicate that the overall risk of PTS-induced vessel failure after 60 years of reactor operation is much lower than previously estimated.
If a licensee chooses to adopt the new approach, the rule would require PWR operators to perform detailed analyses of both reactor vessel surveillance data and the results of regular reactor vessel inspections. If the analyses’ findings exceed certain limits, the operator must take steps to either limit the reactor vessel’s exposure to neutron radiation (which can make the vessel brittle) or determine how the reactor’s systems can be modified to prevent PTS-induced vessel failure.