The US Nuclear Regulatory Commission (NRC) has agreed to re-evaluate seismic risks at California’s two-unit Diablo Canyon NPP following a petition from environmental groups. In a letter to the environmental groups, NRC confirmed its decision to review the seismic risks, saying it would investigate issues including the threat posed by thrust faults beneath Diablo Canyon and the nearby Irish Hills.
The letter said Diablo Canyon owner and operator Pacific Gas & Electric Company’s (PG&E’s) 2018 estimate of the risk of core damage from an earthquake is too low. Updated seismic data suggest this risk is “much higher than previously believed”. NRC said a magnitude 7.5 earthquake in Japan in January 2024, similar to one that could occur near Diablo Canyon, underscores the need for a reassessment. The faults in Irish Hills near the plant are capable of generating large earthquakes approximately every 715 years. However, NRC said a review board had determined that there is no imminent safety concern that warrants the immediate shutdown of Diablo Canyon.
Currently NRC is reviewing a life extension application for the plant’s two 1100 MWe Westinghouse-designed 4-loop pressurised water reactors, which were due to close in November 2024 and August 2025. In November 2023, PG&E submitted an operating licence renewal application to NRC for the two-unit plant. It sought to extend unit 1’s operating licence to November 2044, and unit 2’s to August 2045.
This came after California Governor Gavin Newsom in September 2022 signed a bill enabling the plant to continue in operation until 2030 to ensure energy system reliability and minimise greenhouse gas emissions until additional renewable and zero-carbon energy sources come online. The bill effectively agreed to lend PG&E $1.4bn and help with the process if the company would agree to change course and request a licence
In December 2023 the California Public Utilities Commission (CPUC) also agreed to an operating extension to 2030. The approval was subject to three conditions: NRC must continue to authorise the plant to operate; the $1.4bn federal loan agreement must not be terminated; and that the PUC does not make a future determination that extended plant operations “are imprudent or unreasonable”.
