PG&E's Diablo Canyon vessel report meets NRC surveillance rules
NRC staff found a Unit 1 reactor-vessel report met federal requirements on Sept. 24, months after renewing the plant's operating licences.
Ines Halvorsen
On Sept. 24, the U.S. Nuclear Regulatory Commission found that a report on one Diablo Canyon Unit 1 surveillance capsule submitted by PG&E Corporation NYSEPCG subsidiary Pacific Gas and Electric Company met federal rules. PG&E estimated on June 29 that losing the plant would add US$2 billion a year to California electricity customers' bills. The NRC had already renewed the plant's operating licences on April 2; the capsule assessment grants no new operating term.
At 2:37 p.m. EDT on Sept. 25, PG&E shares traded at US$12.24 in New York, according to NYSEPCG. The shares were below their previous close; the quote does not attribute the move to the NRC report.
Pacific Gas and Electric Company removed capsule B during a spring 2025 refuelling outage, according to its June 29 statement. It submitted the resulting report to the NRC on June 29, 2026, the agency's memorandum says. The capsule's steel specimens help track how neutron exposure changes a reactor vessel's resistance to fracture.
The Sept. 24 memorandum addresses the materials-surveillance reporting requirement in federal regulation 10 CFR Part 50, Appendix H. It covers Unit 1's capsule B. PG&E's earlier safety statement addressed analyses for both Diablo Canyon units and said they supported operation through 2045.
"The NRC staff reviewed the report for surveillance capsule B and finds that the report satisfies the requirements of 10 CFR Part 50, Appendix H," the agency said in its Sept. 24 memorandum.
Capsule Tests Track Vessel Steel
NRC staff examined the report's neutron-exposure calculations and mechanical tests of its steel specimens. Those included Charpy impact and tensile tests. The agency also reviewed PG&E's assessment of whether the capsule data could be used alongside earlier surveillance results.
The staff assessment recorded a 136.9-degree Fahrenheit rise in a transition-temperature measure for transversely oriented Charpy specimens from Lower Shell Plate B4107-1 after irradiation. The group-level shift exceeded a model prediction but remained within the two-standard-deviation range cited by PG&E. The surveillance-weld Charpy group also exceeded its predicted shift, while the surrogate-weld Charpy group fell below its prediction but outside its cited two-standard-deviation range.
NRC staff cautioned that one capsule's readings have limited value without assessing the full set of relevant surveillance data. Its Sept. 24 finding concerns whether PG&E's report satisfies the required monitoring program. The memorandum does not award another licence or set an additional operating date.
PG&E said on June 29 that Diablo Canyon produces enough electricity to meet the needs of nearly four million Californians. It put the potential cost of losing the plant at US$9 a month for an average PG&E residential customer, within its US$2 billion annual statewide estimate. Those are company estimates, separate from the NRC's finding on the capsule report.
Read more: Georgia Power and Google to fund 96 MW of nuclear uprates at Vogtle and Hatch
PG&E has scheduled its third-quarter results call for Oct. 22 at 11 a.m. EDT.
