Federal regulators have concluded that Southern California Edison did not mislead the government about extensive modifications to the San Onofre nuclear power plant's troubled steam generators, where damage has been found on scores of tubes that carry radioactive water.
A special Nuclear Regulatory Commission report released Thursday said faulty computer modeling “inadequately predicted conditions in steam generators at the San Onofre Nuclear Generating Station.”
The report also said manufacturing issues contributed to excessive wear of components, including steam generator tubes.
The NRC report said that computer modeling by the manufacturer, Mitsubishi Heavy Industries, did not result in a design that provides a safety margin for vibrations.
The vibrations were found to cause the excessive tube wear.
Environmental activists, including the group Friends of the Earth, have accused Southern California Edison of duping the Nuclear Regulatory Commission about a series of changes to the generators, including boosting the number of tubes and redesigning internal supports.
“Edison provided the NRC with all the information required under existing regulations about proposed design changes to its steam generators,” said the special NRC Augmented Inspection Team report.
“We are committed to continuing to work with the NRC on the steam generator issues and will continue to use conservative decision making as we work on repairs and planning for the future,” said Southern California Edison Senior Vice President and Chief Nuclear Officer Pete Dietrich. “The number one priority is the safety of the public and our employees.”
Edison said it continues to work on its response the NRC’s Confirmatory Action Letter, which is a formal step in the process of restarting the facility.
The federal agency said its special inspection team was directed, in part, to “identify the circumstances surrounding the tube degradation; review the licensee’s actions following discovery of the conditions; evaluate the licensee’s review of potential causes of the unusual steam generator tube wear; review the computer modeling used in the design of the steam generators; and assess the differences in wear between the Unit 2 and Unit 3 steam generators, manufactured by Mitsubishi Heavy Industries.”
The inspection team report said tube degradation was greater in Unit 3 than Unit 2.
“I recently met with NRC inspectors and believe the team is carefully and thoroughly investigating the cause of tube degradation at San Onofre,” said U.S. Senator Dianne Feinstein (D-Calif.) “The report indicates there is more work to be done before we can fully understand the problem and determine a possible solution.”
The federal regulator said the San Onofre nuclear plant will not restart until Southern California Edison develops a plan to prevent further steam generator tube degradation and the NRC independently verifies the plant can be operated safely.
The plant on the seaside border of San Diego and Orange counties has been shut down since January, when a break in a steam generator tube released traces of radiation into the atmosphere. Inspectors later found extensive and unusual wear of steam generator tubes in both Unit 2 and Unit 3.
Earlier this year, a report issued by Friends of the Earth asserted that equipment and design changes to the generators "created a large risk of tube failure at the San Onofre reactors."
FOE said the company never disclosed that such extensive changes were made, instead describing it as an exchange of similar equipment that allowed Edison "to avoid the requisite NRC oversight of a steam generator replacement."
“The NRC report affirms that Edison made ‘major design changes’ that have led to the loss of steam generator tube integrity (which) is a serious safety issue that must be resolved prior to further power operations,” said Damon Moglen, climate and energy campaign director at Friends of the Earth.
Moglen said Southern California Edison is to blame for the tube issues and safety problems at the San Onofre nuclear plant.
"Edison is the one who recommended these radical, significant design changes," said Moglen. "And it is now clear from this (NRC) report that those design changes have led to serious safety problems at the reactors."
The steam generator tubes are one of the barriers between the radioactive and non-radioactive sides of the plant. If a tube breaks, there is the potential that radioactivity from the system that pumps water through the reactor could escape into the atmosphere.
Serious leaks also can drain cooling water from a reactor.
The NRC said it will schedule a meeting in the near future to hear public comments and questions about the report and the inspection report process.
The agency said there are still ten "unresolved items" in the report that will be subject to follow-up inspections and possible regulatory actions.