By Eileen O'Grady
HOUSTON, March 8 (Reuters) - Southern California Edison officials on Friday disputed findings of a report from Mitsubishi Heavy Industries that indicated both companies were aware of a design problem with steam generator tubes now blamed for an extended shutdown of the San Onofre nuclear station in California.
Earlier Friday, the U.S. Nuclear Regulatory Commission (NRC) released a "root cause analysis" submitted by the Mitsubishi unit that manufactured the replacement steam generators installed in reactors at the 2,150-megawatt San Onofre plant, the largest power plant in Southern California.
Both San Onofre reactors have been shut since January 2012 following the discovery that excessive vibration prematurely damaged thousands of tightly packed tubes inside large steam generators that were installed in 2010 and 2011.
Loss of the output from San Onofre - located halfway between Los Angeles and San Diego - has strained Southern California's power grid and state agencies are planning for a second summer without the plant.
A redacted 135-page report stated that a design team of Southern California Edison (SCE) and Mitsubishi employees recognized that the design for the replacement generator tubes raised an issue called "void fraction," not seen in previous steam generator designs.
Further design modifications to address the consequences were not pursued, in part, because of the possibility that the altered design would trigger a "license amendment proceeding" requiring additional review by the NRC and the public, according to the report.
In a separate statement, Mitsubishi said the report shows that both companies "placed a high priority on minimizing tube wear resulting from vibration."
However, SCE, a unit of Edison International, said it relied on analysis by Mitsubishi that the design changes were acceptable. Some of Mitsubishi's analysis has since been shown to be erroneous.
SCE said Mitsubishi Heavy Industries "repeatedly reassured SCE of the efficacy of the design."
"SCE would never, and did not, install steam generators that it believed would impact public safety or impair reliability," said Pete Dietrich, SCE's chief nuclear officer, in a statement.
SCE said it never rejected a proposed design change to address "void fraction" based on idea that it would require additional NRC scrutiny.
"At no time was SCE informed that the maximum void fraction or flow velocities estimated by MHI could contribute to the failure of steam generator tubes," Dietrich said. "At the time, the design was considered sound."
SCE applied for and was granted two license amendments related to the replacement steam generators in 2008, prior to the first installation in Unit 2.
California Senator Barbara Boxer and Representative Edward Markey of Massachusetts, both Democrats, said the report supported their earlier call for a full NRC investigation before the units are allowed to return to service.
"It is essential that the NRC complete its expansive investigation into whether Southern California Edison fully complied with its legal obligations at the San Onofre nuclear facility," said Boxer in a release.
"A full investigation is critical to any determination on whether it is safe to restart San Onofre Units 2 and 3," she said.
Anti-nuclear group Friends of the Earth is also seeking a full public review of the steam generator design by the NRC.
"Edison clearly knew about design problems with the San Onofre replacement steam generators yet failed to take corrective action," said Damon Moglen, energy and climate director for Friends of the Earth.
The NRC has indicated it may decide in late April or May whether SCE can restart Unit 2 where the tube damage was less severe. SCE has proposed operating the reactor at 70 percent power to reduce vibration believed to have caused the tube damage for a five-month period, then shutting the reactor for inspection.
SCE said it spent $402 million last year for repair work at San Onofre and to buy power to serve its 5 million customers.
San Diego-based Sempra Energy owns a 20-percent stake in the San Onofre station.