HOUSTON, March 8 (Reuters) - Nuclear regulators on Friday released a report from the manufacturer of steam generators installed in Edison International's shuttered San Onofre nuclear plant in California that plant critics said shows the utility was well aware of problems with the generator design.
Both reactors at the 2,150-megawatt San Onofre nuclear station 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 the reactors in 2010 and 2011.
A redacted 135-page version of the "root cause analysis" submitted by Mitsubishi Heavy Industries to the U.S. Nuclear Regulatory Commission states that a design team of plant operator Southern California Edison (SCE) and Mitsubishi employees recognized the design for the replacement generator tubes raised an issue not seen in previous steam generator designs.
Further modifications to address the consequences were not pursued, in part, because of the possibility the altered design would trigger a "license amendment proceeding" requiring additional review by the NRC and the public, according to the report.
Last month, California Senator Barbara Boxer and Representative Edward Markey of Massachusetts cited the Mitsubishi report in a letter to NRC Chairman Allison Macfarlane asking the agency to investigate whether SCE and Mitsubishi knew of the design problems before the steam generators were installed.
The report offers a highly technical explanation of the unprecedented tube damage found in the almost brand-new replacement steam generators at the 2,150-megawatt nuclear station located halfway between Los Angeles and San Diego.
"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, an anti-nuclear group that is seeking a public review of the steam generators.
"It is inconceivable that the NRC would now give approval to restart these damaged reactors without the thorough, public review of a license amendment proceeding," Moglen said in a statement.
Edison officials were not immediately available for comment.
Mitsubishi officials said earlier that all design decisions "were made in accordance with well-established and accepted industry standards and practices, along with our own and third-party operating data and experience."
The NRC 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.
San Diego-based Sempra Energy also owns a stake in the San Onofre station.