By Eileen O'Grady
HOUSTON, March 8 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.
FULL PUBLIC REVIEW SOUGHT
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
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
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.