Southern California Edison Comments on MHI Evaluation of San Onofre Nuclear Plant Steam Generators
Posted March 8, 2013 - 2:00 p.m. PDT
ROSEMEAD,
Calif., March 8, 2013 — An evaluation by Mitsubishi Heavy Industries
(MHI) made public today cites ineffective tube supports, dry steam and
high steam flow velocity as causes of excessive wear in the steam
generators MHI supplied to Southern California Edison's (SCE) San Onofre
Nuclear Generating Station.
SCE previously disclosed these same causes based on its own investigation, and the Nuclear Regulatory Commission's (NRC) augmented inspection team report
last July found that MHI's use of faulty computer modeling in the
design process caused MHI engineers to inadequately predict the dryness
of the steam, measured by void fraction, in the replacement steam
generators.
MHI
repeatedly reassured SCE of the efficacy of the design. During the
design phase of the project, MHI advised SCE that, based on its own
review and analysis, the maximum void fraction that MHI expected to
occur was acceptable, did not require additional design changes or
measures, and that the replacement steam generators would perform as
warranted.
"SCE's
own oversight of MHI's design review complied with industry standards
and best practices," said Pete Dietrich, SCE senior vice president and
chief nuclear officer. "SCE would never, and did not, install steam
generators that it believed would impact public safety or impair
reliability."
In
fact, MHI states in its root cause report (page 41), that its analysis
of conditions in the steam generator during the design phase (which
calculated void fraction and steam flow velocity) concluded that the
thermal hydraulic conditions in the San Onofre steam generators were
acceptable, and specifically that there was no need to reduce void
fraction1.
Additionally, SCE never rejected a proposed design change to address void fraction based on its impact on compliance with 10 CFR 50.59.
"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," said Dietrich. "At the time, the design was considered
sound."
SCE
is disappointed that MHI decided on its own to redact some information
in its evaluation about the flaws in the computer codes. However, the
NRC publicly disclosed the computer code flaws three months before MHI
completed its evaluation. In addition, the corrective actions and other
statements included in the evaluation make it evident that there were
problems with the computer modeling that failed to predict conditions
that led to the tube-to-tube wear.
SCE
has proposed operating Unit 2 at 70 percent to decrease velocity and
decrease steam dryness to increase damping, thus preventing the
conditions that led to excessive wear. The proposed restart plan was
validated using a different computer model and has been reviewed by
independent experts.
The
San Onofre nuclear plant is the largest source of baseload generation
and voltage support in the region and is a critical asset in meeting
California's summer electricity and clean energy needs. Both units at
San Onofre are currently safely shut down. Unit 2 remains shut down
since it was taken out of service Jan. 9, 2012, for a planned outage.
Unit 3 was safely taken offline Jan. 31, 2012, after station operators
detected a leak in a steam generator tube.
NRC
approval is required before SCE can restart Unit 2. The repair,
corrective action and restart plan for Unit 2, along with additional
technical information to address questions from the NRC, are available
to the public at www.SONGScommunity.com
More
information, including videos that explain how a steam generator works
and the role San Onofre plays in providing reliable electricity to the
region, is available at www.edison.com/SONGSupdate and at www.SONGScommunity.com.
San Onofre is jointly owned by SCE (78.21 percent), San Diego Gas &
Electric (20 percent) and the city of Riverside (1.79 percent). Follow
us on Twitter (www.twitter.com/SCE) and like us on Facebook (www.facebook.com/SCE).
About Southern California EdisonAn
Edison International (NYSE:EIX) company, Southern California Edison is
one of the nation's largest electric utilities, serving a population of
nearly 14 million via 4.9 million customer accounts in a
50,000-square-mile service area within Central, Coastal and Southern
California.
1MHI
Root Cause Analysis, page 41: T/H condition "was judged acceptable by
FIV analysis" and "T/H analysis (FIT-III) did not indicate the necessity
to reduce the high steam quality (void fraction)."
By Elaine M. Grossman
