(The following is a reformatted version of a press release
issued by Southern California Edison and received via e-mail.
The release was confirmed by the sender.) 
Southern California Edison Comments on MHI Evaluation of
San Onofre Nuclear Plant Steam Generators 
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 fraction. MHI 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).” 
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 
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 and at San
Onofre is jointly owned by SCE (78.21 percent), San Diego Gas &
Electric (20 percent) and the city of Riverside (1.79 percent).
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About Southern California Edison
An 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. 
Jennifer Manfrè
Principal Manager, Media Relations
Southern California Edison
(sgp) NY 
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