A report on the
root causes of problems at the San Onofre nuclear plant shows that officials
considered making design changes to the plant’s new steam generators before
they were installed but rejected some fixes in part because they would require
further regulatory approvals.
Some of the generators
began malfunctioning a year after they were installed, and the nuclear power
plant has been shuttered for 14 months. The closure has already cost San
Onofre’s operators, Southern California Edison and San Diego Gas & Electric,
across the region are already shouldering some of those costs and could
be on the hook for hefty future repair bills.
The report was
released Friday by the Nuclear Regulatory Commission. It was written by
Mitsubishi Heavy Industries, which built the generators.
provides the most detailed picture to date of how the flawed system was
companies insisted Friday said that they were not aware of the problem that
crippled San Onofre. Mitsubishi argued that the changes officials contemplated
before installation would not have made a major difference.
however, acknowledged that it had made an incorrect input into a computer code that
resulted in underestimating the velocity of steam flow in the plant’s
replacement steam generators. Again, the company said that that error did not cause the
The report comes
amid a furious debate over who is to blame for defects that led to the
Boxer (D-Calif.) and U.S. Rep. Ed Markey (D-Mass.) obtained a leaked version of
the proprietary Mitsubishi report. They wrote to the head of the U.S. Nuclear
Regulatory Commission alleging that the report showed Edison and Mitsubishi
were aware of design defects in the generators before they were installed and
chose not to make fixes.
On Friday, after
weeks of back and forth with Mitsubishi, the NRC released a redacted version of
San Onofre was
shuttered after a tube in the plant’s replacement steam generator system leaked
a small amount of radioactive steam on Jan. 31, 2012. Eight other tubes in the
same reactor unit later failed pressure tests, an unprecedented number in the
industry, and thousands more tubes in both the plant’s units showed signs of
The wear was
blamed on tube vibration caused by excessively dry and high-velocity steam and
inadequate support structures, particularly in one of the plant’s two units.
Tube vibration and wear has been a problem at other plants, but the specific
type of vibration seen at San Onofre had never been experienced before in the