Diablo Canyon nuclear plant 'near miss' in report NUCLEAR ENERGY Diablo Canyon mishap a low risk, PG&E says

PG&E's Diablo Canyon nuclear power plant in Avila Beach, Calif. on Friday, May 26, 2006. The two spent fuel storage pools are nearing its capacity of 2,648 cells so plant officials are constructing a dry cask storage area to hold future radioactive fuel cell waste. PAUL CHINN/The Chronicle less PG&E's Diablo Canyon nuclear power plant in Avila Beach, Calif. on Friday, May 26, 2006. The two spent fuel storage pools are nearing its capacity of 2,648 cells so plant officials are constructing a dry cask ... more Photo: Paul Chinn, The Chronicle Photo: Paul Chinn, The Chronicle Image 1 of / 1 Caption Close Diablo Canyon nuclear plant 'near miss' in report 1 / 1 Back to Gallery

For 18 months, operators at the Diablo Canyon nuclear plant near San Luis Obispo didn't realize that a system to pump water into one of their reactors during an emergency wasn't working.

It had been accidentally disabled by the plant's own engineers, according to a report issued Thursday on the safety of nuclear reactors in the United States.

The report, from the Union of Concerned Scientists watchdog group, lists 14 recent "near misses" - instances in which serious problems at a plant required federal regulators to respond.

The report criticizes both plant operators and the Nuclear Regulatory Commission for allowing some known safety issues to fester.

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"The severe accidents at Three Mile Island in 1979 and Chernobyl in 1986 occurred when a handful of known problems - aggravated by a few worker miscues - transformed fairly routine events into catastrophes," the report notes.

The problem

The problem at Diablo Canyon, which is owned by Pacific Gas and Electric Co., involved a series of valves that allow water to pour into one of the plant's two reactors during emergencies, keeping the reactor from overheating.

The loss of water in a reactor can lead to at least a partial meltdown - a process believed to be under way at Japan's stricken Fukushima Dai-ichi nuclear plant after last week's earthquake and tsunami.

Engineers at Diablo Canyon inadvertently created the problem while trying to solve another issue, according to the report.

A pair of remotely operated valves in the emergency cooling system was taking too long to move from completely closed to completely open. So engineers shortened the distance between those two positions, according to the report.

Unfortunately, two other pairs of valves were interlocked with the first. They couldn't open at all until the first pair opened all the way. No one noticed until the valves refused to open during a test in October 2009, 18 months after the engineers made the changes.

"It was disabled, and they didn't know it," said Jane Swanson, spokeswoman for the Mothers for Peace anti-nuclear group, which frequently spars with federal regulators over Diablo Canyon. "That's unforgivable, and it's not that unusual."

In an emergency, Diablo Canyon operators still could have opened the valves manually.

They could also have used a separate system of pumps to inject water into the reactor, PG&E spokesman Kory Raftery said.

"We want to make sure we put safety first - that's why we have redundant systems," he said.

He added, "The potential is very small for the type of situation where we'd need this system in the first place."

PG&E has asked the Nuclear Regulatory Commission to extend the licenses of Diablo Canyon's twin reactors past their original expiration dates of 2024 and 2025. Mothers for Peace has opposed that move.

The valve problem and the union's report, Swanson said, illustrate how even minor technical issues at a plant have the potential to cause serious problems.

'Domino effect'

"Any given nuclear power plant is such a complex system," she said. "As we've seen in Japan, the domino effect can happen."

With the Japanese crisis riveting world attention, Sens. Barbara Boxer and Dianne Feinstein called on the Nuclear Regulatory Commission on Wednesday to perform thorough safety inspections at Diablo Canyon as well as California's other commercial nuclear plant, San Onofre, in San Diego County.

But the commission's chairman said Thursday there was no immediate need to inspect any U.S. nuclear plants.

Later Thursday, President Obama said the United States faces no danger of radioactive contamination from Japan's nuclear plant and has ordered a comprehensive review of safety at U.S. plants.

Bloomberg News contributed to this report.