Diablo Canyon Nuclear Reactor Near Miss

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.

"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."