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Diablo Canyon control rod malfunctions

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A control rod at the Diablo Canyon Nuclear Power Plant is out of alignment and operators are trying to find out why. But a fix will have to wait until mid-May because doing so now could result in an unwanted shutdown.

Nuclear power plants rely on control rods to stabilize the reaction. Diablo Canyon has 53 such rods and during a routine test in January operators discovered one wasn’t functioning normally. The P-08 rod, or “Papa” 08 as PG&E officials call it, raised properly but when lowered got stuck. The plant is still fully operational and the rod is able to function safely, PG&E spokeswoman Emily Christensen said. The rod’s total range of movement is limited, she said, but emphasized there is no danger to the plant or the public.

Officials worry that attempting to fix the rod could cause it to unexpectedly drop and shut down the reaction in the core. Control rods are supposed to stop the reaction but only when there is a potential hazard. When there’s nothing wrong, however, an all-out shutdown could lead to other problems, explained Michael Peck of the Nuclear Regulatory Commission.

“When we look at it from a probabilistic perspective there’s a slight risk that would start something more,” he said. The risk is very small, he said, maybe a one-in-a-million chance. But in the disaster at Three Mile Island, he said, the reactor shutdown inadvertently damaged the nuclear fuel.

Next month the operators will lower the reactor to half power during routine maintenance on the water-cooling system. Operators will work to fix the malfunctioning rod at that time to reduce the risk of an automatic shutdown. The likely cause of the malfunction is debris or “gunk” that has jammed the control rod’s gears.

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