San Onofre Reactor 2 operated for several months with degraded nuclear fuel cladding

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Three weeks after shutting down, problems continue to surface at San Onofre Unit 2.  Scott Burnell, a spokesman for the Nuclear Regulatory Commission in Washington, said the root cause of the faster-than-expected wear on tubes in replacement generators is yet to be determined.

One of the less-reported stories involves an accident the occurred during the outage, where a worker fell into the San Onofre Reactor 2 reactor pool on January 27th. David Lochbaum, who worked in nuclear plants for 17 years before joining the nonprofit Union of Concerned Scientists, said he believes small particles of uranium fuel —- sometimes called “fuel fleas” or “hot particles” —- could have been in the San Onofre Reactor 2 reactor pool when the worker fell in.

After looking at a set of reactor operation statistics published on the NRC’s website, Lochbaum said the radiation levels in the plant’s cooling system doubled from January to February 2011 and continued to climb through the end of the year.  He shared his findings that increasing levels of radiation in the water that cools the reactor probably indicates that the metal tubes — called cladding — that contain its fuel pellets are cracking.

Those cracks, he said, could have released particles into the reactor pool when uranium fuel was removed from the reactor core during refueling, well before the worker’s fall.

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“The chart shows that San Onofre Unit 2 had operated for several months with degrading fuel cladding,” Lochbaum said. “The opening in the fuel cladding that allowed radioactivity to escape into the water at ever-increasing rates could have also been a pathway for fuel fleas —- small particles of fuel pellets —- to escape.”

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When a nuclear reactor is in operation, the fuel expands on heating, the core of the pellet expands more than the rim.  In an water-cooled reactor the action of radiation on the water (radiolysis) forms hydrogen peroxide and oxygen. These can cause stress corrosion cracking of metal parts which include fuel cladding and other pipework.

The heating of pellets can result in some of the fission products being lost from the core of the pellet. If the xenon can rapidly leave the pellet then the amount of 134Cs and 137Cs which is present in the gap between the cladding and the fuel will increase.


Rochelle Becker, executive director of the Alliance for Nuclear Responsibility, said she was referred to Edison when she asked the NRC for more details about the radiation dose that the San Onofre worker received.

“When you’re telling the public to go back to the utility for any more questions, I think that’s an indication that there is too much trust,” Becker said.


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