Intensely radioactive water from reactor Unit 2 is filling a nearby tunnel faster than workers can empty it, and radioactive levels of seawater near the plant increased sixfold following an aftershock on Saturday.
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The Unit 2 reactor has been shut down since a partial meltdown in 1979.
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The trouble began Jan. 31, when the Unit 3 reactor was shut down as a precaution after a tube break.
Decaying generator tubes helped push San Onofre's Unit 1 reactor into retirement in 1992, even though it was designed to run until 2004.
The future of the heavily damaged Unit 3 reactor, where the radiation leak occurred after a tube break last year, is not clear.
The Unit 1 reactor shut off automatically about 2:20 p.m.
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Southern California Edison wants to change the plant's federal operating rules to permit the Unit 2 reactor to run at no more than 70 percent power, which company engineers and consultants believe will limit unusual tube wear.
That thinking changed on March 28, 1979, when a small valve stuck open, cooling water escaped and the reactor core of TMI's Unit 2 began to melt.
Eight of the more than 9, 700 tubes in one of the unit 3 generators failed a pressure test, while six tubes in unit 2's reactor needed to be plugged, the NRC has found.
Tuesday's explosion at the No. 2 reactor in the Fukushima complex, 150 miles north of Tokyo, for the first time raised the possibility that the key containment structure of the unit, which protects the reactor vessel and keeps dangerous radioactive materials from leaking out, had been damaged.
The No. 4 unit doesn't have live fuel in its reactor, but still holds waste fuel from earlier generation operations.
They were working in Reactor No. 3 which is of particular concern because it is the only unit to use MOX fuel, which is reprocessed spent fuel mixed with plutonium.
The dome in Unit 4, however, was removed in November 2010 and the fuel removed for reactor maintenance and to add new fuel rods.
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The unit was replaced, inspectors found no signs of a problem in two other units and the reactor returned to service on September 5, according NRC records.
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It was only a matter of time before the hydrogen reached a level where it would detonate, and one after another, the first unit, then the third unit, and finally the second unit, suffered hydrogen explosions that blew off the steel panels and left the top of the reactor building exposed.
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