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TEPCO: N-plant xenon not result of criticality
The Yomiuri Shimbun




Tokyo Electric Power Co. said Thursday radioactive xenon detected in the No. 2 reactor at its crippled Fukushima No. 1 nuclear power plant was the result of spontaneous fission, not a nuclear chain reaction known as criticality as had been feared.
TEPCO said spontaneous fission, in which radioactive curium produced during the operation of a reactor splits on its own, is occurring sporadically inside the reactor.
The recent detection of minute amounts of xenon can be explained by the splitting of curium, it said.
Xenon was detected Tuesday in gas from the No. 2 reactor's containment vessel.
Although the amounts were minute--about one-100,000th becquerel per cubic centimeter--TEPCO said a small-scale criticality incident could have taken place temporarily, given the short half-lives of the two types of xenon detected--five days for xenon-133 and nine hours for xenon-135.
However, after analyzing the data, TEPCO concluded criticality did not occur, explaining that even a small-scale criticality incident should produce 10,000 times more xenon than the amount detected.
TEPCO said the spontaneous fission of curium is a normal phenomenon in idled reactors and it would not hamper ongoing efforts to stabilize the reactor.
"There will be no impact on the stabilization of the nuclear reactor and the surrounding environment," the utility said.
(Nov. 4, 2011)