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UPDATE1: Xenon at Fukushima not result of 'critical' nuclear reactions: TEPCO.

TOKYO, Nov. 3 Kyodo

(EDS: ADDING INFO AT 2-5TH GRAFS)

Tokyo Electric Power Co. said Thursday the detection of radioactive xenon at its stricken Fukushima Daiichi power plant, indicating recent nuclear fission, was not the result of a sustained nuclear chain reaction known as a criticality, as feared, but a case of ''spontaneous'' fission.

''It is not leading to instability of the reactor or a rise of the radiation level outside,'' the utility known as TEPCO said, adding that it does not expect the incident to impact its goal of attaining a stable condition of the plant's troubled reactors called cold shutdown.

The plant operator said it has judged that spontaneous fission, which it says occurs at a constant rate, has generated xenon-133 and xenon-135 at its crisis-hit No. 2 reactor, as a criticality would have resulted in the recording of concentration levels 10,000 times higher.

The data at hand match estimated levels of xenon produced by sporadic fission of curium-242 and curium-244 inside melted fuel, it said.

It also cited as evidence supporting its view the detection of xenon even after it poured boric-acid solution to absorb neutrons necessary for a fission reaction, and a lack of abnormal rises in the reactor's temperature or pressure.

Xenon, which was found Tuesday from gas collected from the No. 2 reactor's primary containment vessel, is believed to have leaked out of the fuel's cladding tubes due to their meltdown, it said, adding that it plans to continuously monitor gas inside the furnace.

The nuclear crisis at the plant, the world's worst in 25 years, erupted in the wake of the March 11 earthquake and tsunami, and resulted in the meltdown of nuclear fuel in the six-reactor power complex's Nos. 1 to 3 reactors.

When it revealed Wednesday that it had detected xenon at the No. 2 reactor, it touched on the possibility that melted fuel inside the reactor may have temporarily gone critical.
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Publication:Japan Energy Scan
Date:Nov 7, 2011
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