Kyusaku Natsume (1098) writes "Tokyo Electric Power Co. said Wednesday that some of the melted fuel in reactor 2 at the Fukushima No. 1 nuclear plant may have triggered a brief criticality event. Tsuyoshi Misawa, a reactor physics and engineering professor at Kyoto University's Research Reactor Institute, said that if Tepco's data are correct, "it's clear that the detection (of xenon-133 and -135) comes from nuclear fission."
Tepco spokesman Junichi Matsumoto said the test results suggest that either small-scale fission occurred in the melted fuel, or conditions to trigger criticality were temporarily met for some other reason. He said the same thing could also happen at reactors 1 and 3.
But because the reactor's temperature and pressure level have not changed, the fission would not have been large-scale, Matsumoto said, adding it would not thwart Tepco's schedule for achieving a cold shutdown at the reactors.
In response, boric acid water was injected again in November 2. The Japan Atomic Energy Agency evaluated that the TEPCO's analysis result of the short-half-life radionuclide such as Xe-133 and Xe-135 detection was valid. On the plus side, the concentration of radioactive materials in air is low enough that in some areas inside Fukushima Daiichi for the first time since the accident workers will not use full face masks, starting next week."
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