Nov 02

Fission feared at Fukushima reactor No. 2 (Japan Times, Nov. 2, 2011):

Tokyo Electric Power Co. said Wednesday that particles from melted fuel in reactor 2 at the Fukushima No. 1 plant may have temporarily triggered a criticality incident.

Although the Nuclear and Industrial Safety Agency said there have been no drastic changes in the reactor’s temperature and pressure level, and the reactor itself is stable overall, the discovery may affect the plan by Tepco and the government to achieve cold shutdown of all three crippled reactors by the end of the year.

Suggesting that criticality, or a sustained nuclear chain reaction, may have occurred temporarily or partially, Tepco said one hundred thousandth of a becquerel per cubic centimeter of xenon-133 and xenon-135 was detected in gas samples.

Xenon-133 and xenon-135 are materials created through nuclear fission, and are not usually detected even when a reactor is in operation, as fuel rods are covered with zirconium metal. This means fission may have occurred in the melted fuel.

As the half-life of these two materials is short — five days for xenon-133 and nine hours for xenon-135, the xenon was probably created recently.

Tepco injected boric acid, which can prevent nuclear fission, into the reactor early Wednesday morning.

Tsuyoshi Misawa, a reactor physics and engineering professor at Kyoto University’s Research Reactor Institute, said if Tepco’s data are correct, “it’s clear that the detection (of xenon-133 and -135) comes from nuclear fission.”

“This amount of xenon would not be detected unless there was a certain degree of fission chain reaction,” so it is highly possible criticality took place, Misawa said.

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