THE NUCLEAR COMMUNICATIONS NETWORK News in Brief / No. 225 / 2 November 2011
Thursday, November 3, 2011 at 08:31AM Tepco Confirms Possibility Of Fission At Fukushima-Daiichi Unit 2
2 Nov (NucNet): Plant operator Tokyo Electric Power Company (Tepco) has confirmed that it has detected signs of nuclear fission reactions in unit number 2 at the shut-down Fukushima-Daiichi nuclear plant in Japan.
Tepco said particles from melted fuel in unit 2 at Fukushima-Daiichi might have “temporarily triggered a criticality incident”. This means the particles were of a configuration that could have led to a self-sustaining chain reaction.
In a statement the company said it began spraying boric acid on unit2 at
02:48 Japan time today to prevent accidental chain reactions. Tepco said it may have found xenon, which is associated with nuclear fission, while examining gases taken from the reactor.
Japan’s Nuclear and Industrial Safety Agency (NISA) said there have been no drastic changes in the reactor’s temperature and pressure level, and the reactor itself is stable overall.
Saying that criticality might have occurred temporarily or partially, Tepco said one hundred thousandth of a becquerel per cubic centimetre of
xenon-133 and xenon-135 was detected in gas samples.
Xenon-133 and xenon-135 are materials created through nuclear fission.
They are not usually detected even when a reactor is in operation because fuel rods are enclosed in gas-tight zirconium metal tubes. This means fission might have occurred in the melted fuel, or corium.
Corium, also called fuel containing material (FCM) or lava-like fuel containing material (LFCM), is a molten mixture of portions of nuclear reactor core, formed during a nuclear meltdown. It comprises of uranium fuel, fuel clads, and other core-internal material.
Yoshinori Moriyama, a NISA spokesman, told NucNet there is a possibility fission started from some small pieces of melted fuel. However, the fission was probably small and partial, he said, adding, “large-scale fission was unlikely”.
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