Tokyo Electric Power Co. announced Wednesday that there is the possibility that criticality, a sustained nuclear chain reaction, had occurred “temporarily” and “locally” in the No. 2 reactor of the stricken Fukushima No. 1 nuclear power plant. It detected radioactive xenon-133 and xenon-135, products of uranium or plutonium fission, in gases collected Tuesday from the reactor.

Because the half life of xenon-133 is 5.25 days and that of xeon-135 is 9.14 hours, criticality is very likely to have occurred just before the gases were analyzed.

Although more than seven months have passed since the start of the nuclear fiasco, clearly the reactor has not yet been stabilized. Tepco’s plan to achieve “cold shutdown” of the Nos. 1, 2 and 3 reactors by the end of this year may face difficulty.

The fact that Tepco cannot deny the possibility of criticality irrespective of its scale is a grave situation. The conditions are similar in the Nos. 1, 2 and 3 reactors. It is thought that nuclear fuel in them melted and has collected in the bottom of both the pressure and containment vessels.

Tepco should make serious efforts to accurately grasp the conditions of nuclear fuel inside the reactors.

Even after a reactor is shut down, nuclear fuel fissions occur bit by bit inside cladding tubes without reaching criticality. Experts concur that large-scale criticality will not occur in molten nuclear fuel. But Tepco and the Nuclear and Industrial Safety Agency should take a serious view of the fact that radioactive xenon pointing to criticality was detected from the No. 2 reactor. What happened in it can happen in the Nos. 1 and 3 reactors.

They should strictly watch the conditions of the three reactors and do their utmost to prevent occurrence of criticality. They should not forget the simple fact that a large amount of nuclear fuel exists in these reactors.

Tepco injected 10 tons of a solution containing 480 kg of boric acid into the No. 2 reactor shortly before 3 a.m. Wednesday to restrain nuclear fission. This inversely shows that it has not been injecting a boric acid solution into the reactors in continuously cooling them by circulating water. Its laxness should be criticized. It wasn’t till after 7 a.m. Wednesday that NISA reported the criticality incident to Prime Minister Yoshihiko Noda. NISA clearly lacked the ability to make a correct judgment in this matter.

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