Japan: Tokyo Electric Rules Out Future Criticality at No. 1 Reactor

Jiji Press ^ | 05/13/11

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Tokyo Electric Rules Out Future Criticality at No. 1 Reactor

Fukushima, May 13 (Jiji Press)--Tokyo Electric Power Co. <9501> officials said Friday nuclear fuel inside the No. 1 reactor at the Fukushima No. 1 power plant will not reach a state of criticality even if the current water injection into the reactor pressure vessel stops and the vessel loses most of its water.

Water cools nuclear fuel while it acts as a moderator that slows down neutron, making them more likely to cause nuclear fission. This means that a lack of water makes it more difficult for criticality, or a self-sustained nuclear fission chain reaction, to happen, the officials said.

The No. 1 reactor, whose fuel is believed to have melted at least partially, shut down automatically after the March 11 earthquake, with control rods securely inserted inside fuel assemblies.

As two months have passed since then, decay heat from fission products generated when the reactor was in operation is believed to have decreased to one- to two-thousandths the initial level, the officials said.

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