--The Asahi Shimbun, Nov. 3
EDITORIAL: Fears of fission dispel unwarranted optimism about Fukushima

Tokyo Electric Power Co. (TEPCO) announced Nov. 2 that nuclear fission may have occurred in the No. 2 reactor at the Fukushima No. 1 nuclear power plant.

The operator of the disaster-stricken facility also said that a temporary, small-scale state of criticality might have occurred.

The news is deeply disquieting.

The remaining nuclear materials are still breaking down gradually in a process known as radioactive decay.

Nuclear fission occurs inside nuclear fuel in an operating reactor.

The nucleus of an atom in nuclear fuel splits into smaller parts after being struck by a stray neutron.

When the nucleus of an atom is split apart in this way, it throws off free neutrons, thereby causing the fission of the nucleus of another atom.

When this process continues to occur in a chain reaction, this state is called criticality.

A nuclear power plant generates electricity through the huge amount of energy released when nuclear fission occurs.

In short, criticality must not be allowed to occur in a disabled reactor in which nuclear fuel and control rods are not arranged in an orderly manner but are in a chaotic state.

Nuclear fission in the No. 2 reactor, depending on its scale, could result in a dangerous state of criticality and possibly trigger an unpredictable situation that is impossible to control.

Soon after TEPCO workers began operating a gas control system at the No. 2 reactor to purify gases within the containment vessel, traces of what appeared to be radioactive xenon 133 and xenon 135 were found.

The presence of xenon isotopes indicates that nuclear fission has taken place.
The detected elements that appeared to be telltale signs of nuclear fission were later confirmed by the Nuclear and Industrial Safety Agency to indeed be radioactive xenon isotopes.

Since these isotopes have a half-life of several days, they illustrate what is happening right now inside the No. 2 reactor.
They were not generated in the immediate aftermath of the crisis that flared in March.

The elements were detected soon after the gas purification system was activated. They may point to an ongoing situation in the reactor.

On the other hand, no significant changes in temperature or pressure inside the reactor have been observed.

But just the possibility of nuclear fission occurring requires immediate actions to prevent criticality from happening.

TEPCO's quick move to pump water mixed with boric acid into the reactor to prevent criticality shows that the company takes the discovery of radioactive xenon very seriously.

At the end of September, the government lifted its designation of areas in a radius of between 20 kilometers and 30 km of the plant as an emergency evacuation preparation zone.

The decision was made in the belief that stable cooling of the crippled reators had been making progress, thus almost eliminating the possibility of a fresh nuclear emergency.
It seems that the Fukushima plant is on track to reach a stable state known as cold shutdown by the end of the year in line with the government's timetable for its efforts to bring the nuclear disaster under control.

It must not be forgotten, however, that there is melted nuclear fuel, which has no trace of its original form, inside the wrecked reactors and their containment vessels.

The government is responsible for cleaning up the mess so that residents in areas around the plant can pick up the pieces of their lives.

That said, the government must not become oblivious to the remaining risks simply because its priority is on carrying out the cleanup work according to the timetable.

The discovery that nuclear fission might have occurred should be taken as a serious warning.

by kiyoshimat | 2011-11-04 07:14 | 英字新聞

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