'Fission' scare hits wrecked Japan nuclear plant

The operator of tsunami-hit nuclear power plant in Japan said on Wednesday it had found substances in a reactor which could be a result of nuclear fission, a possible setback in efforts to bring the plant to a safe, cold shutdown.

Analysts said there was unlikely to be any heightened risk of radiation.

The Fukushima Daiichi plant was struck by a devastating earthquake and tsunami in March and has released radiation into the atmosphere ever since in the world's worst nuclear crisis since Chernobyl 25 years ago.

Tokyo Electric Power said that it discovered xenon, a substance produced as a byproduct of fission, in the No 2 reactor, and had poured in a mixture of water and boric acid, an agent that helps prevent nuclear reactions, as a precaution.

"It can be assumed that isolated criticality took place for a short period of time judging from the presence of xenon," Tepco spokesperson Junichi Matsumoto told reporters.

Criticality is a state when controlled nuclear reactions take place. Nuclear power plants harness the resulting heat to produce electricity.

Richard Wakeford at the University of Manchester said his  initial response to the Tepco findings was "scepticism".

"Nuclear criticality requires delicate conditions," Wakeford said in an email. "And it seems unlikely that these exist at Fukushima... Also, criticality would be accompanied by a burst of radiation, which I would have expected to be detected."

Paddy Regan, at the Department of Physics, University of Surrey, said he doubted a major release of radiation.

"But it is worth noting that even if the fuel is cooled, there is still a small amount of residual natural fission of the large amount of uranium fuel in the core. ...As with other reports, this one does not appear to show any new radiological hazard from the disaster."

Tepco, which was widely criticised for its slow release of information in the early days of the disaster, said it was still assessing the find but that it believes any criticality was temporary and finished.                                            The amount of detected xenon was small and the nuclear fuel in the No 2 reactor was unlikely to have melted down again, Tepco said.

The fuel rods in the No 2 reactor and two other reactors melted down early in the crisis after the tsunami knocked out the plant's cooling system.

"We think there won't be an impact on the surrounding environment even if criticality did take place, given that there is no change to parameters from the plant," said an official at the Nuclear Industrial Safety Agency, Japan's nuclear watchdog.

"The amount of detected xenon is so low to have an effect, although we are still in the process of fully analysing the situation," he said.

Tepco said the temperature and pressure at the No 2 reactor remained stable.

The company has succeeded in bringing down the temperatures at the three damaged reactors from levels considered dangerous and hopes to declare a cold shutdown -- when temperatures are stable below boiling point -- this year.

Tepco said in October that the amount of radiation being emitted from the complex had halved from a month earlier in the latest sign that efforts to bring the facility under control are progressing.

To show that decontamination efforts were progressing, Japanese cabinet official Yasuhiro Sonoda on Tuesday drank a glass of purified water taken from the Daiichi plant after being challenged by journalists to prove it was safe.