But analysts said there was minimal risk of added 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 spokesman Junichi Matsumoto told reporters.
Criticality is a state when controlled nuclear reactions take place. Nuclear power plants harness the resulting heat to produce electricity.
Tepco, which was widely criticized 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 analizing the situation,” he said.
Steve Jones, an independent nuclear and environmental consultant in Cumbria, north England, said if xenon had been present, it must have been produced by fission.
“However that doesn’t mean that a nuclear chain reaction is occurring,” he said in an e-mailed comment.
“In the reactor core, there will be nuclides present that undergo spontaneous fission and emit neutrons... The xenon may be produced by the spontaneous fission itself or by other fissions prompted by the emitted neutrons. However, so long as the system is well below the critical level needed for a self-sustaining chain reaction, all will be well.”
Tepco said the temperature and pressure at the No 2 reactor remained stable.
Tepco 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 decontaminaton 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.