Operators of Japan's crippled nuclear plant say a radioactive gas found inside one of the plant's reactors most likely was produced by normal causes, not an uncontrolled nuclear reaction.
The Tokyo Electric Power Company reported Wednesday it had detected small amounts of xenon gas inside the Fukushima plant's No. 2 reactor, possibly indicating that nuclear fission was taking place.
But plant officials said Thursday they believe the gas was produced by "spontaneous" fission which is normal in any idle nuclear reactor.
The officials said the amount of xenon detected was thousands of times less than would have been created by an uncontrolled and sustained nuclear chain reaction - an occurrence known as "criticality."
The officials said they are explaining their findings to the government's nuclear watchdog agency.
TEPCO says Japan is still on track to achieve a so-called cold shutdown of all the plant's reactors by the end of this year. Three of the six reactors suffered presumed core meltdowns after a March 11 earthquake and tsunami destroyed cooling systems at the plant.
Tens of thousands of people have been evacuated from areas around the plant, and elevated radiation levels have been detected in food and water as far away as Tokyo.
Some information for this report was provided by AP and AFP.