A substance produced as a byproduct of fission has been found in one of the reactors at the tsunami-struck Fukushima nuclear plant in Japan.
Junichi Matsumoto, a spokesman for Tokyo Electric Power (TEPCo) which owns the plant, said xenon had been discovered in the No 2 reactor.
Scientists poured into the reactor a mixture of water and boric acid – a mixture 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,” Matsumoto said.
Criticality is defined as a state during which controlled nuclear reactions occur. Nuclear power plants use the resulting heat to produce electricity.
TEPCo said temperature and pressure at the No 2 reactor has remained stable. The company hopes to declare a cold shutdown later this year, when temperatures are stabilized below the boiling point.
Fuel at three reactors at the Fukushima Dai-ichi nuclear power plant went into partial meltdown after a massive, 9.0-magnitude earthquake followed by a 23-foot tsunami damaged the plant's cooling system earlier this year.
The resulting incident was considered the world's worst nuclear crisis since Chernobyl, the nuclear disaster that occurred in the former Soviet Union 25 years ago.