Radiation detected by operator of a tsunami-hit nuclear power plant in Japan was not the result of sustained nuclear fission, the utility said on Thursday.

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

Analysis suggests that it was not a criticality, Ai Tanaka, a spokeswoman for the company, commonly known as Tepco, said.

The nuclear facility 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.

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

The company, which was widely criticised for its slow release of information in the early days of the disaster, has since bought down the temperatures at the three damaged reactors from levels considered hazardous.

It 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.

(Reporting by Tim Kelly; Editing by Yoko Nishikawa)