The operator of Japan's tsunami-crippled Fukushima nuclear power plant said Wednesday it feared nuclear fission had resumed inside one of the reactors.

Tokyo Electric Power Co. (TEPCO) said it had begun injecting water and boric acid into Reactor No. 2 at the Fukushima Daiichi plant, which began leaking radiation after the March 11 earthquake and tsunami.

"We cannot deny the possibility of a small nuclear fission reaction," TEPCO spokesman Hiroki Kawamata said, adding that the injection was a precautionary measure.

He said there was no fresh danger at the plant, as the reactor's temperature and pressure, as well as radiation levels at monitoring posts, showed no substantial changes.

Fission is the process by which an operating nuclear reactor produces power.

The reactor automatically shut down in the wake of the disaster but nuclear fuel is believed to have melted through its container onto the bottom of the outer vessel when the tsunami knocked out the plant's cooling systems.

The injection was ordered after preliminary analysis of gas samples from the reactor building showed the possible presence of xenon 133 and xenon 135, byproducts of a nuclear reaction.

The two substances have short half-lives -- five days for xenon 133 and just nine hours for xenon 135 -- indicating that any nuclear fission was a recent phenomenon.

"Considering the half-life of xenon 133 and 135, we believe nuclear fission may have occurred in the recent past," said Junichi Matsumoto, TEPCO official in charge of nuclear operations.

There is a possibility that criticality, a self-sustaining nuclear chain reaction, occurred temporarily, Matsumoto said, but added it would not have been long enough to pose any risk.

The temperature at the reactor No. 2 had been brought to below 100 degrees centigrade (212 Fahrenheit), TEPCO said, one of the conditions for the utility to declare so-called "cold shutdown".

The possible fission "has no major impact on the reactor's state of being cooled down," Matsumoto told a news conference.

Technicians have been battling since the tsunami to achieve cold shutdown of the reactors, a stable condition in which temperatures drop and no reaction takes place.