Power began flowing from the Genkai nuclear power plant reactor in the south of Japan, Kyushu Electric Power said, less than a month after the facility automatically shut down following a safety alert.
The reactor is expected to reach its normal level of power generation on Friday, a spokesman for the plant told.
The reactor is the first to resume operations since the massive earthquake and tsunami of March 11 sparked an atomic emergency at the Fukushima Daiichi plant in the northeast of the country.
The restart came as the operator of the tsunami-crippled Fukushima plant denied that signs of a new nuclear reaction at the stricken plant were a setback to recovery efforts there.
Tokyo Electric Power Co. (TEPCO) said it had begun injecting water and boric acid into Reactor No. 2 after scientists detected 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 recent.
“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.
There is a possibility that criticality, a self-sustaining nuclear chain reaction, occurred temporarily, Matsumoto said, but added it would not have lasted long enough to pose any risk.
The temperature at the reactor No. 2 had been brought to below 100 degrees centigrade, TEPCO said, one of the conditions for the utility to declare so-called “cold shutdown”.