Japan on Wednesday restarted its first nuclear reactor since the Fukushima disaster in March, in a boost to its beleaguered atomic power industry faced with a deeply sceptical public.

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 AFP.

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 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 lasted 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.

Since the nuclear disaster struck in March, Japan's atomic industry has been struggling to overcome the scepticism of a public largely unwilling to allow operations to restart at dozens of stalled reactors nationwide.

Wednesday's restart at Genkai was seen as a boost to the industry, even though Kyushu Electric officials said the situation there was different from that at other suspended reactors, which have to undergo government-mandated stress tests.

The Genkai reactor was halted after an "abnormality" in its steam condensing unit emerged on October 4, Kyodo news reported, adding that Industry Minister Yukio Edano, who oversees the sector, had attributed the fault to "human error".

Edano said on Tuesday the final decision on restarting the reactor rested with Kyushu Electric because the country's nuclear regulator had found its procedures to be sound, but he urged the company to consult the local community.

Reactors halted for checks must pass stress tests before they are allowed to resume operation, but local authorities have the power to veto the restarts.

Public officials in Saga gave their assent to the restart there after the nuclear safety agency gave its stamp of approval.

Saga Governor Yasushi Furukawa told reporters: "If the state made the judgment after a full examination, we'll accept it," Kyodo reported.

Before the disaster at Fukushima, Japan relied on nuclear power for around a third of its electricity, and has since had to ramp up its imports of thermal fuels to plug the supply gap as the number of active reactors dwindles.

A nationwide campaign to save energy over the summer is to continue over the winter, amid warnings of a shortfall that could be particularly acute in western Japan, which is more heavily dependent on nuclear power.