The operation to stabilise the Fukushima Daiichi nuclear power plant has suffered a setback after officials said they had detected small traces of a radioactive gas that is a byproduct of nuclear fission.

The discovery was made as a nuclear reactor in south-western Japan became the first to start generating electricity following a series of shutdowns in the wake of the 11 March earthquake and tsunami.

The operator of Fukushima Daiichi, Tokyo Electric Power (Tepco), said it had found signs of recent nuclear fission inside the No 2 reactor, one of three at the plant that suffered core meltdown in March.

Japan's nuclear safety agency said the situation was stable and that the small amounts of radioactive material did not present a risk to public health.

The utility dismissed the possibility of a "major criticality accident", in which a sustained nuclear reaction occurs, but has not ruled out localised criticality inside the reactor.

The reactor's pressure and temperature had remained stable, it said, adding that radiation levels in the vicinity had not risen.

Workers began injecting water containing boric acid into the reactor via a cooling pipe to prevent a possible fission chain reaction.

"Given the signs, it is certain that fission is occurring," Junichi Matsumoto, a general manager at Tepco, told reporters.

Evidence that even partial or temporary nuclear fission had occurred underlined the fragile state of the plant almost eight months after it was struck by a magnitude-9 earthquake and tsunami that knocked out vital cooling systems.

The accident, the worst in the history of the Japanese nuclear power industry, forced the evacuation of 80,000 residents from a 12-mile radius around the plant. People living closest to Fukushima Daiichi have been told their old neighbourhoods could remain unsafe for decades.

Decommissioning the plant is expected to take at least 30 years, according to a recent report by the country's nuclear energy commission.

A Tepco spokesman insisted that the latest setback would not affect the company's roadmap towards making the plant safe by the end of the year. "We have confirmed that the reactor is stable and we don't believe this will have any impact on our future work," Osamu Yokokura told Associated Press.

Tepco said it had detected low densities of radioactive xenon-133 and -135 in gas samples extracted from the No 2 reactor.

The substances have relatively short half-lives of five days and nine hours, respectively, so their presence suggests that nuclear fission has occurred undetected inside the reactor very recently, raising the possibility of lingering activity inside its melted core.

The news coincided with the restart of a reactor at the Genkai nuclear plant in Saga prefecture on the island of Kyushu.

The reactor, which shut down automatically after an abnormality was spotted early last month, went back into service on Tuesday night and began generating electricity on Wednesday, according to Kyushu Electric Power.

The reactor will shut down again in December to undergo safety checks. Only 19 of Japan's 54 nuclear reactors are currently in operation due to regular inspections or earthquake damage.

They must pass recently introduced stress tests before they can go back online, although strong local opposition could delay or prevent the restart of reactors even after they are given the all-clear.

The government approved Genkai's restart after attributing the fault to human error and approving the utility's response.