Radioactive particles associated with nuclear fission have been detected at Japan's tsunami-damaged atomic power plant, officials said Wednesday, suggesting one of its reactors could have a new problem.

The fresh concerns at the Fukushima Daiichi nuclear facility came as a reactor in southern Japan was restarted and brought back online, marking a first since the March 11 disaster created an outcry over the safety of Japan's nuclear power sites.

Utility officials said gas from inside the Fukushima plant's No. 2 reactor indicated the presence of radioactive xenon, which could be the byproduct of unexpected nuclear fission. Boric acid was injected through a cooling pipe as a precaution because it can counteract nuclear reactions.

Tokyo Electric Power Co., or TEPCO, said there was no rise in the reactor's temperature or pressure. The company said the radioactive materials had not reached the point when nuclear reactions are self-sustaining and the detection of the xenon would have no major impact on workers' efforts to keep the reactor cool and stable.

Officials at the tsunami-damaged Fukushima Daiichi nuclear power plant detected a radioactive gas associated with fission on Wednesday. (Tokyo Electric Power Co./Associated Press)

Because the half-life of the isotopes detected is short, the xenon was likely created recently. But officials said the level was so low that further tests would be required to confirm the measurements were not an error.

"We have confirmed that the reactor is stable and we don't believe this will have any impact on our future work," said TEPCO spokesman Osamu Yokokura. He said no radiation leaks outside the plant were detected.

Hiroyuki Imari, a spokesman with the Nuclear Industrial Safety Agency, said the detection of the gas was not believed to indicate a major problem, but its cause was being investigated.

The plant is the site of the worst nuclear disaster since Chornobyl in 1986. A 20-kilometre exclusion zone has been in effect since the earthquake and tsunami crippled the facility northeast of Tokyo, sending three of its reactors into meltdowns, touching off fires and triggering several explosions.

TEPCO had reported significant progress toward stabilizing the facility, saying that it has essentially reached a "cold shutdown," meaning the temperatures at the reactors are constant and controlled.

Even so, a Japanese government panel says it will take at least 30 years to safely decommission the facility.

The Fukushima disaster has severely impacted Japan's nuclear power supply.

Most Japanese reactors suspended

Forty-three of Japan's 54 reactors are now suspended for inspections or mechanical troubles and public opposition to restarting them since the disaster has cast doubts on the nation's overall nuclear future.

Before the tsunami, Japan relied on nuclear power for about one-third of its electricity. If power companies cannot win local approval, which is required to restart reactors shut down for glitches or inspections, all of Japan's plants could be offline by next May.

But, in a first since the disaster, a nuclear reactor in southern Japan has resumed operation after a monthlong shutdown for a technical problem. A reactor in Hokkaido, northern Japan, was brought back online in August, but it had not been completely shut down and was out of commercial service only for a regular inspection.

The Kyushu Electric Power Co. says No. 4 reactor at the Genkai nuclear power plant in southern Japan restarted late Tuesday and was generating electricity Wednesday. It automatically shut down Oct. 4 following an abnormality in a steam condenser, but that didn't cause any radiation leaks or injuries.

The reactor will be closed again in January for routine inspections.