2:01 AM
Tokyo - Officials detected a radioactive gas associated with nuclear fission at Japan's tsunami-damaged atomic power plant Wednesday, indicating there could be a new problem at one of its reactors.

They injected a substance that neutralises nuclear reactions as a precaution.

Gas from inside the reactor indicated the presence of radioactive xenon, which could be the byproduct of unexpected nuclear fission. Boric acid was being injected through a cooling pipe as a countermeasure because it can counteract nuclear reactions.

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

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

Hiroyuki Imari, a spokesperson 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 was the site of the worst nuclear disaster since Chernobyl in 1986.

A 20km exclusion zone has been in effect since the earthquake and tsunami on March 11 crippled the facility north of Tokyo, sending three of its reactors into meltdowns, touching off fires and triggering several explosions.

The latest setback comes as TEPCO had reported significant progress toward stabilising the plant. TEPCO says it has essentially reached a "cold shutdown" of the plant, meaning the temperatures at the reactors are constant and under control.

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