A building housing an experimental nuclear reactor in Japan caught fire Tuesday, but there was no leak of radioactive materials, officials said, amid nervousness over Japan's atomic industry.

The quasi-public Japan Atomic Energy Agency said sound insulation on the ceiling of a building housing a reactor in central Ibaraki prefecture caught fire around 9:30 am (0030 GMT).

Sparks from welding tools ignited the glass wool insulation as a maintenance crew worked to place a covering over the roof, said an agency spokesman.

The reactor has been stopped for routine inspection since February.

Firefighters were at the scene, but the fire died out on its own two hours after starting, the spokesman added.

"There was no change to monitoring posts around the reactor," the spokesman said.

The news came days after Japan announced it had achieved a state of cold shutdown at the crippled Fukushima nuclear plant which had released massive amounts of radioactive materials following the country's March quake and tsunami.

The world's worst nuclear accident in a quarter of a century did not directly claim any lives, but has badly crimped the public's appetite for atomic power, the source of around a third of Japan's energy at one time.

Despite last week's declaration, the decommissioning of reactors at Fukushima Daiichi is expected to take another three decades, as is the decontamination of land around the plant that was once home to tens of thousands of people.