A fire Tuesday partially burned the ceiling of a building housing a nuclear reactor in Tokai, Ibaraki Prefecture, the prefectural government and the reactor's operator said.
The fire broke out at around 9:30 a.m. and was extinguished two hours later, they said, adding the blaze at the research reactor facility did not result in a radiation leak and no one was injured.
The ceiling under the building's steel plate roof was set alight by sparks during welding work on the roof, the Japan Atomic Energy Agency said.
A 110-sq.-meter area of glass-fiber acoustic absorbent covering the ceiling was burned, the agency said.
The reactor core is located in the underground section of the 22-meter-high building. The reactor has been undergoing a periodic check since September.
There was no change in the airborne radiation levels inside and outside the building, the agency said.
Part of the roof was corroded and "there is a high possibility that the sparks fell through that part onto the ceiling," an agency official said.
The agency started operating the reactor in 1975 for research on nuclear fuel safety. It suspended operations in February, before the Fukushima nuclear criris.