Officials said it was not immediately clear what had happened at the plant, but there were no danger of radiation leakage and no reported injuries.

One report said the fire had started in an incinerator outside the reactor building used to burn low-level radiation items like workers clothing.

A local government spokesman said smoke emerging from the building quickly cleared and fire fighters did not need to turn on their hoses.

The nuclear plant, near Tsuruga, 350 kilometres (220 miles) west of the capital Tokyo, is a research reactor which stopped generating power three months ago and is undergoing decommissioning.

The Fugen reactor is owned by the government-funded Japan Nuclear Cycle Development Institute.

The BBC Tokyo Correspondent says it has suffered a number of safety alerts over the past year, including a leak of radioactive steam.

Japan's nuclear industry has been hit by a series of accidents and safety scandals in recent years - including the country's worst ever nuclear accident in 1999, when two people were killed because of lax safety standards.