An electricity transformer caught fire at the No 3 reactor at Takahama plant in Fukui, said a Kansai Electric Power spokesman, adding the blaze was rapidly brought under control.
"No one was injured and there was no impact from the incident - such as a radiation leak - on the outside environment," he added.
Japan has become increasingly nervous about nuclear power in the years since the March 2011 disaster at Fukushima. Currently, none of its 50 viable reactors, including the Takahama plant, is in operation.
It sees this as the best way to plug the country's energy gap and reduce the yawning trade deficit caused by the need to import mountains of fossil fuels.
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Tokyo Electric Power is struggling with the clean-up at Fukushima Daiichi, where cooling systems were swamped by the monster waves of a tsunami, sparking reactor meltdowns and radiation leaks.
Tens of thousands of people were evacuated from around the plant and many still cannot return because of elevated radiation levels.
Decommissioning of the site is expected to take decades.