TOKYO (Jan 30): A nuclear reactor in Japan was unexpectedly shuttered, delivering a new challenge to the nation’s energy supplies in the middle of winter.

Kansai Electric Power Co’s Takahama No. 4 nuclear reactor in Japan’s Fukui prefecture was automatically halted earlier on Monday, according to a spokesman.

The utility is investigating the reason for the halt, he said.

The reactor automatically stopped after an alarm went off, the nation’s Nuclear Regulation Authority said in a statement. The reactor is being cooled normally and no irregularities have been detected, and a local inspector is checking the plant, it said.

The closure of the plant could boost Japan’s demand for natural gas and coal as a replacement, and further tighten power supplies during the frigid winter months. The reactor was just one of nine units currently online.

Still, Japan is sitting on ample fuel stockpiles and is unlikely to see power generation drop to levels that would lead to blackouts.

It’s uncertain when the reactor can restart, as there needs to be a thorough investigation, Akihiro Yamamoto, a nuclear safety division official from Fukui prefecture, said in a press conference carried by local broadcaster NHK. “It’s not something that can be resolved in a day,” he said.

Kansai Electric restarted Takahama No. 4 reactor in November, following planned maintenance work. The utility has said that it will seek approval from the nuclear watchdog to operate the No. 4 reactor, which was commissioned in 1985, for 60 years.