The system keeping spent atomic fuel cool at the Fukushima Daiichi nuclear plant stopped on Friday, its operator said, the latest glitch to hit the crippled site.
Tokyo Electric Power (TEPCO) said an alarm sounded at the facility at 2:27 pm (0527 GMT), and technicians soon confirmed that the cooling system was not working for the pool attached to reactor 3, a spokesman said.
"We have no information at hand about the cause," the TEPCO spokesman said.
Although the breakdown is not thought to be immediately dangerous, it served as a reminder of the precarious state of the Fukushima plant, more than two years after it was crippled by the giant tsunami of March 2011.
Last month, a wide power outage at the plant stopped cooling systems for four pools storing spent nuclear fuel after a rat interfered with the electrics.
As of 2 pm on Friday, the temperature inside the pool attached to reactor 3 was 15.1 degrees Celsius (59.18 Fahrenheit), indicating that spent fuel remained stable and was not posing immediate danger to the environment, according to TEPCO.
Fukushima was the site of the worst nuclear crisis in a generation. Reactors went into meltdown and spewed radiation over a wide area, forcing tens of thousands of people from their homes and polluting farmland.