The cooling system for the spent fuel pool at the crippled Fukushima nuclear power plant in northeast Japan was halted Monday after several dead rats were discovered at the facility, the Kyodo news agency reported.
The plant operator, Tokyo Electric Power Co. (Tepco), said the system was stopped for inspection after dead rats were found near a transformer.
The inspection is expected to take several hours, after which the cooling system will be restarted, Tepco said.
There have been several radioactive water leaks recently at the nuclear power plant, which was hit by a massive earthquake and an ensuing tsunami in 2011.
Earlier this month, Tepco announced it would stop using underground tanks to store radioactive water used in the plant's cooling systems and transfer the water to existing surface storage tanks.
The operator previously said about 120 tonnes of contaminated water might have escaped from underground reservoir tanks.
In March 2011, Japan was hit by a massive 9.0-magnitude quake which caused a tsunami, claiming over 15,000 lives and triggering a number of explosions at the Fukushima Nuclear Power Plant.
The tsunami caused a partial meltdown at three of the nuclear plant's reactors.
Radiation leaked into the atmosphere, soil and seawater, making the accident the world's worst nuclear disaster after Chernobyl in the USSR in 1986.
Japan will need at least 40 years to recover fully from the nuclear catastrophe, scientists say.
Some 315,000 victims are still living in a temporary housing as they are unable to return to their houses near the plant.
Fukushima's cooling systems sustained a one-day power outage in late March. That incident was tentatively blamed on a rat which short-circuited a switchboard with its body.