Japan's crippled Fukushima nuclear plant halted cooling of a spent fuel pool at the site this week to remove two dead rats.
Plant operator Tokyo Electric Power (Tepco) said it halted cooling of the No. 2 unit pool, which stores spent uranium fuel rods at the Fukushima Daiichi site, for a few hours to remove the rats and install a net to stop further such intrusions.
Last month, Tepco lost power to cool fuel rods for 29 hours, an outage it later blamed on a rat that had shorted a temporary switchboard.
Two weeks later, workers attempting to install a net tripped the system again.
A tsunami crashed into the plant in March 2011, causing fuel-rod meltdowns at three reactors and triggering the evacuation of 160,000 people in the world's worst nuclear disaster since Chernobyl in 1986.
Monday's incident follows a string of mishaps including four leaks of contaminated water from underground storage pits.
The problems at the plant, 240 km (150 miles) north of Tokyo, attracted a rebuke from the government and the nuclear regulator, reviving public debate over whether Tepco was up to the task of a decommissioning project expected to last decades.
The International Atomic Energy Agency (IAEA) said it believed Tepco could handle the job, but said the contaminated water was its "biggest challenge".
Juan Carlos Lentijo, an IAEA team leader, said decommissioning would be an enormous task.
"It will be near impossible to ensure the time for decommissioning such a complex facility in less than 30, 40 years as it is currently established in the roadmap," he said after a week-long IAEA tour of the site.
Tepco has been waging a constant battle to filter and store groundwater that continues to flood the basements of the reactor buildings at a rate of 400 tonnes a day.
The IAEA said that Tepco had achieved the stable cooling of the reactors and spent fuel pools, but cautioned that it needed to improve systems to treat the toxic water and find reliable ways to monitor and store it on site.
More than 80 per cent of available storage capacity has been filled, forcing Tepco to scramble to build new tanks.