TEPCO announced on March 20 that it had restored the cooling systems for the four spent fuel pools which were knocked out for nearly 30 hours before restoration. All in all, the outage caused 9 systems to fail, after three power distribution boards stopped working.
TEPCO officials have said that one of the power distribution boards which was temporarily installed on a truck may hold the secret as to why the outage occurred. Littered around the temporary terminal were the carcasses of small rodents and mice, leading some TEPCO workers to worry about the possibility of a short circuit.
Source: Enformable Nuclear News