According to TEPCO and the Japanese Nuclear Regulation Authority, around 19:00 on Monday night, the seismic building at the Fukushima Daiichi nuclear power plant had experienced a power failure. While the outage did not affect the monitoring instruments in the reactor containment, it did affect the spent fuel pool cooling system at Unit 1, Unit 3, and Unit 4.
TEPCO data shows that the Unit 4 spent fuel pool was at the highest temperature at the time of the outage, but has a margin of about four days before the temperatures would be expected to reach levels of concern. The contaminated water treatment system was also knocked out by the power failure.
TEPCO workers are still investigating the cause of the outage.
Source: The Daily Yomiuri