No electricity to pumps that feed ponds where spent nuclear fuel rods must be kept underwater at a safe temperature

Four fuel storage pools at Japan's Fukushima nuclear plant have been without fresh cooling water for more than 15 hours due to a power outage. The plant's operator has said it is trying to repair or replace a broken switchboard that might be the problem.

The 11 March 2011 earthquake and tsunami destroyed the plant's power and cooling systems, causing three reactor cores to melt and fuel storage pools to overheat. The plant has since been using makeshift systems.

Plant operator Tepco said pool temperatures remained within safe levels and the pools would remain safe for at least four days without fresh cooling water.

Tepco was preparing a backup system in case the repairs didn't work, said Masayuki Ono, a company official. "If worse comes to worst we have a backup water injection system."

Yoshihide Suga, the chief government spokesman, sought to allay concerns. "In a sense we have put in place measures that leave no room for worry," Suga said.

The command centre at the plant briefly lost power before 7pm on Monday. Electricity was quickly restored there but not to the pool pumps.

The temperature in the four pools had risen slightly but was well below the utility's target control temperature of 65 degrees Celsius, Tepco said.

The fuel pool for Unit 4, which contains spent and new fuel rods, had risen to 30.5C as of 10am Tuesday from 25C before the power outage. A common pool storing spent fuel for all reactors was at 28.6C, while the Unit 1 pool was at 17.1C and Unit 3 was at 15.9C.

Tepco said the reactors were unaffected and no other abnormalities were found.