TOKYO (AP) — Four fuel storage pools at Japan's tsunami-damaged nuclear plant have been without fresh cooling water for more than 15 hours due to a power outage, but the plant's operator said Tuesday morning it was trying to repair a broken switchboard that might have caused the problem.

Tokyo Electric Power Co. said that pool temperatures were well within safe levels at the Fukushima Dai-ichi plant, and that pools would remain safe for four days without fresh cooling water.

The utility was preparing a backup system in case the repairs didn't fix the problem, Masayuki Ono, an official at operator Tokyo Electric Power Co., told reporters.

"If worse comes to worst, we have a backup water injection system," said Ono.

The command center at the plant suffered a brief power outage before 7 p.m. Monday. Electricity was quickly restored to the command center but not to equipment pumping water into the fuel pools.

The utility was investigating the cause of the power outage and believes it might be due to problems with a switchboard, which it is trying to repair. At the same time, the company is preparing to connect another switchboard if repairs cannot fix the problem.

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 30.5 degrees as of 10 a.m. Tuesday from 25 degrees before the power outage. A common pool storing spent fuel for all reactors was at 28.6 degrees, while the Unit 1 pool was at 17.1 degrees and Unit 3 was at 15.9 degrees.

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

The March 11, 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 is now using makeshift systems.

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Associated Press writer Malcolm Foster contributed to this report.