TOKYO: A power outage has left four fuel storage pools at Japan's tsunami-damaged nuclear plant without fresh cooling water for nearly 20 hours, raising concerns about the fragility of a facility that still runs on makeshift equipment.

Tokyo Electric Power Co. said that pool temperatures at the Fukushima Dai-ichi plant were well within safe levels, and that pools would remain safe for at least four days without fresh cooling water. The utility said the reactors were unaffected and no other abnormalities were found.

The cooling system was restored at one of the four pools by mid-afternoon Tuesday, and the systems for the three other pools are expected to resume by Wednesday morning as workers complete repairs and try to determine the cause the problem, TEPCO officials said.

Workers fixed the last of the three switchboards that they suspect as a possible cause of the problem and the utility was preparing a backup system in case the repairs didn't fix the issue, TEPCO spokesman Masayuki Ono told reporters.

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

Japan's 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 current power outage is a major test for TEPCO to show if it has learned anything from the disaster. TEPCO, which has repeatedly faced cover-up scandals, was slammed by local media Tuesday for waiting hours to disclose the blackout.

Ono acknowledged the plant was vulnerable.

"Fukushima Dai-ichi still runs on makeshift equipment, and we are trying to switch to something more permanent and dependable, which is more desirable," he said. "Considering the equipment situation, we may be pushing a little too hard."

Ono said the utility did not immediately try to switch to a backup cooling system because doing so without finding and fixing the cause could lead to a repeat of the problem. There is a backup cooling system but no backup outside power.

Regulators previously have raised concerns about the makeshift equipment and urged the plant to switch them to a more permanent arrangement. The operator still has to remove melted, fatally radioactive fuel from the reactors before fully decommissioning the plant, which officials say could take 40 years.

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 told reporters in a regular briefing.

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