Tokyo Electric Power Co. said it could not use a backup pump to restart cooling immediately after the pump that served the No. 1 unit, one of three reactors destroyed in the massive earthquake and tsunami in March 2011, stopped running, The New York Times reported.
The pump that stopped Monday was part of a provisional cooling system Tepco developed after the accident to dump hundreds of tons of water per day onto the three damaged reactor cores to prevent them from reheating. In 2011, the fuel cores in the overheating reactors melted down, causing explosions that destroyed the reactor buildings.
Tepco said the stoppage could have occurred because of a faulty electric switchboard, which has forced the plant's cooling system to shut down before.
Japan's Nuclear Regulation Authority, however, suggested a worker may have shut down the main pump accidentally by hitting the stop button during a routine checkup.
The plant has been plagued by mishaps. Last week, the company said workers spilled 114 gallons of radioactive water when they mistakenly tried to put water in an already-full tank.
Japan welcomes foreign expertise as it seeks to contain radioactive water leaks at the Fukushima-Daiichi nuclear power plant, Prime Minister Shinzo Abe said.
Since its damage in by the earthquake and tsunami, Japanese experts have been working to decommission the plant, which could take decades. The work, however, has been seriously hampered of late by frequent leaks of contaminated water.
"My country needs your knowledge and expertise" in coping with the issue, Abe said in a speech in English Sunday at an international science conference in Kyoto.
"We are wide open to receive the most advanced knowledge from overseas to contain the problem," Kyodo News quoted him as saying.
Last week, Katsuhiko Ikeda, president of Japan's nuclear regulator, ordered Tepco to bring the radioactive water problem under control at the No. 1 nuclear complex.
The leaking contaminated water is entering into the reactor complex and mixing with water being injected to keep the damaged reactors cool.