Emergency workers on Sunday restored the cooling system of a reactor which had come to a halt after escaping major damage from the March 11 quake and damage at the crippled Fukushima nuclear power plant.
The water pump to cool the reactor and the pool for spent nuclear fuel at the facility's No. 5 unit was found to be at a standstill late Saturday, the plant's operator Tokyo Electric Power Co. (TEPCO) said.
The work began at 8.00am on Sunday (2300 GMT Sarurday) to replace the pump and it was completed in four and a half hours.
"There was a motor problem in the pump and we replaced it with a backup pump which is operating now," TEPCO official Ryoko Sakai said.
The temperature of water in the reactor, which was 68 degrees Celsius when the trouble was found, reached 93.7 degrees Celsius before the backup pump was activated, the official said.
The 9.0-magnitude quake and monster tsunami ravaged cooling systems at the Fukushima Daiichi plant, causing it to leak radiation from damaged reactors into the environment, including the Pacific Ocean, in the worst nuclear accident since the 1986 Chernobyl disaster.
Of the plant's six reactors, the No.1, 2 and 3 units are presumed to have suffered a meltdown, TEPCO has said.
The No. 5 and 6 reactors were in a cold shutdown for regular checkups at the time of the disaster. They have remained stable as an emergency power generator continued supplying electricity to them.