Four twin rotor CH-47 Chinooks of the Self-Defence Forces ran the first mission to empty large buckets that hold more than seven tonnes of water each onto the facility damaged by Friday's massive earthquake and tsunami.
The operation aims to keep the fuel rods inside reactors and containment pools submerged under water, to stop them from degrading when they are exposed to air and emitting dangerous radioactive material.
The helicopter mission started in the morning under a clear sky after a similar effort the previous day was called off by officials citing strong radiation and high winds, shortly before darkness fell.Police water cannon were also set to support the effort Thursday, along with firefighting equipment already in use over recent days at the plant, located some 250 kilometres (155 miles) northeast of the Japanese capital.
The plant's operator, Tokyo Electric Power Co. (TEPC0), said earlier it was concentrating on restoring the power supply to reactivate its crippled cooling systems, which were knocked out in the dual calamity.
"We cannot tell when, but we want to restore the power source as soon as possible," TEPCO spokesman Naohiro Omura told AFP.
TEPCO was preparing to restore outside power lines from Tohoku Electric Power Co., which serves the region, and connect its damaged electric transmission system with unaffected lines.
"At the moment, we are concentrating our efforts on this work," he said.
"If the restoration work is completed, we will be able to activate various electric pumps and pour water into reactors and pools for spent nuclear fuel."
The 9.0 magnitude quake, the biggest on record to strike Japan, knocked down electricity pylons which Tohoku had used to supply power to the TEPCO plant.
Some 70 workers have been using fire pumps to pour seawater to cool
reactors at the plant, according to media reports. They are using electricity from power supply cars which the company has borrowed from different places.