Specially equipped vehicles from the Tokyo Fire Department resumed water-spraying operations at the number three reactor at the stricken Fukushima nuclear plant, NHK reported.
Tonnes of water have been used to douse overheating fuel rods at the Fukushima No.1 power station, which suffered critical damage in the massive earthquake and tsunami that ravaged northeastern Japan eight days ago.
Engineers, military personnel and rescue workers have scrambled to avert a catastrophe in what the head of the UN International Atomic Energy Agency has called "a race against time".
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On Friday, six military fire trucks unleashed some 50 tonnes of seawater at the plant, located 250 kilometres (155 miles) northeast of Tokyo.
The twin disasters knocked out the plant's reactor cooling systems, sparking a series of explosions and fires. Authorities have since struggled to keep the fuel rods inside reactors, and fuel storage containment pools, under water.
If they are exposed to air, they could degrade further and emit large amounts of dangerous radioactive material.
On Thursday, four twin-rotor CH-47 Chinook military helicopters ran the first mission to empty large buckets that hold more than seven tonnes of water each onto the facility. Five military fire trucks later joined the effort.
