By Harumi Ozawa

TOKYO, Mar 18, 2011 (AFP) - Japan resumed water cooling operations at a quake-hit nuclear plant Friday using a fleet of fire trucks, as workers racing against time to avert catastrophe ran a power supply cable to the site.

Tonnes of water are being used to douse fuel rods and prevent a calamitous radiation release at the Fukushima No.1 power station, which suffered critical damage in the massive earthquake and tsunami that rocked Japan a week ago.

The trucks unleashed high-powered streams of water on the facility, located 250 kilometres (155 miles) northeast of Tokyo, NHK footage showed. A defence ministry official told AFP they were targeting the number three reactor.

Japan's military, the Self-Defence Forces (SDF), has deployed 11 trucks to the scene, while the Tokyo metropolitan fire department has sent 30 vehicles and 140 personnel to try to bring the situation under control, Kyodo reported.

The twin disasters knocked out the plant's reactor cooling systems, sparking a series of explosions and fires. Authorities have since raced to keep the fuel rods inside reactors and containment pools under water.

If they are exposed to air, they could degrade further and emit even more 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 special SDF fire trucks later joined the effort.

"We poured water onto the number three reactor yesterday. There is no doubt that water entered the pool, but we have not confirmed how much water is in there," chief government spokesman Yukio Edano told a news conference.

The fuel-rod pools at reactors three and four may be boiling and are not fully covered by roofs that would reduce radiation leaks.

"The number three reactor is the top priority," Edano said.

But the defence ministry said the military helicopters would not be used Friday, without giving a reason.

Fluctuating radiation levels at the complex have hindered the cooling operation, repeatedly sparking delays.

Edano put radiation levels on Friday at about 100 microsieverts, and said that those levels did not indicate an immediate threat to human health.

Officials at Tokyo Electric Power Co (TEPCO), which operates the plant, said overnight that they believed the effort was bearing fruit.

"When we poured water, we monitored steam rising from the facility. By pouring water, we believe the water turned down the heat. We believe that there was a certain effect," a TEPCO spokesman told reporters.

Workers were also continuing with the crucial task of trying to restore power lines, Edano said, in a bid to reactivate the plant's crippled cooling systems.

The nuclear safety agency said early Friday that TEPCO had managed to get a line from a regional power firm into the plant site which would allow it to restore the cooling system.

"The power cable is near. We would like to speed up this operation as we can then use it to speed up the rest of what we have to do," Edano said.

The 9.0-magnitude quake, the biggest on record to strike Japan, knocked down electricity pylons used to supply power to the plant.

Edano meanwhile denied that Tokyo had rejected an offer of US assistance.

Paving the way for a more direct role by the US military, the Pentagon said it had sent a team of experts to evaluate what assistance US forces could provide to the effort to control the situation at the Fukushima complex.

The French nuclear authority has said the disaster equated to a six on the seven-point international scale for nuclear accidents, ranking the crisis second only in gravity to Chernobyl.

"I had always thought the plant's structure was very strong. I was surprised that it was destroyed like that," a 41-year-old plant worker who was working at the number one reactor at the time of the quake, told NHK.

Yuko Okabe, 42, who was on duty at reactor number three on March 11, said: "Right now, there are people still working inside the facility. I'm sure their families are very worried."

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Copyright AFP 2011.