MILITARY HELICOPTERS DUMPED seawater onto Japan’s stricken nuclear complex on Thursday, turning to combat-style tactics while trying to cool overheated uranium fuel that may be on the verge of spewing out more radiation.

Plant operators also said they were racing to finish a new power line that could restore cooling systems and ease the crisis at the Fukushima Dai-ichi plant on the country’s northeast coast.

The top US nuclear regulatory official gave a far bleaker assessment of the situation than the Japanese, and the US ambassador said the situation was “deteriorating” while warning US citizens within 80 kilometres of the complex to leave the area or at least remain indoors.

The Japanese government said it had no plans to expand its mandatory, 20-kilometre exclusion zone around the plant, while also urging people within 30 kilometres to stay inside.

The troubles at the nuclear complex were set in motion last week’s 9.0-magnitude earthquake and tsunami knocked out power and destroyed backup generators needed for the reactors’ cooling systems. That added a nuclear crisis on top of twin natural disasters that likely killed well more than 10,000 people and left hundreds of thousands homeless.

Four of the plant’s six reactors have faced serious crises involving fires, explosions, damage to the structures housing reactor cores, partial meltdowns or rising temperatures in the pools used to store spent nuclear fuel. Officials also recently announced that temperatures are rising in the spent fuel pools of the last two reactors.

Two Japanese military CH-47 Chinook helicopters began dumping seawater on the complex’s damaged Unit 3 at 9.48 am (0048 GMT), defense ministry spokeswoman Kazumi Toyama said. The choppers dumped at least four loads on the reactor in just the first 10 minutes, though television footage showed much of it appearing to disperse in the wind.

Chopper crews were flying missions of about 40 minutes each to limit their radiation exposure, passing over the reactor with loads of about 7,500 litres of water.

The dousing is aimed at cooling the Unit 3 reactor, as well as replenishing water in that unit’s cooling pool, where used fuel rods are stored, Toyama said. The plant’s owner, Tokyo Electric Power Co, said earlier that pool was nearly empty, which would cause the rods to overheat and emit even more radiation.

Defense Minister Toshifumi Kitazawa told reporters that emergency workers had no choice but to try the water dumps before it was too late.

- AP