FUKUSHIMA, Japan » Emergency workers racing to cool dangerously overheated uranium fuel scrambled Saturday to connect Japan’s crippled reactors to a new power line, with electricians fighting tsunami-shattered equipment to restart the complex’s cooling systems.

Though the power line reached the complex Friday, making the final link without setting off a spark and potentially an explosion means methodically working through badly damaged and deeply complex electrical systems at the Fukushima Dai-ichi plant on Japan’s northeast coast.

“Most of the motors and switchboards were submerged by the tsunami and they cannot be used,” said Hidehiko Nishiyama of Japan’s Nuclear and Industrial Safety Agency.

Operators of the plant, which have prompted global worries of radiation leaks, hope to have power reconnected to four of the complex’s six units on Saturday, and another on Sunday. However, even once the power is reconnected, it is not clear if the cooling systems will still work.

The storage pools need a constant source of cooling water. Even when removed from reactors, uranium rods are still extremely hot and must be cooled for months, possibly longer, to prevent them from heating up again and emitting radioactivity.

Along the northeast coastline, meanwhile, where the tsunami killed thousands of people and obliterated entire villages, rescuers pulled a man alive from a wrecked house eight days after a massive 9.0-magnitude earthquake began the cascade of catastrophes.

He was too weak to talk and transferred immediately to a hospital, a military official said, declining to be named because he was not authorized to speak with reporters. Japanese media reports said, however, that the man returned to the house a week after the disaster and was trapped only for one day.

As Japan crossed the one-week mark since the twin natural disasters spawned the nuclear crisis, the Japanese government conceded Friday it was slow to respond and welcomed ever-growing help from the United States in hopes of preventing a complete meltdown at the Fukushima Dai-ichi power plant.

The natural disasters claimed more than 7,200 lives, with many thousands more missing.

Emergency crews at the nuclear plant faced two continuing challenges: cooling the nuclear fuel in reactors where energy is generated and cooling the adjacent pools where thousands of used nuclear fuel rods are stored in water.

“In hindsight, we could have moved a little quicker in assessing the situation and coordinating all that information and provided it faster,” Chief Cabinet Secretary Yukio Edano said Friday.

Also Friday, Japan’s government raised the accident classification for the nuclear crisis from Level 4 to Level 5 on a seven-level international scale. That put it on a par with the Three Mile Island accident in Pennsylvania in 1979, and signified its consequences went beyond the local area.

Edano also said Tokyo was asking Washington for additional help, a change from a few days ago, when Japanese officials disagreed with American assessments of the severity of the problem.

A U.S. military fire truck was among a fleet of Japanese vehicles that sprayed water into Unit 3, according to air force Chief of Staff Shigeru Iwasaki, sending tons of water arcing over the facility in an attempt to prevent nuclear fuel from overheating and emitting dangerous levels of radiation.

Additionally, the United States also conducted overflights of the reactor site, strapping sophisticated pods onto aircraft to measure radiation aloft. Two tests conducted Thursday gave readings that U.S. Deputy Energy Secretary Daniel B. Poneman said reinforced the U.S. recommendation that people stay 50 miles (80 kilometers) away from the Fukushima plant. Japan has ordered only a 12-mile (20-kilometer) evacuation zone around the plant.

American technical experts also are exchanging information with officials from the Tokyo Electric Power Co. which owns the plants, as well as with Japanese government agencies.

The tsunami knocked out power to cooling systems at the nuclear plant and its six reactors. In the week since, four have been hit by fires, explosions or partial meltdowns. The events have led to power shortages and factory closures, hurt global manufacturing and triggered a plunge in Japanese stock prices.

Most of Japan’s auto industry is shut down. Factories from Louisiana to Thailand are low on Japanese-made parts. Idled plants are costing companies hundreds of millions of dollars. And U.S. car dealers may not get the cars they order this spring.

But Prime Minister Naoto Kan vowed that the disasters would not defeat Japan.

“We will rebuild Japan from scratch,” he said in a nationally televised address, comparing the work with the country’s emergence as a global power from the wreckage of World War II.

While nuclear experts have been saying for days that Japan was underplaying the crisis’ severity, Hidehiko Nishiyama of the nuclear safety agency said the rating was raised when officials realized that at least 3 percent of the fuel in three of the complex’s reactors had been severely damaged. That suggests those reactor cores have partially melted down and thrown radioactivity into the environment.

Low levels of radiation have been detected well beyond Tokyo, which is 140 miles (220 kilometers) south of the plant, but hazardous levels have been limited to the plant itself.

The Science Ministry said radiation levels about 30 kilometers (19 miles) northwest of the Fukushima Dai-ichi plant briefly spiked Friday to 0.15 millisieverts per hour, about the amount absorbed in a chest X-ray. While levels fluctuate, radiation at most points at that distance from the facility have been far below that. The ministry did not have an explanation for the rise. Police said more than 452,000 people made homeless by the quake and tsunami were staying in schools and other shelters, as supplies of fuel, medicine and other necessities ran short.