TOKYO (Reuters) - More than 300,000 people living near the site of Japan's worst nuclear accident were told it was safe to venture outside their homes Friday as officials probed for answers to why it ever occurred.

Road and rail links around the Tokaimura uranium processing plant, 90 miles northeast of Tokyo, were re-opened by late Friday and schools were notified they could open as usual Saturday.

Military vehicles deployed in case of a possible mass evacuation were taken off standby and sent home.

Authorities said people living within a 10 km (six mile) radius of the plant were now free to go outside without risk some 30 hours after the accident.

Fifty-five people, mainly plant workers and emergency personnel who responded to the accident, were exposed to radiation. Three were in serious condition with some experts saying their chances of survival were slim.

The advisory excluded 80 people living within 350 meters (1,150 feet) of the plant, who were evacuated soon after the accident Thursday morning and were expected to be barred from returning until Saturday.

Residents welcomed the lifting of the order, which had turned Tokaimura, with a population of 34,000 with 15 nuclear facilities, into an eery ghost town where police in white protective suits roamed the streets.

After a sleepless night followed by a tense day penned in their homes or lining up for checks for radiation exposure, many ventured out to shop or walk dogs.

Some of those unable to go home, however, remained ill at ease. ``Even if they say it's all right, we can't tell with our own eyes,'' one young man at the community center said.

Authorities said that 10,000 area residents were tested for radiation exposure at community centers -- all proving negative.

ACCIDENT RATED LEVEL FOUR, COULD GO HIGHER

The government declared the incident to be ``level four'' on a scale of nuclear accidents, making it Japan's worst ever.

The United Nations' atomic watchdog said in Vienna that the accident could be given a higher ratings after a comprehensive investigation.

Level four means that there has been a leak of a small amount of radioactive material outside a nuclear facility.

The 1979 U.S. Three Mile Island incident was level five, and the 1986 Chernobyl accident in the Ukraine, the worst in history, was at the maximum, level seven.

Authorities scrambled to assure residents they were safe.

An official in the nuclear safety division of the government's Science and Technology Agency said they had detected no spread of radiation beyond the immediate area that could adversely affect humans.

``This is not anything like an explosion. And considering the quantities of radiation leaked from the facility, we believe it unlikely that high levels of radiation could spread to wider areas,'' he told Reuters.

Officials said it was safe to drink tap water but warned against drinking water from local wells which was being tested for possible contamination, a process which would take days.

AUTHORITIES UNDER FIRE, ASHAMED

But the government came under fire for its handling of the accident amid questions about how it all happened in the same town that suffered a nuclear accident only two years ago.

Chief government spokesman Hiromu Nonaka admitted the government was slow to respond to the accident, in which a chain nuclear reaction began, causing radiation levels to rise around 15,000 times the normal amount in the area near the plant.

``As a modern nation, it's shameful that this kind of accident happened,'' Nonaka said in what has become a refrain from top officials after a string of similar disasters.

Prime Minister Keizo Obuchi talked with President Clinton. Obuchi told reporters late Friday that Clinton had offered assistance in helping to determine the cause of the accident or in helping with the clean-up.

Obuchi also apologized for the incident and noted that it had received widespread international attention.

FATE OF ILL WORKERS UNCERTAIN

A hospital spokesman said the three seriously ill workers were in stable condition Friday evening but added ``It's hard to say what will happen next.''

A spokesman for the United Nation's atomic watchdog said in Vienna that the three were unlikely to survive, given how close they were to the blue flash of light which signals a nuclear chain reaction -- similar to what occurs in a reactor.

Officials suspect that a worker at the facility, which reprocesses uranium into pellets for use in nuclear power plants, had loaded 16 kg (35 lb) of uranium into a container, nearly eight times the normal amount.

This may have created the ``flash criticality'' which was not halted until Friday morning.

An official at JCO Co Ltd, the firm operating the facility, said the workers may have failed to follow company rules. The company is a subsidiary of Sumitomo Metal Mining Co.

NUCLEAR PROGRAM SEEN STALLED AFTER FRESH MISHAP

The accident, the latest in a string of mishaps for Japan's trouble-plagued nuclear industry, is likely to spur renewed public opposition to its nuclear program.

``It is an accident of unprecedented seriousness,'' said Hideyuki Ban, co-director of the anti-nuclear Citizens Nuclear Information Center. ``The situation (facing the industry) is more serious than ever before.''

Japan is heavily dependent on nuclear power, with some 35 percent of its electricity coming from its 51 commercial nuclear reactors.