A nuclear accident at a uranium-processing plant 125 km northeast of Tokyo on Thursday reached criticality, injuring three and pushing radiation levels up to 20,000 times beyond normal in Tokai, Ibaraki Prefecture.

The accident, Japan's worst, may expose thousands to radiation and remained out of control as of 1:30 a.m.

Criticality is the point at which neutrons produced in the fission process are sufficient to sustain an independent chain reaction, and early Friday morning it was feared that reaction was still simmering.

The nation's most recent nuclear accident has left three workers hospitalized in serious condition, forced nearby residents to evacuate, and moved the government to ask some of Tokai's 33,000 residents to stay indoors if they lived within 10 km of the accident site.

Just after midnight, it was reported another 11 workers had been exposed to radiation during the accident, which began at 10:35 a.m.

"There is a strong possibility that abnormal reactions are continuing inside even now," Chief Cabinet Secretary Hiromu Nonaka said around 10 p.m. "We believe that it is a severe situation, and there are concerns about (high radiation levels) in the surrounding areas.

He also said the Ground Self-Defense Force's chemical warfare unit is ready to be deployed at the nuclear accident site but admitted, "the unit, however, is not capable of coping with that kind of nuclear accident."

The government set up a task force earlier in the night, led by Prime Minister Keizo Obuchi, to cope with the accident, the likes of which "our country has never experienced," Nonaka said.

France's nuclear safety institute said Thursday that the critical accident would be the 60th in the world since 1945, following 33 such accidents in the United States and 19 in the former Soviet Union.

"Generally criticality accidents have more significant consequences at the site of the installation involved than on the environment," the institute said.

Radiation levels at and around the plant remained high in the evening, officials said.

In the afternoon, the Science and Technology Agency said the radiation level outside the private nuclear fuel processing company, JCO Co. Ltd., was about 4,000 times that of normal levels. The Ibaraki Prefectural Government said the level was about 15,000 times higher than normal.

JCO, a fully owned subsidiary of Japan's Sumitomo Metal Mining Co. Ltd., processes uranium into an oxidized powder form for burning at nuclear power plants.

According to the Japan Atomic Energy Research Institute, also in Tokai, as of 5 p.m., radiation levels on the plant grounds were between 10,000 and 20,000 times normal.

Prefectural authorities in the adjacent town of Naka, meanwhile, said the radiation level after 4 p.m. was rapidly increasing, based on observations of monitoring posts inside the town.

They also said the reaction was still critical.

A critical accident occurs when a malfunction causes a concentration of nuclear fission.

At a news conference at the science agency earlier in the day, Makoto Ujihara, head of JCO's Tokyo office, said, "We are still trying to find out what exactly happened but we believe the uranium reached the critical point."

Ujihara said he had received reports that said blue flames were spotted when the accident occurred.

According to Ibaraki police, the workers became ill after they saw the blue flame, which flared while they were mixing a uranium compound with a nitric acid solution to produce fuel for a nuclear plant.

They are believed to have mixed too much uranium in the tank -- pouring 16 kg, far above the limit of 2.3 kg, they said.

In the evening, the Tokai Municipal Government said that cesium 138, a chemical element created by nuclear fission, was detected from the ground inside the complex of the Japan Atomic Energy Research Institute, located next to the accident site.

Nonaka said the situation turned out to be worse than an initial report he had received Thursday afternoon. He had told a news conference earlier in the day that the government believes that "damage will not expand any further."

In his early report, Nonaka had said he received a report on the accident shortly before noon.

Asked why it took more than an hour for the report to reach top government officials and for the government to act on the accident, Nonaka said, "It took some time for us to understand the situation because it was an accident that occurred at a private company."

The science agency said it has set up a crisis management headquarters to deal with the incident.

The facility is located about 2 km northwest of JR Tokai Station. A primary school, kindergarten, houses and shops are located nearby.

The Tokai Municipal Board of Education instructed kindergartens, primary and junior high schools in the village to close all windows of their buildings and not to let students outdoors.

At 10:30 p.m., Ibaraki Prefecture began urging residents living within 10 km of the accident site to stay indoors to avoid the risk of radiation exposure.

In addition, the Tokai Municipal Government, with cooperation from the governmental Japan Nuclear Cycle Development Institute, began health checkups of the evacuees.

The checkup showed that some of the residents may have been exposed to radiation, local officials said.

The first three workers reported to be exposed to radiation were identified as Hisashi Ouchi, 35, Masato Shinohara, 39, and Yutaka Yokokawa, 54. All are employees of JCO Co., previously called Japan Nuclear Fuel Conversion Co., which runs the Tokai plant.

One worker is in serious condition with diarrhea and nausea, police said, while the other two are in extremely serious condition.

The three were taken to a state-run hospital in Mito, Ibaraki Prefecture.

Later they were transported by helicopter to the National Institute of Radiological Sciences in Chiba Prefecture.

Immediately after the accident, police sealed off a 200-meter area around the facility. From that area, they evacuated 150 people from 50 families after radiation increased to 4,000 times the normal level.

Traffic was banned in a 3-km radius around the accident, and television helicopters showed the zone like a ghost town with no cars or people.

"My God, I don't know what to do," said one middle-aged woman as she arrived on a government bus at the town's community center after being evacuated from her home.

Later in the afternoon, most readings near the plant were said to be about 10 times above normal, though they had climbed as high as 0.87 sievert an hour -- about 4,000 times the normal 0.0002 sievert, said a senior science and technology agency official.

Residents were told to stay indoors.

"We have asked all residents in Tokai to stay indoors until inspections by the prefectural and the science and technology agency's officials can confirm that radiation has fallen to a safe level," a local government official said.

The warning was broadcast on loudspeakers in the district, home to about 20,000 residents, according to officials.

Later, residents of Naka were also told to stay inside.

"We asked some of the town's 5,000 households not to go out after winds from the direction of Tokai became stronger and radiation readings began to go up," a town official said.

"Radiation meters placed around the company's facility showed radiation levels went back to normal in less than two hours after the accident, but we want to take precautions for absolute safety," said a Tokai government official.

Tokai has been the scene of nuclear accidents in the past.

An explosion and fire on March 11, 1997, at a different nuclear complex in Tokai irradiated 37 workers. Some of the escaped radiation spread to Tokyo and neighboring prefectures.

It was Japan's worst nuclear accident.

Greenpeace issued a statement Thursday saying the accident "exposes once again the lack of an adequate safety culture in Japan."

The accident came a day before a British-flagged ship was expected to deliver 225 kg of mixed plutonium-uranium oxide fuel to a plant in Takahama, Fukui Prefecture.