Workers accidentally set off a nuclear-fission chain reaction at a uranium processing plant about 75 miles northeast of Tokyo on Thursday morning, sending three employees to the hospital, two of them in critical condition, and exposing an unknown number of nearby residents to at least low levels of radiation.

Responding to what they called Japan's worst nuclear power accident, officials asked more than 300,000 people in the surrounding area to remain in their homes until at least this morning. Schools were closed, and highway traffic and trains were halted.

More than 40 people, most of them plant employees, were being treated for exposure to radiation, officials said. Police ordered about 150 people living near the plant to evacuate, and the area was sealed off. By early today, the reaction was contained, according to Masaru Hashimoto, the governor of Ibaraki prefecture where the accident occurred.

The privately run plant is in Tokaimura, which has a population of about 34,000 and is home to 15 nuclear-related facilities. There are 52 nuclear reactors in Japan, all but one of them operated by private companies that are government-regulated.

Radiation around the outer boundary of the plant reportedly reached 4 millisieverts per hour. By contrast, the average American gets about 3.6 millisieverts per year from natural sources. A millisievert is a standard measure of radiation exposure.

By this morning, radiation levels had fallen to one-third or one-fourth of their peaks, Hashimoto said. Workers injected boric acid into the container to completely shut down the reaction.

In Washington, President Clinton pledged that the United States would "do whatever we possibly can" to aid the Japanese.

Energy Secretary Bill Richardson, who is visiting Russia, said in a telephone interview that the incident was "serious, but the good news is that it's only a limited area. The contamination does not extend over a wide area. But it's important that [they] move rapidly to contain the problem."

Richardson said that he and the Russians were offering Japan "a joint American-Russian team" of experts in radiation monitoring and radiation health issues, along with emergency response teams and robots that can crawl into radioactive areas.

Japanese Foreign Minister Masahiko Komura said that Japan asked the United States and Russia for technical assistance in dealing with the accident.

Uranium emits dangerous radiation of various kinds, most of which can be fairly easily blocked by walls and other cladding. But when enough uranium is put together in the same place under the right conditions, it can reach a "critical mass" in which neutrons emitted from the nuclei of some of the atoms cause the nuclei of neighboring atoms to split apart in a chain-reaction process called nuclear fission.

Uncontrolled, explosive fission reactions can cause the kind of devastating energy release seen in the bombs dropped on Hiroshima and Nagasaki in 1945. When fission is employed in a controlled form in nuclear power reactors, the energy released by the uranium-rich fuel rods is used to generate heat that in turn produces electric power.

"Criticality" accidents such as this one in Japan occurred in the United States about two dozen times between 1945 and 1964, and numerous times in other countries. Ordinarily, the first remedial step is to spread out or dilute the mass of radioactive material, halting the fission reaction.

The Japanese plant turns uranium into fuel for nuclear power plants. The accident happened when workers put too much uranium -- 35 pounds instead of about 5.3 pounds -- into a liquid mixture of uranium and other compounds. The excess was enough to create a critical mass and begin a fission chain reaction.

Makoto Morita, an executive with JCO Co., which operates the plant, said that the workers manually bypassed part of a required procedure that would have prevented them from using too much uranium, Kyodo news service reported. "We have no words to express our apologies," he said. "We cannot escape our responsibility."

The workers involved in the accident told their colleagues "they saw blue flame rising from the fuel" and complained of nausea, according to Makoto Ujihara, head of the Tokyo office of JCO. Fission reactions are often accompanied by release of a blue light called Cerenkov radiation, a kind of sonic boom for light.

In a fission reaction, the greatest public health threats often come from radioactive material released into the air or water. An explosive fission reaction would have created a number of radioactive gases that may have escaped the building.

Of the workers inside the plant, Hisashi Ouchi and Masato Shinohara had abnormally high white blood cell counts, diarrhea and were barely conscious, according to a hospital official. A third worker was able to walk on his own. They were flown to a hospital specializing in radiological sciences near Tokyo.

After the accident, the first people to approach the site were three two-man teams who went in between 2:30 and 3:30 a.m. today for a few minutes each to take photographs, confirm that water circulation was still taking place and then to turn off the water supply and drain out the water coolant surrounding the uranium. Water can encourage nuclear fission reactions by slowing emitted nuclear particles to the speed at which they break up nuclei in surrounding atoms.

"The situation is one our country has never experienced," a government spokesman said. A chemical warfare unit of the Self-Defense Forces, consisting of about a dozen personnel and two chemical-proof vehicles, were sent to the area. Defense agency officials said that they were on standby but that the group was not capable of coping with an accident of this magnitude, according to Reuters news agency.

Several experts in the United States, however, noted that the event appeared to be nowhere near as dangerous as the catastrophe at Chernobyl, Ukraine, or even the meltdown at Three Mile Island in Pennsylvania. And 35 pounds of uranium, while not a trivial amount, is relatively small compared with the 50 to 70 tons of uranium in a typical commercial nuclear power plant.

"The hazard is primarily to the workers who are in the room or in very close proximity to the event," said Thomas B. Cochran, director of the nuclear program at the Natural Resources Defense Council. "You do have to be concerned about leakage of the gases and volatile material out of the building, where it can be blown off-site." But the Japanese event appears to be "much smaller than the types of reaction meltdown accidents" familiar to the public, he said. "This is a million times" less energy than was released at Chernobyl, with a likely "TNT energy equivalent of about 20 grams to 20 kilograms" -- that is, less than an ounce to about 44 pounds of dynamite. "My guess is that the off-site risk was probably pretty small."

Robert Bari, chairman of the department of advanced technology at Brookhaven National Laboratory on Long Island, agreed. "It's hard to believe there would be a widespread public health problem," he said. However, rain in the area prompted warnings to residents to wipe off any drops that fell on them.

Japan relies on nuclear power for nearly one-third of its electric energy needs, but has been plagued by a series of mishaps in the nuclear power industry.

A fire at another facility at Tokaimura in 1997 exposed 37 people to radiation, until Thursday the country's worst nuclear power accident. France's nuclear safety institute said Thursday's incident would be the 60th reported criticality accident in the world since 1945, according to Reuters.

Sumitomo Metal Mining Co., parent company of JCO, issued an apology today for the accident. It said it had set up a special task force at its Tokyo headquarters to deal with what officials said was the "unprecedented" accident.

Arjun Makhijani, president of the Institute for Energy and Environmental Research in Takoma Park, which studies nuclear problems worldwide, called it a "very unusual accident" at a plant that is "badly managed" and has a recent history of radiation exposure problems.

"The whole place should be investigated," Makhijani said. "This is a site that needs to be shut down."

Staff writer Curt Suplee in Washington contributed to this report.

During Nuclear Fuel Processing, an Accident

Workers and residents were exposed to high radiation levels in and near the JCO Co. nuclear fuel processing plant in Tokaimura, 75 miles northeast of Tokyo. Two workers were in critical condition in the hospital.

More than 300,000 people within six miles of plant told to stay indoors.

40 people, most of them employees, exposed.

At least 150 nearby residents evacuated.

An accidental chain reaction occurred during a

CRITICAL STAGE of a fuel-processing cycle.

1) Uranium ore converted to powder.

2) Powdered uranium oxide treated with fluorine.

3) Uranium compound mixed with liquid for purification.

Accident occurs when too much uranium is added in step 3.

4) Uranium oxide enriched with plutonium.

5) Purified uranium extracted.

The finished fuel rods typically contain 3 or 4 percent fissionable uranium, compared to more than 90 percent in weapons-grade uranium.

SOURCES: Daily Yomiuri, Mainichi Shinbun, wire reports