Nine daring forays by repair workers into a uranium processing plant Friday ended Japan's worst nuclear accident, but the search for blame is likely to deepen public suspicion about the nation's nuclear industry.

Japanese officials declared the danger over as monitors showed radiation levels around the plant, 75 miles northeast of Tokyo, had returned to normal. At midafternoon Friday, officials said more than 300,000 people who had been urged to stay indoors could safely move about.

The spread of radiation was stanched early Friday morning after teams of plant workers finally succeeded in breaking a pipe outside the plant after repeated attempts. That served to drain water that had accelerated the nuclear fission of 35 pounds of uranium that workers had mistakenly poured together.

The teams, which could work for only a few minutes at a time to avoid overexposure, had to smash a drainpipe after their efforts to open a water valve failed to stop the nuclear reaction occurring inside. Five hours later, three other teams entered the building with hoses to shoot boric acid into the tanks to make sure the chain reaction was over.

Two workers who were handling the material when it erupted into a nuclear fission chain reaction Thursday remained in critical condition, and the condition of a third was listed as serious. Approximately 46 others, including firemen and civilians who live near the factory, were exposed, but none remained hospitalized.

About 150 residents who live next to the plant were advised to stay away Friday night while inspectors continued measurements of the radiation levels inside the people's homes.

Officials ran Geiger counters over nearly 2,000 residents at area community halls, and said they found no one with increased levels of irradiation. They also said tests of crops in the area showed they are safe for consumption.

Although radioactivity inside the processing plant in the town of Tokaimura had soared to lethal levels, the surrounding residential area received radiation doses only slightly above natural levels, according to officials.

"Based on studies by experts, we have concluded that there is no problem with residents going about their normal life," said government spokesman Hiromu Nonaka.

Even as the fears of widespread contamination lessened, pressing questions arose about how and why why the accident happened. Makoto Morita, a top official of Tokyo-based JCO Co., which has operated the plant since 1973, confirmed Friday that three employees were being questioned by local police on suspicion of negligence.

Morita acknowledged that the company bypassed procedures, and the company has apologized for the incident.

The chain reaction began when workers in the plant apparently skipped a step in the processing of uranium for nuclear fuel. Normally, the liquefied uranium would pass automatically into a small cylinder before it is processed further, ensuring that only a small amount -- about five pounds -- is used.

Instead, according to Morita, the workmen used a metal bucket to mix by hand about 35 pounds of uranium with a nitric acid solution and then poured it directly into a settling tank. When enough uranium is put together under the right conditions, it can reach critical mass and begin nuclear fission. That apparently happened, and the workers reported an eerie blue glow indicative of fission.

Japanese television reported that the plant was producing fuel for the Joyo fast breeder reactor instead of for standard nuclear power plants, so the process was different. The Japan Broadcasting Corp. said that two of the three workers involved in the accident had no experience in producing this type of fuel, which the plant last made three years ago.

Jinzaburo Takagi, an analyst for the Nuclear Power Information Research Institute, suggested Friday the procedures were probably commonly used, but the workers were accustomed to using a lighter grade of uranium that would not begin fission so easily.

"These workers had no idea of how dangerous this material is," Takagi said.

The government, too, has been criticized for its failure to monitor more closely uranium processing facilities, which are privately owned but subject to regulation, and for the five-hour delay in notifying Tokaimura-area residents of the problem.

Nonaka acknowledged the government was slow to respond. "As a modern nation, it's shameful that this kind of accident happened," he said.

Despite the limited reach of the contamination, the incident was officially classified as Japan's worst nuclear accident, and the first to involve an uncontrolled reaction of nuclear materials.

Prime Minister Keizo Obuchi acknowledged after a telephone call from President Clinton that the incident is bound to increase public skepticism of nuclear energy, which the government has strongly endorsed in an effort to reduce dependence on imported oil.

But the nuclear industry here, which operates 51 plants and supplies about one-third of Japan's electricity, has been plagued by an embarrassing succession of mishaps in recent years.

Indeed, what had been Japan's worst nuclear accident until Thursday occurred in the same town two years ago. A fire and explosion at another nuclear processing plant in Tokaimura exposed 37 employees to radiation.

"There's a general failure of the safety culture in Japan on nuclear issues," insisted Shaun Burnie, nuclear research director for Greenpeace Japan. "There are probably hundreds of these facilities handling nuclear materials throughout Japan, and in terms of inspections, there's practically nil."

Unlike nuclear power plants, which are built with thick concrete to contain any accidental release of radioactivity, factories such as the one at Tokaimura are not expected to face fission, and they operate with far fewer safeguards.

But even defenders of the industry say they are appalled at the apparently casual way the uranium was handled at Tokaimura.

"It was quite unbelievable," said Nobuo Ishizuka, deputy secretary general of an industry group, the Japan Atomic Industrial Forum.

Ishizuka acknowledged that "it's natural" there will be a slip in public confidence in nuclear energy after the accident. And he said the industry has been hurt by lax management and sloppy work procedures. "Even my wife and daughter told me last night they didn't think it was safe."

But the Japanese public may be getting jaded about professed surprise over laxity in the nuclear field. After a 1997 leak of radioactive waste in Tokaimura, prime minister Ryutaro Hashimoto called the errors "unbelievable" and ordered increased government diligence.

The latest accident came just as the nuclear industry is beginning to implement a plan to use mixed-oxide fuel from Europe. Japan has commissioned Britain and France to produce the so-called MOX fuel, made by blending uranium and plutonium recycled from spent nuclear fuel.

As the first ships delivering the MOX fuel docked this week, police used water cannons to keep away Greenpeace protest boats.

Special correspondents Shigehiko Togo and Akiko Kashiwagi contributed to this report.