For at least the past two years, the uranium processing plant that was the site of Japan's worst nuclear accident had been using an illegal procedure to handle the dangerous material because it was faster, an official of the company operating the plant said today.

Workers were following a company manual when they poured the uranium mixture from a bucket into a settling tank on Thursday, the official said. But this time the head of the three-man crew instructed his colleagues to use a 35-pound capacity tank instead of a smaller one.

As a result, the men put enough uranium together to set off a chain reaction that could not be stopped until 18 hours later.

If they had used the government-approved process, the material would have passed through a measuring cylinder that would have limited the amount to well below the danger level.

"From at least two years ago, we had an internal manual which we did not present to the government and which called for using buckets," said a JCO Co. official in Tokaimura, who asked not to be quoted by name. "We knew the practice was illegal but it's faster."

The accident forced the evacuation of about 150 residents who lived near the plant and kept more than 300,000 others indoors for more than 36 hours. The government allowed the evacuees to return to their homes Saturday evening when monitors showed a drop in the lingering radioactivity at the site, 75 miles northeast of Tokyo.

The three workers are in a hospital, two of them listed in critical condition, and approximately 46 others, including plant employees and residents who live near the factory, have been treated for radiation exposure.

The workers were involved in processing 126 pounds of uranium for use in an experimental breeder reactor program. The process was used occasionally; two of the workers were inexperienced, but crew leader Yutaka Yokokawa, 54, had done this work before, according to Yutaka Tatsuta, another company official.

From his hospital bed, the supervisor "admitted that a process had been skipped," Tatsuta said.

Two workers apparently skipped the procedure in which they were to pour a small amount of uranium into the measuring cylinder, and instead used a bucket to dump seven times as much uranium into a larger processing tank.

Company executives are mulling over what to do with the contaminated building. Although officials said the danger has passed for most of the town's residents, they said two radioactivity monitors on the plant's perimeter showed higher than normal readings today.

But Tatsuta said later that the company had erected a barrier of bags filled with a mixture containing aluminum around the building, and the two high readings had dropped to normal.

"We don't know exactly what the conditions at the site of the accident are right now," said Shizuo Hoshiba, an official of the Nuclear Power Safety Bureau. "We don't have any other readings that we think are high."

Work crews wearing shielded clothing entered the factory briefly Friday while workers tried to stop the nuclear reaction. But the radiation levels are high and workers have not returned to the building.

Some experts said it may be years before the radiation within the building drops to a safe level. Government officials said they may consider using robots to try to remove contaminated equipment.

Among the unanswered questions is why the Tokyo-based JCO Co. had no contingency plans to deal with such an accident. In a document given to government regulators in 1983, the company maintained that "critical fission chain reactions could not occur" at the plant, according to the Japanese daily Yomiuri Shimbun.

Such assertions apparently were accepted by the government's Science and Technology Agency, which licensed the plants and required minimal safeguards, all on the assumption that uranium would not be placed together in quantities sufficient for a fission reaction.

Chief cabinet secretary Hiromu Nonaka called the failures that led to the accident "unthinkable," and declared that "we must examine how nuclear facilities are being managed." But critics have been quick to scoff at the government's protestations, noting the string of mishaps at nuclear facilities in recent years. Some of the facilities were run by the government, which has been embarrassed when its attempts to cover up problems were exposed.

Not unexpectedly in Japan, the nuclear incident already has led to much hand-wringing introspection. Pointing to various train wrecks, satellite failures and nuclear mishaps, officials including Prime Minister Keizo Obuchi have fretted over what they see as a laxity in the nation's work ethic.

A nation that has built a reputation for quality in high-tech manufacturing should not have such workmanship errors, Obuchi observed. The head of the Science and Technology Agency warned of the "moral disintegration among engineers" in Japan. Even officials of the nuclear industry said there are morale problems among workers and mismanagement in companies.

Commentators are seeking to reconcile the reputation of the meticulous Japanese worker with the scenario that led to Thursday's accident.

"In Japan, manuals include subjects ranging from how new company workers should talk on the phone, to economizing toilet paper at company offices," observed the Japanese daily Asahi Shimbun. But "in a country obsessed with manuals, the one for uranium processing was ignored."

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