The Nuclear Regulatory Commission yesterday shut down a plant in Erwin, Tenn., that makes nuclear fuel elements for atomic submarines after discovering that enough weapons-grade uranium to make at least one atomic bomb is missing from the factory.

"Right now, we suspect no diversion [of the uranium] but we're not ruling it out," an NRC spokesman said. "We have alerted the Federal Bureau of Investigation and its regional office is already on the job."

The plant at Erwin employs 600 people and is operated by the Nuclear Fuel Services Corp., which fabricates uranium fuel elements for U.S. nuclear-powered submarines and surface vessels. The fuel is enriched with as much as 97 percent of the isotope known as U-235, the same one used to make nuclear weapons.

The NRC refused to say how much of the enriched uranium is missing from Erwin, except to say that it exceeds 20 pounds, the approximate amount of this type of uranium that is necessary to make a single atomic weapon.

The absence of the uranium was discovered after an inventory was taken at the plant Aug. 14. Results of the inventory were reported Monday to the NRC, which immediately called the plant's managers and ordered them to shut down.

"We expect the plant to be closed for at least 45 days," an NRC spokesman said. "An exercise like this takes at least that long to look into the plant's accounting procedures alone."

Sources at the NRC said that the agency and the Navy have not been happy with the accounting methods used at the Erwin plant, which was closed briefly once before, in 1976, when an inventory showed a large amount of uranium missing. NRC regulations mandate that inventories of weapons-grade uranium and plutonium be taken every two months at factories handling them.

In the 1976 case, the NRC concluded that faulty accounting procedures were the main reasons the uranium had disappeared. At the time, the NRC ordered accounting procedures tightened up at Erwin to prevent a recurrence.

"The Erlin plant has always had a serious accounting problem," one NRC source said. "They handle uranium in gas, liquid and solid form, which makes it very hard to get a measure of how much total uranium you've handled at the end of each fabrication job."

Besides the possibility of theft, an NRC source said the FBI also will investigate the possibility that the missing uranium may have been mischievously hidden or disposed of by workers at Erwin before or after a strike slowed down production for four weeks during July and August.

"A physical security force was kept on duty through the whole strike and there was never any indication of any gaps in the security system." an NRC, source said. "But nevertheless, we will look into the possibility that the security system was compromised during the strike."

Still another area for investigation, the NRC said, will be the production procedures used by management personnel who filled in on production lines during the strike. It is possible, the NRC source said, that management personnel who filled in on production lines during the strike. It is possible, the NRC source said, that management personnel was inexperienced enough on the production lines to lose some uranium inadvertently.