The FBI is investigating attempts by European commodity traders to sell in the United States 239 pounds of uranium - possibly from the black market - said to be enriched enough to be turned into nuclear weapons.

From documents on file at the Nuclear Regulatory Commission and interviews with people in the United States who were approached as possible buyers, The Washington Post has reconstructed this sequence:

Last summer, Washignton commodities dealer Herb Waters was contacted by a man identifying himself as Max Aculi, an exiled Albanian, and by an unidentified Swiss businessman living in Geneva. They said they had uranium to sell. Their asking price was $2,650,000 a kilogram, which would have brought them $288 million if they had 239 pounds of enriched uranium.

The two men offered the uranium to anyone who wanted to buy it, according to the documents on file with the NRC, but the offer apparently was only passed on to Wstinghouse Electric Crop. c because it is the largest supplier of uranium in the United States and was short of uranium last summer.

The offer was turned down by Westinghouse and immediately reported to the FBI and the CIA.=t"We're looking into it under authority of the Atomic Energy Act," an FBI spokesman said.

"The FBI has requested through our legal attache in Berne for the Swiss authorities to assist us in this investigation."

Under the Atomic Energy Act of 1954, it is illegal to "transfer, receive, deliver, acquire, own, possess or import" any special nuclear material such as weapons-grade uranium. The Atomic Energy Act also orders the FBI to investigate all "alleged or suspected criminal violations" of this act.

The attempt is believed to be the first serious instance of an effort to sell black market uranium, if there is black market uranium. Westinghouse said it was not given proof the uranium existed and was not told its source.

Nonetheless, the fact that uranium described as high-enriched uranium was even offered for sale has triggered the FBI investigation to find out where the uranium might have come from . Sources close to the investigation said that one fear was that the uranium had been smuggled out of the Soviet Union.

If the United States could test a sample of the alleged uranium offered for sale, it could identify what country and what factory produced the enriched material. Evey country has its own way of enriching uranium which leaves telltale chemical fingerprints.

There are five countries in the world capable of turning uranium ore into uranium rich enough in the isotope called U-235 to make nuclear weapons: the United States, the Soviet Union, France, Great Britain and China. India has also exploded an atomic weapons but did it with plutonium and not enriched uranium.

The documents at the NRC show that the attempts to peddle the uranium began last August, when Aculi contacted Waters by telephone from Geneva and asked him if he could "find a market" for uranium.

"Aculi is a man I know from the import-export business of farm commodities," waters said in an interview. "I told him I knew nothing about uranium and would have to know what kind of uranium he had and what he was proposing."

Next came a teletyped message from a Swiss businessman Waters would not identify. The cable was garbled but offered for sale 10 bars of 10.9 kilograms each of enriched U-235 that was described as "99.3 percent pure." Price-would be $2,650,000 per kilo. A kilogram is 2.2 pounds.

Waters said he called his lawyer, Lewis Rivlin, a partner in the firm of Peabody, Rivlin, Lambert & Meyers. One of Rivlin's clients is Westinghouse Vice President A. L. Bethel in Pittsburgh and told himof the offer.

Bethel said he told Rivlin to express what he called "conditional interest" and to find out whether the 99.3 percent purity meant chemical purity of the uranium or U-235 concentration.

Back came a cable from Geneva that the purity of the uranium referred to the U-235 content, suggesting it was weapons-grade uranium. The cable included a report from an unidentified Swiss laboratory attesting to the enriched uranium.

The same cable said that seven more bars might be available for sale in addition to the 10 bars being offered. The cable suggested a meetin in Geneva to discuss price but added that the expected payment must be in cash.

Rivlin said another cable came to him stating that ownership of the uranium was "legal" and that notarized testimony attesting to its legality could be produced.

At this time, Bethel told Rivlin to re jects the deal and to notify the FBI and the CIA. All copies of the cables sent to Rivlin and Waters were turned over to the FBI and the CIA. Summaries of what took place were sent to the Nuclear Regulatory Commission and the Department of Energy.

Even if the highly enriched uranium existed and were diluted with natural uranium to make uranium for electricity it would have a market value of no more than $87,000 a kilogram and not the $2,650,000 asked for it.

"The only interpretation I can make of their asking price," Bethel said in an interview, "is that they had no idea of what they were dealing with . . . if their offer was in fact bona fide."

Bethel said he doubts there was any weapons-grade uranium for sale but suspects an offer might have been made to test what a company like Westinghouse might be willing to pay for enriched uranium. The FBI and CIA declined comment.