By law, U.S. utilities can't import uranium from South Africa. Yet, South African uranium manages to find its way into U.S. nuclear reactors.

The uranium is converted to its gaseous form in France, England and occasionally even in the Soviet Union before being sold to U.S. electric utilities by European dealers.Under current law, there is nothing illegal about these transactions. The United States and many other nations have imposed economic sanctions on South Africa because of its apartheid system of racial separation, but they continue to allow importation of South African commodities diverted through a third country.

If a group of U.S. congressmen have their way, South African goods no longer will be able to enter the United States this way.

The House Committee on Foreign Affairs Subcommittee on Africa, expected to meet today, is consolidating a bill that would significantly toughen existing anti-apartheid legislation.

The last round of sanctions - covering steel, uranium, coal, textiles and other specific products - cut U.S. imports of South African products about 40 percent, to $1.4 billion.

The new bill, sponsored by Rep. Ronald V. Dellums, D-Calif., and 102 co- signers, will try to outlaw the importation of any South African-sourced commodity except those strategic items - notably chromium and platinum - deemed irreplaceable by the president.

Although details of the bill are not final, the ban would apply to all products originating in South Africa, regardless of their transshipment point, said Robert Brauer, special legislative consul to Rep. Dellums.

Country-of-origin rules, as applied to South African goods, would be changed under the new legislation, Mr. Brauer said. A South African diamond

cut in Bombay, which is defined as Indian in origin under current

interpretation of value-added rules, would be treated as a South African export.

Gold, often minted in Europe, would also be affected.

Special provisions concerning imports of the uranium gas UF-6 are also likely, Mr. Brauer said.

While few industry and government officials doubt the good intentions of the legislation, many are skeptical it can be enforced.

Researchers at the General Accounting Office say there are no hard numbers about how much comes from South Africa through third countries into the United States.

The complexities of international trade - where products are transshipped any number of times and commodities from different nations get mixed together en route - make it nearly impossible to trace a path back to Johannesburg, industry officials said.

European countries with trade sanctions against South Africa see their attempts to limit South African access to their markets easily foiled.

South African coal is diluted with coal from other parts of the world in the huge hoppers of Amsterdam, Rotterdam and Antwerp, and the resulting Dutch Blend is resold on the European market.

Even comprehensive tracking systems, such as the Port Import Export Reporting Service affiliated with The Journal of Commerce, can only trace a product back to its last lading point.

When asked about this difficulty, Mr. Brauer acknowledged that enforcement was a problem under existing anti-apartheid laws. Accordingly, he said, the new legislation will include establishment of a special office in the State Department to track South African exports and enforce stricter sanctions.

But trade experts doubt that anyone, even a full-time staff, can truly track the convolutions of South African exports.

We get questions about tracking South African commodities, said LeRoy Norfolk, chief of chemicals and sundries, in the foreign trade division of the Bureau of the Census. But my answer is that I don't have at my fingertips anything that can tell me the country of origin. The trail just sort of peters out, he said.

Further complicating identification of South Africa-sourced products is the lack of distinguishing characteristics of South African commodities, said Cecelie Counts, political director for the Washington-based foreign policy lobby, TransAfrica.

This creates fears in the minds of U.S. diamond merchants, for example, that all diamonds imports would be threatened by anti-apartheid legislation.

Diamonds are almost impossible to identify according to where they were dug up, said Lloyd Jaffe, chairman of the American Diamond Industry Association in New York and a diamond merchant himself.

If, for some reason, imports of South African diamonds were stopped, there is worry that all diamonds might be stopped, he said.

There was confusion when the Comprehensive Anti-Apartheid Act of 1986, the last move to impose sanctions, was implemented, Mr. Jaffe said.