The Carter administration is holding up 28 export licenses involving the shipment of almost 2,900 pounds of uranium fuel to research institutions and power plants around the world while it reassesses the threat of nuclear weapons spread.

All 28 export licenses involve the shipment of what nuclear experts call "highly enriched uranium," which is uranium fuel that is 93 per cent saturated with the isotope called U-235. This isotope is used in research reactors that make radioactive medicines and hybrid wheat and corn seeds but could also be used to make atomic weapons.

The Nuclear Regulatory Commission, which has licensing authority over the shipments of uranium fuel, declined yesterday to comment on the delays in licensing approval for the 28 shipments. Sources outside the NRC said, however, that the 28 licenses were being held up by the White House and the State Department pending studies of the weapons proliferation threat.

The shipments being held up cover 2,887 pounds of highly enriched uranium fuel to 13 countries, including most of the countries of Western Europe, Japan, Bolivia, Brazil and Canada.

One license application calls for the shipment of 220 pounds of highly enriched uranium to a firm called Nukem in West Germany, which will fabricate the uranium into fuel rods and ship them to General Electric Co. in Pleasanton, Calif.

The GE research reactor awaiting the fuel shipment uses the uranium to create radioactive isotopes like cobalt-60 to treat cancer patients. A GE spokesman said the Pleasanton facility is one of the largest of its kind in the world and will have burned up the uranium fule in its reactor right now in less than a year.

"It takes time to get the fuel to Germany, get it fabricated and get it back for tests to Pleasanton," the GE spokesman said. "We can't wait forever for the export license."

Of the 28 shipments awaiting an export license, the one held up the longest is for 53 pounds of highly enriched uranium for a research reactor in South Africa. That license has been pending for two years.

More than half the licenses have been abeyance for about one year, which is as long as Congress has been seriously debating the issue of nuclear exports. Jimmy Carter took up the debate when he was nominated by the Democratic Party as its presidential candidate, and he promised a tough stance on nuclear exports if he was elected.

As President Carter has called for a complete re-examination of U.S. nuclear export policy. Fueling the re-examination is the growing fear that terrorists might try to steal highly enriched uranium or plutonium to make a weapon or that countries would divert nuclear materials from peaceful purposes to weapons manufacture.

Uranium saturated 93 per cent with the fissionabel isotope called U-235 is a strong source of neutrons, which can be used to promote a quick chain reaction as in a weapon or which can be used to permanently irradiate another metal like cobalt to make it useful as a treatment for cancer.

There are 261 research reactors around the world using the highly enriched uranium fuel, most of them supplied by the United States. The holdup of the 28 export licenses for this fuel is the first time the United States has delayed uranium fuel shipments to research reactors in the 23 years it has supplied them.