The Carter administration is studying a plan to mix uranium with plutonium as a means of minimizing the threat of nuclear weapons spread and keeping in place the fast breeder nuclear plant that would use plutonium as a fuel.

The plan would involve reprocessing spent uranium fuel to recycle it but would not involve extracting pure plutonium from the spent fuel. Leaving reprocessed uranium in with reprocessed plutonium would mean that the mixture could be used again as a nuclear fuel but could not be used to make nuclear weapons, which plutonium by itself could be.

Nuclear critics have long identified the worst hazard from nuclear power as being reprocessed plutonium, which is a by-product of burned-out uranium and which can be used itself as a nuclear fuel but also can be used to make nuclear weapon.

President Carter has implied that he agrees with that assessment, and last week a panel of 21 scientists and economists sponsored by the Ford Foundation recommended that the United States defer the use of plutonium as a nuclear fuel until the 21st century.

If the White House follows this recommendation, it would mean the indefinite postponement of the $2 billion demonstration fast breeder plant the United States plans to build at Clinch River, Tenn. An estimated $400 million has been spent on the plant, which would use plutonium as a fuel and serve as a model for commercial breeders.

Much of the money has been spent on design and engineering of a plant that will be built to burn plutonium. Any plant that would burn a mix of uranium and plutonium would have to be redesigned, meaning delays and more money.

In a report not yet made public, the Energy Research and Development Administration called the breeder "the potentially lowest cost, long-term option" the country has to generate electricity. The report states flatly that commercial breeders burning plutonium fuel could save the country $31 billion in electrical costs.

"The question we're asking now is can you have an economic fast breeder without using separated plutonium," ERDA's acting administrator, Robert W. Fri, said recently in an interview. "We want to find out if we still get economic fuel benefits by keeping the uranium and plutonium together instead of separating the plutonium out by itself."

Fri said he hopes to have an answer to this question before April 20, when Carter will announce his energy policy to the nation. Fri said it is a question that is critical to which direction Carter takes on the future development of atomic power.

The study being conducted by the Carter administration now centers on whether a uranium-plutonium mix can generate cheap power. The trouble with the mix is that it must be "spiked" with another form of uranium enriched to a high degree with an isotope called U-235.

The "spike" would be done with uranium enriched with up to 20 per cent of U-235, an isotope that is used to make weapons but in concentrations of 80 to 90 per cent. Theoretically, a bomb could be fashioned out of uranium enriched with 20 per cent U-235, but it would have to weigh as much as five tons to get it explode.

This means the fuel would cost more to produce and would not generate power as cheaply as plutonium would on its own. What the Carter administration wants to find out is will the spiked mixture of uranium and plutonium still be more economical than uranium by itself.

The Ford Foundation study concluded that the fast breeder is not needed during this century because the United States has enough uranium to satisfy its need for nuclear power. The Carter administration shares the concern about the use of plutonium but does not agree that uranium reserves are plentiful.

For this reason alone, White House energy aides are looking for ways to use plutonium as a fuel without making it available for weapons.