France is about to begin full-scale testing of a new uranium fuel for light-water nuclear research reactors that unlike present fuels could not be used to produce an atomic bomb.

The test, according to top officials, will start "within the next month" using the Osiris reactor at the French Atomic Energy Commission's Saclay nuclear research center.

If the operational test proves as successful as preliminary experiments indicate, it could signal a major break-through in the nonproliferation efforts of the major nuclear powers.

Unlike the fuel now used in the Osiris reactor, in which the uranium is enriched to 93 percent, the new fuel will have an enrichment of only 7 to 8 percent.

U.S. specialists regard 20 percent as the "break point," beyond which looms the capability of fashioning a nuclear weapon, albeit an extremely crude and unwieldy one.

If the new fuel which the French call "caramel" proves satisfactory in operation of the 70 megawatt Osiris reactor, officials indicate that they intend to provide it, instead of highly enriched uranium, to developing countries operating research reactors.

This step, where carried out, would eliminate the possibility of a country diverting fuel provided for a research reactor to produce a nuclear weapon.

India used fuel diverted from a Canadian research reactor in building the nuclear device that it tested in 1975.

French officials said the United States is also working to develop a uranium fuel of "a maximum 20 percent enrichment" for use in research reactors.

(Fuel enriched with about 19 percent of the fissile isotope U-235 is being tested at the Argonne National Laboratory outside Chicago, U.S. sources said in washington).

(The tests at Argonne are aimed at creating a fuel that packs the equivalent of 70 percent U-235 in uranium but is enriched to less than 20 percent. This way the fuel would produce enough neutrons to be useful in a broad number of research purposes but not enough to trigger a weapons-type chain reaction.)

"If research reactors around the world are fueled with uranium of less than 20 percent enrichment, it will of course diminish the risk of proliferation," a top official in the French nuclear program said.

French officials said that preliminary experiments indicate that research reactors using the carmel fuel can be used to perform most of the experiments currently conducted with reactor ussing highly enriched uranium.

"The difference in performance is not noticeable," one French expert said.

If this proves true when caramel is given a full-scale test in the Osiris, a pool-type reactor which has been in operation here since 1966, developing countries using similar reactors would appear to have little scientific reason to insist on being provided weapons-grade uranium.