The United States and other major nations have been trying to win French and West German agreement on a new and tougher system of inspection for customers who buy nuclear plants and know-how, it was learned today.

The move, which is supported by the Soviet Union and Britain, is another step aimed at preventing nuclear customers from building bombs.

Paris and Bonn, however, have resisted the urgings of the others, according to authoritative sources here. The French are said to regard closer inspection as an intringement of national sovereignty. The West Germans are reportedly to fear that tougher rules might discourage potential sales.

The issue was discussed again here last week at the meeting of the Nuclear Supplies Group, made up of the 15 nations that sell nuclear materials and technology abroad. It is expected to come up at future gatherings until it is resolved.

The London meeting, however ,as reported last week, did agree for the first time to publish a code governing foreign sales. It embodies the existing inspection producers of the International Atomic Energy Agency, a U.N. body in Vienna.

Reports that the group almost broke up were dismissed by knowledeable sources here as "nonsense." The disagreement over tighter inspection is a long-standing one, and it was shelved to produce the guidelines on which agreement could be reached.

At issue, according to experts, is what is called "full fuel cycle safeguards." The United States, Britain and the Soviet Union want them; France and West Germany do not.

The proposed safeguards would enable the atomic energy agency's inspectors to examine an imported item - for example, a reactor - and the plant it goes into as well as any other nuclear facilities that the country has or plans to build. This would include all power plants plus all fuel rod fabrication plants and all nuclear fuel reprocessing plants.

The current system is far from foolproof, and the nations seeking tougher rules aim at frustrating a country bent on cheating.

The French, who have never signed the treaty that bans the spread of nuclear weapons, are said to fear that tougher inspection would compel Paris to disclose more than it wants the world to know. It would be difficult for a nation exporting nuclear materials to deny inspectors the visiting rights imposed on customers.

The West German support for France, it is thought, rests on purely commerical grounds. The slacker the inspection the greater the number of customers.

The argument, however, did not detract from the important nature of last week's agreement, it is said here. It is disagreement of long standing.

Although the guidelines governing nuclear sales will not be published for several months, the principal elements are known. The United States and the other 14 exporters agree that every nation buying nuclear materials or technology must: accept existing inspection procedures for nuclear operations pledge that it will not attempt to set off any explosion, even for peaceful purposes; promise it will not cheat by copying plants it has bought; pledge that any re-export or sale to a third nation will be governed by these same rules.