WHEN THE French freighter Mont Louis sank in the English Channel last weekend, it was carrying a cargo of uranium on its way to the Soviet Union. The Soviets were to enrich it under a commercial contract for use as reactor fuel and ship it back to customers in France, Belgium and West Germany. The sinking raised two questions. First, is the uranium dangerous? And why were the French sending it to the Soviets for enrichment?

No, it's not dangerous. When uranium is in its natural state, as it is mined out of the earth, about seven-tenths of 1 percent of it is U-235. That is the isotope that can be made to fission to generate heat -- or to explode. Most of the uranium in this cargo was at the natural level or slightly below. Some of it was enriched slightly to nine-tenths of one percent U-235. The Soviets were to have brought it up to 4 percent, the customary level for civilian reactor fuel. Weapons require enrichment closer to 90 percent.

Why go to the Soviets? Up to the early 1970s, the United States enriched just about all of the fuel for all reactors worldwide except the Soviets'. The Western Europeans were uneasy about total dependence on the Americans for a crucial supply, and they began to turn to the Soviets as a second source. The cargo of the Mont Louis was going to Riga under a contract signed in 1973, just before the oil crisis began.

While the oil crisis made the Europeans more apprehensive than ever about security of supply, access to American enrichment over the following years seemed to become less dependable. In the Nixon-Ford period, the United States talked about turning the enrichment business over to private enterprise and raising the charges. In the Carter administration, it imposed unilateral conditions on the fuel to try to prevent diversion to weapons. The Europeans considered the conditions ineffectual and demeaning. They responded by developing their own enrichment capacity and maintaining their Soviet contracts as a further alternative. To the Europeans, security of supply means having a choice of sources.

There's a lesson here for the United States in its struggle to prevent the proliferation of nuclear weapons. As more countries around the world rely on reactors for electricity, it's important for Americans not to pursue policies that encourage them to build more and more enrichment plants. Enrichment, remember, is the link between reactor fuel that won't explode and bombs that will. Earlier this year Secretary of Energy Donald Hodel promised other countries cheap and reliable enrichment here in the United States. That's a notable incentive to nonproliferation.