The United States has proposed using a Pacific island as storage facility for spent nuclear fuel, intensifying a debate between Tokyo and Washington over nuclear power generation.

The United States first proposed the idea for a regional storage base last month during negotiations here and asked Japan to share the costs of studying its feasibility, according to Japanese officials.

Japan has reacted warily to the idea because it could contradict this country's long-term policy of reprocessing spent nuclear fuel for use in its commercial power program. The United States is opposed to that reprocessing.

Japan has not rejected the proposal outright and is expected to give its answer this spring. But officials said today that if it is not compatible with the Japanese hopes of large-scale reprocessing it would be rejected.

If the idea gains favor, it would become the first American-sponsored regional storage facility. One purpose would be to provide a dumping ground for the Asian nations that are entering the age of nuclear power generation.

According to Japanese sources, State Department negotiators last mont suggested several islands that might be used and they proposed studies to determine which one might meet technical, economic and political tests.

A leading newspaper, Nihon Keizai, identified the American-owned Wake Island as the probable choice, but neither U.S. nor Japanese sources would confirm that and said several islands are still being considered. They said that although studies could begin promptly it would take at least five years to prepare the island for storage.

It was understood that the island would be designed as a storage facility for fuel used by Japan, South Korea and Taiwan.

A number of American islands are being studied as the possible site for spent fuel storage, some inhabited and some uninhabited, administration sources said in Washington. No names of islands were mentioned by U.S. officials, although Wake Island was ruled out.

(The reason a Pacific island has been mentioned as a possible longterm storage site for spent fuel is that geologic storage of nuclear fuel is not suitable in Japan, Korea or Taiwan where earthquakes often strike, typhoons often hit and volcanoes occasionally erupt.

Only Japan is being asked to take part in its planning. Japan already has become the world's second-largest nuclear power-generating country, behind the United States, and has vast plans for shifting more of its capacity from conventional fuels.

The storage facility fits in with the U.S. policy of inducing other nations to store their spent uranium fuel instead of reprocessing it to extract plutonium that can be used in nuclear reactors. Plutonium also can be used in nuclear weapons.

That policy conflicts with Japan's ambitous plan to use reprocessed fuel in its power plants. It currently has contracts with France and England for reprocessing its spent fuel but hopes to build its own major reprocessing facility.

The United States repeatedly has tried to discourage Japan from reprocessing and the issue has been a touchy one between the two countries for years. Because it buys all of its enriched uranium from the United States, Japan must seek U.s. permission for all reprocessing.

The latest source of irritation is the U.S. refusal so far to grant permission for two Japanese power companies to send spent fuel to Europe for reprocessing. The deadline for the shipments is early April and unless Japan receives permission it is liable for stiff financial penalties.

[U.S. officials in Washington denied they were withholding any permission for Japanese reprocessing shipments to Britain and France, at least three of which have been requested. One U.S. official said all three requests had been received by the State Department and are being "staffed" to see if approval can be granted.]

Japanese officials said today that they are seriously considering cooperating in the venture even though they may never use it.

Kunisada Jume, science division director in the Foreign Ministry's U.N. bureau, said there would obviously be a need for storage in the future if the reprocessing of spent fuel is totally prohibited by the United States.

Furthermore, he said, Japan believes that for some time countries that want to reprocess will not have the facilities for reprocessing all of the spent fuel turned out by nuclear power plants. In that context, he said, the American proposal to enlarge the world's storage facilities is sensible.

But Kume also said that his government questions whether the storage idea might not be inimical to Japan's long-term intention to reprocess as much fuel as it can for domestic use.

"It is a long-standing policy for Japan to have its own reprocessing capability," he said. "If this [island storage base] is not compatible with that policy, we cannot participate in it. My personal view is that it is worth studying further.

"It is possible that spent fuels from countries other than Japn could go there and if so it is in Japan's interest to cooperate." he added. "Add if Japan's reprocessing capability is not sufficient to accommodate all of its own spent fuel, the excess will have to be stored some place."

At this point, the United States is not asking Japan to commit itself to sto4re spent fuel on the island if the facility is built. It is only asking for Japan's help in conducting studies.

So far, Japan has reprocessed only a small amount of used uranium at a pilot plant under a special agreement with the United States. But the plant has been closed since August because of a radiation leak. Japan intends to build another major reprocessing plant as soon as legislation clears the parliament.