Japan is preparing to seek removal of the major U.S. restrictions on its nuclear fuel reprocessing plant, thus providing an early test of the Reagan administration's nuclear nonproliferation policy.

Officials here say they will probably ask the new administration to lift limits on the amont of fuel that can be reprocessed in an experimental plant and to do away with time limits imposed at the insistence of the Carter administration.

The issue was one of the most controversial between Japan and the United States throughout the Carter years. Initially, Jimmy Carter had strongly opposed Japan's experimental plant at Tokai-Mura because it had the potential for producing plutonium, from which nuclear explosives are made.

Japanese officials say they do not know who the Reagan administration will react to the proposals, but Japanese press reports indicate that some expect it to take a less rigid posture than did Carter's, which had little support among Republican leaders in the United States. So far the Reagan administration has made no definitive statement of its policy on nonproliferation.

The enriched uranium used in Japan's nuclear power plants is purchased from the United States, which reserves the right to set restrictions on any reprocessing of the spent fuel.

Although the Tokai-Mura pilot plant is small in output, the question of U.S.-imposed restrictions has great importance for future Japanese nuclear development. The government last year opened the door for private comercial reprocessing and one company, Japan Atomic Fuel Service, is planning to open a full-scale operation about 1990 to process about 1,200 tons a year.

If agreement is not reached by this summer, the plant at Tokai-Mura theoreticaly could be shut down. It began full-scale operations in January and is legally able to operate only until June 1, when an agreement with the United States is due to expire.

Japan insisted that the pilot plant was essential for its long-range nuclear energy development. In September 1977 the two countries reached agreement on a two-year operating plan that was extended until June 1 of this year.

Foreign Ministry officials said they expect to open "informal contacts" on the issue soon with the Reagan administration but do not expect formal negotiations until spring because it is not yet clear who will be the responsible officials in Washington.

Atsuhiko Yatabe, director general for scientific and technological affairs at the Foreign Ministry, said no specific plan for the negotiations has been prepared.

But he noted that the present ceiling on spent nuclear fuel reprocessing will be reached by late spring. "In veiw of this, there is a general feeling in Japan that the time has come to talk with the Americans on the reprocessing issue and to reach a basic agreement" on what can be done after June 1, he said.

This would mean asking for an open-ended agreement that would end the practice of setting fixed periods of time for Tokai-Mura's plant to operate, he said, and it would also mean removng ceilings on the amount of fuel that can be reporcessed there.

From September 1977 until the end of 1980, the Tokai-Mura plant reprocessed 79 tons of spent fuel. In theory, it could reprocess about 140 tons a year and there are reports that Japan would want to reprocess about 100 tons a year.

Yatabe said there have been no clear indications from Reagan administration officials what their policy will be on rules related to reprocessing of fuel by foreign customers. But he noted that "it seems much more positive than the previous one on the question of nuclear power in general."

In an agreement signed last month, the United States permitted Japan to process 50 additional ton before the June 1 deadline and the Tokai-Mura plant theoretically could be shut down after that. it will be closed anyway then for about three months for cleaning and repairs, so the practical deadline will be about Sept. 1.

Japan, the only target of atomic bombings since the dawn of the nuclear age, disclaims any intention of building nuclear weapons, although it insists that it must have more nuclear energy to make it less dependent on imported oil.