With its first experimental fast-breeder reactor successfully kindled this week, the Japanese government is opening a diplomatic offensive against President Carter's proposed curb on the nuclear fuel, plutonium, which can be used to make weapons as well as for peaceful purposes.

Concern over Carter's proposed suspension of plutonium-fueled nuclear energy is especially strong in Japan. This country's future energy plan is predicated on extensive use of fast-breeder plan is predicated on extensive use of fast-breeder reactors. Despite American pressure to shelve them indefinitely, Japanese officials are determined to go ahead.

Prime Minister Takeo Fukuda will instist on Japan's right to nuclear reprocession in private meetings with President Carter during the May 7-8 London summit of advanced industrialized nations, plutonium. The two leaders agreed on the need for a compromise solution when Fukuda visited Washington last month, but an initial round of working level talks was held later with little progress.

We are not in a position to talk about technical compromises. We need a political decision," said a senio Japanese negotiator. "We are going to take all possible means to appeal to the U.S. government."

The joyo (meaning everlasting sun) experimental ractor is to provide the foundation for Japan's future generation of fast-breeded rectors. It employs British-supplied uranium and plutonium fuel and became operational Sunday.

Meanwhile, Japan has discussed its reservations over the U.S. nuclear policy with Britain, France and West Germany and will bring them up at the conference of 14 nuclear technology exporting nations which starts in Londaon today. Foreign Ministry officials have denied newspaper accounts that Japan was fashioning a "joint front" to seek modification of Carter's plutonium policy, which is designed to halt the spread of nuclear weapons.

Nonetheless, Japan and the West European nations have strong common interests that will tend to unite them. West Germany, like Japan, had no nuclear weapons and is committed to fast-breeder development. Britain and France are negotiating lucrative contracts to reprocess Japan's nuclear waste into plutonium in the 1980s.

American and Japanese diplomats are locked in a difficult imnpasse over the issue because the U.S. supplies all the enriched uranium used in Japan's nuclear power stations and under a 1958 agreement can preveant subsequent reprocessing into plutonium.

Japan has completed a $200 million reprocessing plant at Tokai village northeast of Tokyo, but has been unable to obtain U.S. approval for the scheduled start-up in July.

A decision is due soon on a much larger, commercial reprocessing plant to be completed in eight years.

Refusal by the United States to allow operation of the Tokai plant "would be discrimination against Japan compared with West Germany which can reprocess U.S.-supplied fuel without limitation," said a Japanese diplomat.

The dispute carries the potential for serious tension between Japan and the United States. The Japanese believe nuclear reprocessing and fast-breeder reactors are indispensable to the most economic use of costly uranium and their long-term goal of greater self-reliance in energy production. An American commitment to guarantee supply of enriched uranium or possibly even allow Japan to have the spent waste reprocessed by France or Britain does not satisfy the strong Japanese desire to reduce its dependence on others.

Newspapers here criticize Carter's plan as "self-righteous" and "self-centered." The Amerian argument that reprocessing is uneconomic, unnecessary and a threat to world peace has made almost no headway in Japan.

The Japanese emphasize the difference between Americans substanial energy resources and their own almost total reliance on imports. Carter contradicts himself, they argue, by warning of a world energy crisis while restraining Japan's attempts to find its own solution through nuclear energy.