Protests from governments in the South Pacific are forcing Japan to postpone plans to dump part of a two-decade accumulation of nuclear waste into an ocean trough 550 miles southeast of Tokyo.

The protests, part of a general antinuclear sentiment in the South Pacific, touch a sensitive nerve in Japan, which with Hiroshima in its past considers itself a special authority on anything nuclear.

But Prime Minister Yasuhiro Nakasone promised publicly last week that Japan would not put any waste into the ocean "unless the countries of the South Pacific agree."

Nakasone leaves later this month for a week-long tour of Australia, New Zealand, Fiji and Papua New Guinea, all of which oppose the plan. He is expected to press them to reverse their positions.

But with Japan again campaigning for an old objective of its diplomacy, fostering of closer ties among the Pacific Basin community, he is unlikely to push them too hard and risk alienating them.

Meanwhile, Japan is also studying creation of a land dump at the northern end of its main island, Honshu. But officials are concerned that the country's own antinuclear activists could block that plan.

The object is permanent disposal of the more than 500,000 drums of "low-level" nuclear waste -- radiation-exposed clothing and water used to wash floors in nuclear plants, for instance -- that are stored around Japan.

Japan, the only country in history to suffer a nuclear attack, has committed itself never to acquire nuclear weapons. But it is pursuing an ambitious and often controversial program to develop nuclear power to reduce dependence on oil imports.

Low-level waste has been accumulating since before 1966, when the first commercial plant opened. Today, 28 plants are in operation, providing about 20 percent of the electricity generated in Japan, and 17 more are under construction or planned.

The waste involved is not the more highly radioactive spent fuel, which Japan currently ships to France and Britain for reprocessing. A separate land dump on Japan's northernmost main island, Hokkaido, is being studied for materials produced by this reprocessing.

In 1976, the Japanese government adopted plans to dump the low-level waste into the ocean. Mixed with cement and encased in metal drums, the materials would be dropped into a four-mile-deep trough 550 miles southeast of Tokyo.

When Japan announced plans for a test dumping, opposition came from the South Pacific Forum, a grouping of Australia, New Zealand and nine other countries and semi-independent territories in the region.

For more than 30 years, nuclear tests have been conducted in the South Pacific, with France continuing underground tests at an atoll in French Polynesia. Antinuclear feelings are particularly strong in New Zealand and the smaller islands.

In 1983, countries discussing the so-called London Dumping Convention, which governs marine pollution, called for a moratorium on ocean disposal pending a safety study due out this year. Japan protested, but agreed to hold off on its plans until the study was completed.

Last summer, the South Pacific Forum reiterated its opposition in a communique that called any disposal "unacceptable."

Japanese officials point out that the site is thousands of miles north of the countries protesting it. "The area selected for sea dumping is much closer to Japan than the image the South Pacific countries seem to have," said a Foreign Ministry official here.

They also cite studies that have concluded that even if all the containers broke open, the radiation inside is so weak it would pose no significant hazard to health.

An Australian diplomat in Tokyo said that the South Pacific countries are unlikely to be swayed by this logic. The prevailing feeling among them, he said, is that "we just don't like the idea of people dumping their garbage in our back yard."

Nakasone's visit comes as the South Pacific Forum is discussing a treaty to declare the region a nuclear-free zone, with no testing, storage or use of nuclear weapons and no dumping of nuclear waste. As currently envisaged, however, it would not affect passage of U.S. warships carrying nuclear weapons or individual countries' rights to receive them at their ports.

The treaty is to be debated at a meeting in the Cook Islands in August.