The Energy Department yesterday abandoned its plan to renew the processing of plutonium for use in nuclear weapons next year and pledged to conduct a two-year study of the environmental hazards at the sole U.S. plutonium processing plant in Hanford, Wash.

Energy Secretary James D. Watkins announced the decision during a visit to Hanford, where local citizens and officials have mounted protests against the plan to reopen the Plutonium Uranium Extraction (Purex) plant after a two-year shutdown for safety and mechanical repairs.

His move to block the plant's reopening was the latest in a series of government actions to sharply curtail operations within the aging nuclear-weapons complex, including a decision last year to cancel construction of a new Idaho facility for purifying some of the plutonium to be produced at Hanford.

Experts said the decision will not directly impinge on the production of new nuclear weapons, however, because the government has access to vast stocks of plutonium that can be recycled from existing weapons.

Watkins left open a government option to operate the Purex plant solely to treat 2,100 metric tons of nuclear wastes generated by Hanford's N-reactor, a weapons facility that has been closed since 1988. But Watkins said no decision on this plan would be made for at least two years and that the department planned to study alternative waste plans as well as any associated environmental problems.

"Such a long, planned shutdown means that Purex will never operate again," predicted Jim Thomas, research director of the Hanford Education Action League, an activist group that opposed reopening the plant. He said the decision "marks the effective end of more than 45 years of plutonium production for nuclear weapons in the United States."

"They've recognized that with the end of the Cold War, the United States doesn't need to produce any new plutonium for nuclear weapons," said Dan Reicher, a senior attorney of the Natural Resources Defense Council, a private environmental group here. Reicher notified the department in January of his group's intention to seek a legal injunction against the plant's reopening.

At the time of its 1988 closure due to safety problems, the 1,000-foot-long Purex plant was regarded as the world's largest nuclear fuel reprocessing facility. It operated from 1956 until 1972 and was restarted in 1983 to provide material for the Reagan administration's expansion of the U.S. nuclear weapons arsenal.