WHILE THE dramatic reduction in tensions between the United States and the Soviet Union brought much of a peace dividendin cash to the budget, other kinds of peace dividends are beginning to be visible. The secretary of energy, James D. Watkins, has now announced that the extraction plant at Hanford, Wash., will not be restarted for the production of plutonium for nuclear weapons. The United States has a lot of plutonium on hand, more than present circumstances require.

The diminished nuclear rivalry with the Soviet Union comes at a fortunate time. For decades the American weapons plants had operated behind a heavy shield of secrecy. But after the Soviet reactor disaster at Chernobyl in 1986, the Energy Department commissioned a series of unprecedentedly thorough and public assessments of its facilities. These reviews showed that its weapons plants, built a generation ago, had suffered from hard use during their long lives and had fallen far behind current standards of nuclear safety.

The plutonium extraction plant at Hanford is typical. It was first put into service in 1956, was mothballed in the early 1970s, then was started again in 1983 for the Reagan administration's build-up of armaments. When it was shut down two years ago, its safety and environmental defects were well known.

Hanford is not an isolated example. The department is anxious to get the reactors at its Savannah River Plant, in South Carolina, back into operation to produce tritium, another ingredient of nuclear explosives and, unlike plutonium, one that decays over time. The three Savannah River reactors were shut down for reasons of safety, but renovations are under way and the department is scheduled to restart one of them in December. Congress will need to take a close look at conditions there.

Even harder decisions lie ahead at Rocky Flats, Colo., where plutonium and other materials are fabricated into weapons components. It's another site where obsolescence and environmental pollution are severe, the dangers in this case being compounded by the plant's proximity to a large city, Denver.

Patching and modifying old equipment is, beyond a certain point, dangerous. In most instances entirely new installations are going to have to be built from the ground up in conformity with the standards that four decades' experience have produced. It's going to be a huge job, but the relaxation of the arms race gives the United States more time than it thought it had, as recently as a year ago, to consider how best to proceed. That's not a trivial kind of a peace dividend.