THE work proceeded quietly, largely out of public view, but it ground on relentlessly: For more than 40 years, America's nuclear-weapons industry churned out tens of thousands of warheads, bombs, artillery shells, and the gases and devices needed to set off their lethal chain reactions. The industry has consolidated from its peak in the '50s and '60s, when the macabre but seemingly necessary work went on at about 100 research, manufacturing, and test sites in 30 states; but as recently as six years ago -with the cold war still being waged - few Americans foresaw a day when the United States actually could start to dismantle its nuclear infrastructure. Yet that day has arrived. The announcement this week by Energy Secretary James D. Watkins that the Bush administration will accelerate plans to shrink the nation's nuclear-weapons program is dramatic further proof that the cold war is over and that the US has entered a new phase in its security thinking. Mr. Watkins said that by 1996 the nuclear-weapons infrastructure would be cut to just four production plants in Kansas City, South Carolina, Tennessee, and Texas, and a test site in Nevada. Production will be phased out at major plants in Colorado, Florida, and Ohio. Equally encouraging is the evidence of DOE's heightened commitment to environmental cleanup at nuclear production sites, especially at a number of facilities that were closed in the late '80s after extensive contamination came to light. Within a few years, fully half of the nuclear-weapons budget and personnel will be devoted to cleanup. America's nuclear agenda remains full. With thousands of nuclear weapons still in place in Russia and other republics of the former Soviet Union, as well as on Soviet submarines, the US is obligated to maintain a substantial - though greatly lowered - nuclear capability. Also, the full scope of the cleanup at leaky nuclear-weapons plants still isn't known. And the worldwide danger of weapons proliferation is greater today than ever before in the nuclear age. Nonetheless, it's heartening to see evidence that the trajectory of mankind's experience with these terrible weapons needn't forever be upward.