THE cold war may be over, but the business of producing new nuclear weapons goes on.United States warhead research and production activities, run by the Department of Energy (DOE), appear likely to receive around $7 billion in 1992 under spending bills recently approved by key House and Senate panels. That's a 1 or 2 percent real decrease from 1991, depending on the final figure. Environmental cleanup at troubled DOE production plants will likely receive a further $4 billion-plus, bringing total US atomic energy defense spending to slightly over $11 billion. The changing world security situation has already put pressure on the nuclear warhead budget, admit Energy Department officials. In addition, environmental compliance, weapons safety improvements, and rebuilding of old facilities have become top priorities and could well eat up more nuclear budget dollars in years ahead. In recent congressional testimony, Richard Claytor, assistant secretary of energy for defense programs, said: "We are facing increasing constraints on the core mission, which is weapons research, development, and testing. We want to preserve that." Mr. Claytor's comments were made at a closed hearing of a House Appropriations panel. The transcript of this annual budget hearing, declassified and published in a 500-plus page book, is among the most detailed accounts of US nuclear weapons activities available. It is avidly scoured by experts outside government for hints about what is happening in this mysterious, closed world. Throughout this year's transcript, nuclear weapons officials insist that the US atomic arsenal will need updating in coming years even if the number of targets has greatly decreased along with the chances for nuclear war. But due to a Joint Chiefs of Staff (JCS) scrub of requirements, the Energy Department is planning for "a significant reduction in the size of the stockpile over the next decade," said Claytor. Exact figures for the planned reduction are deleted in the transcript, though supporting documents reveal that weapons-retirement plans have been accelerated for 1991 and 1992. Last year's transcript reported that the number of US nuclear warheads had shrunk 10 percent since 1985. The START long-range arms treaty now in the final stages of superpower negotiation would mandate cuts of US strategic nuclear weapons of about one-third. Among other items of note contained in the transcript: * An underground nuclear test of the X-ray laser concept is scheduled for 1992. The X-ray laser, a weapon that would channel the fury of a nuclear explosion in space into directed beams of energy capable of shooting down rising rocket boosters, was part of President Reagan's original vision for the Strategic Defense Initiative. SDI has since been reoriented toward less-exotic weapons, and DOE says its work with directed energy weapons now focuses on making sure the Soviets don't beat America to the punch . * A new warhead intended to burrow into the earth and destroy buried targets such as command bunkers has entered the final stage of development. * The secretaries of Defense and Energy have ordered nuclear weapons officials to review whether modern safety features need to be introduced more quickly into those weapon types which do not now have them. Among things under consideration: more use of insensitive high explosives, fire-resistant plutonium pits, upgraded protection against unauthorized use, and safer warhead electronics. Last December, a panel of experts reported to the House Armed Services Committee that while nuclear weapons were generally safe against accidental detonation, they could be safer. Weapons produced before 1970 don't have the Electrical Nuclear Detonation Safety system, which reduces the chances of accidental explosion due to electronics failure to one in a million. "We are confident that we can achieve modern safety in the stockpile by the year 2000," says a written Energy Department response to a congressional inquiry.