By a vote of 387-36, the House of Representatives passed a spending bill for energy and water programs that contains 7.5 million dollars to study the feasibility of the so-called Robust Nuclear Earth Penetrator, which one Energy Department official insisted "would enhance the nation's ability to hold hard and deeply buried targets at risk."
The Senate approved the measure by voice vote later Tuesday night. It will now go to President George W. Bush for his signature.
US scientists are looking into the possibility of converting into bunker-busters two existing warheads - the B61 and the B83, according to Bush administration officials.
The B61, which has selectable yields ranging from 0.3 kilotonnes to 300 kilotonnes, is a tactical thermonuclear gravity bomb that can be delivered by strategic as well as tactical aircraft -- from B-52 and B-2 bombers to F-16 fighter jets.
The B83, whose yields range from one to two megatonnes, is designed for precision delivery from very low altitudes, most likely by B-2 stealth bombers, military experts said. It has a nose cone capable of withstanding a supersonic-speed collision with concrete or steel and a delayed detonation to allow the aircraft to escape the blast.
The main task now is to find a way of hardening these bombs' shells to allow them to survive penetration through layers of rock, steel and concrete before detonating close to their deep underground targets, the officials said.
But the 7.5 million allocated for the penetrator represent a 50-percent reduction from the 15 million requested by the administration.
An additional six million dollars have been earmarked to study low-yield nuclear weapons that some experts believe could be useful in high-precision strikes. Experts say that a five-kilotonne or smaller nuclear explosive detonated, for example, right on a missile silo door will vaporize the door as well as the missile inside.
Low-yield weapons could also be effective against other types of underground facilities such as command posts and hardened ammunition dumps.
According to the Defense Intelligence Agency, at least 10,000 such bunkers currently exist in over 70 countries around the world.
More than 1,400 of them are used as strategic storage sites for weapons of mass destruction, concealed launch pads for ballistic missiles as well as leadership or top-echelon command and control posts, the DIA estimates.
The bill also contains 24.9 million dollars to heighten readiness at the Nevada test site to enable it to conduct a nuclear test 24 months after the White House decides to do so. The administration had been insisting on an 18-month readiness window, down from the current 36 months.
Overall, Congress allocated 6.3 billion dollars for nuclear weapon activities in fiscal 2004 -- 303 million dollars more than last year, but 94 million below Bush's request.
Research projects focusing on new types of nuclear weapons have been roundly criticized by many congressional Democrats and independent disarmament experts as potentially destabilizing.
"Congress and the Bush administration have made a mistake by opening the door to a new wave of global nuclear weapons competition" Daryl Kimball, executive director of the Arms Control Association, said in a statement. "This sends a dangerous message that will hamper US efforts to prevent other nations from developing nuclear weapons."