In swapping out an older nuclear warhead for a newer, safer one, weapons scientists suggested they had bolstered the feasibility of a Bush administration plan to replace the entire U.S. nuclear arsenal.

That plan calls for designing and manufacturing simpler, hardier bombs and warheads, called "reliable replacement warheads," some perhaps generic enough to fly on different missiles or aircraft.

Scientists worked through part of that problem over the last three years in deciding that the W87 warhead, designed for the defunct Peacekeeper missile, could fly on the Minuteman III missile, the nation's last silo-based ICBM. The same computer simulations and experiments, they argued, might be used to put replacement warheads on a variety of missiles.

"I think it does establish that this is a do-able thing," said Derek Watman, weapons engineering chief for Lawrence Livermore National Laboratory.

For Princeton University physicist Frank von Hippel, "that's a stretch."

Debate over the new replacement warheads instead has been focused on a tougher question: Should the nation make and deploy redesigned H-bombs without ever having tested them in a nuclear explosion?

"The question really is whether this thing will work, not whether you can fasten it on a missile," said von Hippel, co-director of Princeton's Program in Science and Global Security. "It's whether you can have confidence in an untested warhead."