The new warhead would work much the same as any other fusion bomb. The fissile nuclear pit, or primary, explodes and floods surrounding chemical compounds, known as the secondary, with radiation. This radiation triggers a fusion reaction between the tritium and deuterium isotopes of hydrogen produced by the irradiated compound. A thermonuclear explosion follows.
Only a limited number of such primaries have been tested. "It's the SKUA9 design," Goodwin says, one of a series of primaries created by Livermore during the nuclear testing program simply to test the viability of secondaries, and never produced as a weapon. As a result of this prior testing, this first Reliable Replacement Warhead (RRW1), if built, would require no further detonations, according to the NNSA and Livermore.
It will also provide increased confidence in the weapon's "margin," says J. Stephen Rottler, vice president for weapons engineering and product realization at Sandia National Laboratories in Albuquerque, N.M., which will be responsible for integrating the nuclear explosive into weapons systems such as missiles. Margin is the term used to describe a weapon's ability to avoid failure, such producing as a smaller explosive yield than for which it was designed.
Some scientists argue that the W76 has a low margin of failure because of the thin uranium shell that surrounds its core explosive. It could weaken, particularly as its plutonium core bombards it with radiation over time, subsequently failing to contain the primary fission explosion long enough to generate the high temperatures needed for fusion to take place in creating the secondary hydrogen detonation.
The new warhead will be bigger, thicker and heavier than the W76s, and therefore less likely to allow for that kind of failure, according to both Rottler and Goodwin. "That provides a margin, if you will, as the warhead ages," Rottler says. "The chances of us going underground [to test] again are remote." This is key to the appeal of RRW, because the U.S. government, complying with its treaty obligations, has mandated no return to underground testing.
But critics note that no nuclear weapon in the current U.S. arsenal has ever been manufactured without being tested. "Is there a military commander out there who will ever rely on something that has not been fully tested?" the Federation of American Scientists' Kristensen asks. "So far that has not been the case."
Building a Better Bomb
During the Cold War, the military emphasized packing as many warheads into one weapon as possible to generate maximum explosive yield, while also minimizing the overall weapon's weight to enable maximum range, resulting in weapons like the W76. Now that the Cold War has thawed such considerations are no longer as crucial, weapons designers say, allowing them to add new, heavier features—insensitive high explosives and advanced security technology—to the RRW1.
Insensitive high explosives, which resist detonation except when properly triggered, would improve the safety of handling these weapons in storage. "We have taken insensitive high explosives and slammed it into reinforced concrete blocks at Mach 4. It will not detonate," Livermore's Goodwin says. It is so secure, "you can put a gasoline fire out with it. If you put a blowtorch to it, you can get it to molder."
Further, the W76 lacks permissive action links (PAL), a computerized system that requires appropriate authorization to fire the weapon. "Under refurbishment, if we wanted to improve security interior to the warhead, we would have had to retrofit that into the warheads, which is difficult to do without nuclear testing," NNSA's Harvey says.
The W76 spends the majority of its life aboard submarines or in heavily secured stockpiles, reducing its need for such features, critics note. And the Life Extension Program for other nuclear weapons, such as the B61 gravity bomb, has incorporated added security measures, such as increased encryption, FAS's Kristensen argues. "Here was a weapon that was designed back in the 1960s and 1970s, and when it was first deployed it did not have safety features," he says. "They refit it all on the weapon itself without having to rebuild it. This suggests that you can achieve extraordinarily high levels of safety in current designs without going to a new design."