In February 2019 the first new W76 Mod 2 nuclear warhead was produced, several hundred more will follow. This is a lower yield (explosive power) version of the standard W76, which is built to provide 100 kilotons (equivalent to 100,000 tons of conventional explosives) of blast effect. The new W76-2 is a minor modification of the current W76 Mod 1 design. The Mod 2 achieves its lower yield by eliminating the second stage of the nuclear detonation (that basically amplifies the yield). The result is a warhead with a yield of 5-10 kilotons. This provides the option to respond to some kinds of nuclear attack (like a few ICBMs aimed at the United States that are intercepted) with a low-yield nuke to demonstrate that actions (firing nuclear armed missiles at the U.S.) have consequences and that now it is time to talk rather than face a much more devastating nuclear response (many more higher yield warheads). This capability addresses the question “what options do I (an American president) have short of a major nuclear response.” It was suggested that some SLBMs (submarine-launched ICBMs) be equipped with a conventional (high explosive) warhead for “demonstration of willingness” purposes. It was pointed out that using a high explosive warhead on an ICBM just demonstrates that you were willing to pay a lot to deliver a non-nuclear warhead.
The W76-2 warhead will equip one or two missiles on each SSBN (nuclear sub carrying ballistic missiles) with the rest of the missiles having the standard W76. Since the current SLBMs carry up to eight warheads, the ones carrying the W76-2 might just have one warhead. Several hundred W76-2 will be produced and the first W76-2 warheads will be in service (mounted on Trident SLBMs carried by SSBNs) in the early 2020s.
The W76 is the standard nuclear weapon used on SLBM ballistic missiles carried by American SSBNs. In 2011 Britain decided to use the W76-1 upgrade to the older W76 nuclear warhead, as older generations of nuclear warheads are updated before they become dysfunctional from old age. Previous to this, Britain had used its own nuclear weapons designs for these warheads, although the current British SLBM (sea launched ballistic missile) warhead is believed to be similar to the American W76, but with some different features (like selectable yield, or how big a nuclear explosion there will be).
Over the last decade, the U.S. has produced 2,000 W76-1 warheads. This is a minor upgrade of the original W76 and has the same yield of 100 kilotons. Upgrading these older W76 warheads was not easy. For example, American efforts to refurbish the elderly W76 nuclear warheads was held up by difficulties in manufacturing several components. The warheads were originally manufactured between 1978 and 1987. Since that time it was discovered that the necessary details for manufacturing some of the unique components had been lost. One of those items, a chemical codenamed Fogbank, could not be created with surviving documents. This problem was eventually overcome, but then similar problems were discovered with some other components. This sort of thing was largely the result of manufacturing details being so highly classified. Normally, manufacturing details for older items can afford to be a little vague, because unclassified components have lots of similar items either still in production, or many people and documents you can consult to quickly reconstruct the needed materials and process details. Not so with highly classified components for nuclear weapons.
While the W76 is an old design it is not the only warhead used by Trident SLBMs. There is also the W88 warhead. These were designed in the 1980s and 400 were manufactured in 1988-89. The W88 has a yield of 475 kilotons and American SSBNs carry one or more missiles armed with the W88.
All this W76 activity got started because the earlier ones were fast approaching the point where they would be useless. Nuclear warheads have a lot of components (explosives, various exotic chemicals and electronics) that degrade over time. The W76 was the oldest warhead in service for a long time. In 2007 the nuclear weapons industry proposed a new warhead design for the navy's sea-launched Trident D5 ballistic missiles. This involved replacing 3,000 W76 warheads that equipped 336 missiles. That would cost about $100 billion. The navy preferred to refurbish the W76s and save a lot of money, rather than coming up with a new design.
The case for a new warhead is that this would provide a nuclear weapon that is more reliable, less likely to go off by accident, cheaper to maintain and more difficult to use if one is stolen by terrorists. The navy insisted that the current W76 warheads, designed in the 1960s, were adequate. The W76s are old, but like any piece of expensive machinery, they are carefully maintained. Parts wear out and are replaced. It was components that don't wear out quickly that caused the problem with the refurbishment. These items have been out of production since the 1980s.
Most importantly, the W76 has been tested. So we are sure that a W76 will explode when ordered to. Because of a 1992 treaty, nuclear weapons may no longer be tested, even underground. The new warhead would have to be "tested" via simulation. That is not a major obstacle. Simulation of complex systems is now quite common, and reliable. It's one of those unseen technologies that make life so much better for everyone. The nuclear weapons designers, however, believe they have discovered several flaws in the W76 design, things that could be eliminated with a new warhead, even one that will never actually be detonated. One of the flaws is apparently the difficulty of reviving the manufacture of key W76 components like the mysterious Fogbank chemical.
Times have been tough for the nuclear weapons crowd since the Cold War ended in 1991. After that several treaties were signed that reduce the American nuclear arsenal. Thus it is bad politics to try and get lots of money for new warheads. This is especially true because most people would like for there to be even fewer warheads. It's the old debate over "how many warheads do you need to get the job done." The U.S. currently has 6,000 nuclear warheads. There are another 7,000 out there (most of them Russian).
Over 15,000 warheads have been taken out of service since the 1990s. The U.S. and Russia had so many because both nations had developed tactics that included attempting to knock each other’s land-based missile silos out of action. Any exchange of that many warheads, even if only ten percent of them actually went off, would have destroyed Eurasia and North America. Those tactics are no longer popular, thus you only need a few hundred warheads to pose a credible nuclear threat. The U.S. and Russia have since agreed to get each of their warhead inventories down to less than 2,000.
As a result of all this, getting $100 billion for a new generation of warheads was not going to happen. The decision was made to refurbish. Then along came, or didn't, fogbank and other components that were more difficult to recreate than expected. That added some extra cost but not to the extent that the upgraded W76s were not a bargain (as W76 Mod 1) compared to a new design.
Maintaining existing warheads costs over a billion dollars a year, with or without crises like lost manufacturing knowledge. That includes money needed for maintaining and upgrading facilities, as well as work on the warheads themselves, and research and development of maintenance requirements and techniques. Nukes are still a big business. But they are not likely to get a lot bigger. A new treaty is proposed that will reduce the nuclear arsenal even further but that does not make warhead maintenance any easier or much cheaper.