W80 (nuclear warhead)



The W80 is a small thermonuclear warhead (fusion or, more descriptively, two-stage weapon) in the enduring stockpile with a variable yield of between 5 and 150 kt of TNT.

It was designed for deployment on cruise missiles and is the warhead used in the majority of nuclear-armed US Air Force ALCM and ACM missiles, and their US Navy counterpart, the BGM-109 Tomahawk. It is essentially a modification of the widely deployed B61 weapon, which forms the basis of most of the current US stockpile. The very similar W84 warhead was deployed on the BGM-109G Gryphon GLCM.

Dimensions
The W80 is physically quite small: the "physics package" itself is about the size of a conventional Mk.81 250 lb bomb, 11.8 in in diameter and 31.4 in long, and only slightly heavier at about 290 lb.

Armorers have the ability to select the yield of the resulting explosion in-flight, a capability sometimes referred to as "dial-a-yield" but more properly variable yield. At one end of the scale, perhaps using just the boosted fission primary, the W80 has destructive power equivalent to around 5 kilotons of TNT; at the other end, the yield is equivalent to around 150 kt.

Early development
The Los Alamos National Laboratory began development on the W80 in June 1976, with the brief of producing a custom weapon for the cruise missiles then under construction. The main differences between the W80 and W61 appear to relate to the physical packaging of the device, and to the removal of the 0.3 kt yield mode; the W61 apparently needed this feature when deployed as a depth charge, a role for which the W80 was not intended.

Production of the W80 Model 1 (W80-1 or Mod 1) to arm the ALCM started in January 1979, and a number of warheads had been completed by January 1981 when the first low-temperature test was carried out. To everyone's surprise the test delivered a much lower yield than was expected, apparently due to problems in the TATB based insensitive high explosives used to fire the primary. This problem turned out to affect several models of the B61-based line, and production of all weapons was suspended while a solution was worked on. Production restarted in February 1982.

In March 1982, designers began working on a W80 variant intended for the Navy's Tomahawk program. The W80 Model 0 (W80-0 or Mod 0) used "supergrade" fission fuel, which has less radioactivity, in the primary in place of the conventional plutonium used in the Air Force's version. "Supergrade" is industry parlance for plutonium alloy bearing an exceptionally high fraction of Pu-239 (>95%), leaving a very low amount of Pu-240 which is a gamma emitter in addition to being a high spontaneous fission isotope. Such plutonium is produced from fuel rods that have been irradiated a very short time as measured in MW-Day/Ton burnup. Such low irradiation times limit the amount of additional neutron capture and therefore buildup of alternate isotope products such as Pu-240 in the rod, and also by consequence is considerably more expensive to produce, needing far more rods irradiated and processed for a given amount of plutonium. Submarine crew members routinely operate in close proximity to stored weapons in torpedo rooms, in contrast to the Air Force where exposure to warheads is relatively brief. This justified the additional costs of the premium supergrade alloy used on the Navy weapon. The first models were delivered in December 1983 and the Mod 0 went into full production in March 1984.

Production of the W80 was completed by September 1990, although the exact date at which the respective Mod 0 and Mod 1 runs ended is not clear. A total of 1750 Mod 1 and 367 Mod 0 devices were delivered; 1,000 Mod 1 devices were deployed on the original ALCM, another 400 on the later ACM, and 350 Mod 0s on the Tomahawk.

A number of the original ALCMs equipped with the Mod 1 later had their warheads removed in order to use them with conventional explosives (the CALCM conversion), and under START II only 400 ACMs would retain their warheads and the rest would be removed, apparently with all remaining ALCMs converted to CALCMs and their warheads removed to the "inactive stockpile". With the fall of the START II treaty it is not clear what the current plans are, but it is highly likely they will take place as originally planned in order to remove the ALCM from service. Mod 0 armed Tomahawks are currently stored onshore, but there are/were plans to make this the primary US Navy nuclear weapon.

2007 nuclear weapons incident
On August 30, 2007, six cruise missiles armed with W80-1 warheads were mistakenly loaded onto a B-52 and flown from Minot Air Force Base, North Dakota, to Barksdale Air Force Base, Louisiana, on a mission to transport cruise missiles for decommissioning. It was not discovered that the six missiles had nuclear warheads until the plane landed at Barksdale, leaving the warheads unaccounted for over 36 hours. This was the first time since 1968 that nuclear warheads were publicly revealed to have been transported on a US bomber. The munitions crews involved in mistakenly loading the nuclear warheads at Minot were temporarily decertified from performing their duties involving nuclear munitions.