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W47 (nuclear weapon)

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Deployed as the warhead for the first two versions of the UGM-27 Polaris submarine-launched ballistic missile, the W47 warhead both made breakthroughs in miniaturization, but also had significant reliability problems. Some of those problems were discovered only with an actual nuclear test, so the W47 remains one of the controversies as to whether the United States nuclear surety program can be viable under a nuclear test ban.[1]


The entire Polaris program was under intense time pressure. An initial "emergency capability" EC-47 warhead was in brief use between April and June 1960, replaced by the 660 kt[note 1] Y1 model, which, in turn, was replaced by the 1.2 Mt[note 1] Y2. Both airburst and impact fuzing were available.

All models were 46.6 inches (118 cm) long and 18 inches (46 cm) in diameter. The Y1 weight is given as between 717 - 720 pounds (325 - 327 kilograms) , while the Y2 was 732 pounds (332 kilograms).[2]


It represented several breakthroughs in compact design, several of which probably involved new ideas in modular design. The reentry vehicle and beryllium reflector, for example, were a single unit.


In 1966, 75% of the stockpiled Y2s were inoperable, correction took until September 1967.[2] Three problems were identified with the W47 warhead: corroding fissile material, vulnerability of a material to an ABM environment, and a defect in a mechanical safing device. Remanufacturing issues were associated with the second and third problems, and nuclear testing played a necessary role in correcting them.

One-point safety

As originally designed, a mechanical safing device was used in the W47 warhead to comply with the one-point safe criterion. This was probably a neutron absorbing safing wire that was withdrawn from the core during weapon arming. This device was part of the Primary, not of the overall two-stage thermonuclear weapon. "However, chemical corrosion in the W47 caused a serious reliability problem in the safing mechanism that did not lend itself to a viable engineering solution. In a large fraction of the sampled warheads, the mechanism would not fully complete its arming operation; this indicated that a large fraction of the W47 warheads would be duds, and the number of duds was increasing with age. Rather than remanufacture the warheads with rebuilt safing mechanisms that would fail again, we solved the problem by replacing the primary with one known to be inherently one-point safe." One-point safety was confirmed in the only live test of a U.S. SLBM warhead, "Frigate Bird" in Operation Dominic on 6 May 1962.[3] "A nuclear test was required to certify this new design. All W47 warheads in the stockpile were retrofitted with the new primary. Although these problems with the W47 did not involve exact replication of the warhead, they all nvolved dismantling warheads and rebuilding them with improvements. Each of the improvements was necessitated by serious unforeseen changes in vulnerability or reliability. The W47 illustrates that in order to maintain the capability of a weapon, exact replication may not be desirable or even possible."[4]

ABM vulnerability

The W47 was not designed at a time when ABMs were a threat. While the details of the vulnerability have not been disclosed, there is evidence that the W47 may have held its boosting tritium inside the outer shell rather than inside the levitated pit, since the safety wire withdrawal did maintain a gas-tight seal. Tritium boosting is known to prevent predetonation due to radiation bombardment from external explosions. A modified design was needed to correct the ABM vulnerability problem. The extent of the design modification was large enough to require a nuclear test to certify the change, and weapons were rebuilt with the modification.


  1. 1.0 1.1 The energy yields of nuclear weapons are commonly expressed in units of TNT equivalent, meaning the energy yield from the explosion of a stated amount of trinitrotoluene (TNT). The commonly used units are a kilotonne or a megatonne of TNT equivalent. A kilotonne (kt) of TNT equivalent is equal to 1012 joules and a megatonne (Mt) of TNT equivalent is equal to 1015 joules. The kilotonne and megatonne are often taken to be synonymous with kiloton and megaton.


  1. George Miller, Report to Congress on Stockpile Reliability, Weapon Remanufacture, and the Role of Nuclear Testing, Lawrence Livermore National Laboratory, pp. 18-20
  2. 2.0 2.1 Carey Sublette (14 October 2006), Complete List of All U.S. Nuclear Weapons
  3. Operation Dominic: 1962 - Christmas Island, Johnston Island, Central Pacific, 3 January 2005
  4. George Miller, Report to Congress on Stockpile Reliability, Weapon Remanufacture, and the Role of Nuclear Testing, Lawrence Livermore National Laboratory, p. 29