ET.317 was a

thermonuclear weapon A thermonuclear weapon, fusion weapon or hydrogen bomb (H bomb) is a second-generation nuclear weapon design. Its greater sophistication affords it vastly greater destructive power than first-generation nuclear bombs, a more compact size, a lo ...

of the

British Royal Navy The Royal Navy (RN) is the United Kingdom's naval warfare force. Although warships were used by English and Scottish kings from the early medieval period, the first major maritime engagements were fought in the Hundred Years' War against Fra ...

, developed for the UK version of the

UGM-27 Polaris The UGM-27 Polaris missile was a two-stage solid-fueled nuclear-armed submarine-launched ballistic missile (SLBM). As the United States Navy's first SLBM, it served from 1961 to 1980. In the mid-1950s the Navy was involved in the Jupiter missi ...

missile.

Development

The US Polaris A-3T warhead was the US

W58 The W58 was an American nuclear bomb, thermonuclear warhead used on the UGM-27 Polaris, Polaris A-3 submarine-launched ballistic missile. Three W58 warheads were fitted as multiple warheads on each Polaris A-3 missile. The W58 was in diameter a ...

. Britain considered but never used the W58 because the British safety authorities considered it unsafe in several respects. Instead they fitted a hybrid of a US W59 fusion secondary, triggered by a new British primary based on a Cleo boosted-fission device tested in

Nevada Nevada ( ; ) is a U.S. state, state in the Western United States, Western region of the United States. It is bordered by Oregon to the northwest, Idaho to the northeast, California to the west, Arizona to the southeast, and Utah to the east. N ...

as PAMPAS and TENDRAC. Variants of this basic design were used or intended for several other delivery systems, including the

WE.177 The WE.177, originally styled as WE 177, and sometimes simply as WE177, was a series of tactical and strategic nuclear weapons with which the Royal Navy (RN) and the Royal Air Force (RAF) were equipped. It was the primary air-dropped nuclear we ...

bomb, UK

Skybolt ALBM The Douglas GAM-87 Skybolt (AGM-48 under the 1962 Tri-service system) was an air-launched ballistic missile (ALBM) developed by the United States during the late 1950s. The basic concept was to allow US strategic bombers to launch their weapons ...

, and the

Blue Water Maritime geography is a collection of terms used by naval military units to loosely define three maritime regions: brown water, green water, and blue water. Definitions The elements of maritime geography are loosely defined and their meanings hav ...

SSM (a tactical battlefield missile), and several others. Documents declassified and released into the public domain in 2012 disclose that ET.317 was a warhead that used the fission-fusion-fission process, where a boosted-fission device codenamed '' Jennie '' triggered a fusion secondary codenamed '' Reggie ''The Real Meaning of the Words: A Very Pedantic Guide to British Nuclear Weapons Codenames
which in turn was encased in U-238 depleted uranium.Public Record Office, London. AVIA 65/1845 In the conditions created in the fusion process, vast quantities of free neutrons are liberated, and the normally unfissionable U-238 casing fissions. This results in a smaller, lighter, cheaper, dirtier weapon than one using only a fission-fusion process. Fission of the depleted uranium casing can generate a significant proportion of total weapon yield, and compared to other fissile material U-238 is cheap and plentiful as a waste product from HEU manufacture. '' Jennie '' was based on the '' Cleo '' device tested at PAMPAS and TENDRAC. '' Reggie '' was based on the US W59 fusion secondary. Since the '' Reggie '' secondary manufactured in the UK from British-owned materials was a copy of the US W59, by implication, the W59 thermonuclear weapon deployed in some

Minuteman I The LGM-30 Minuteman is an American land-based intercontinental ballistic missile (ICBM) in service with the Air Force Global Strike Command. , the LGM-30G Minuteman III version is the only land-based ICBM in service in the United States and re ...

ICBMs also used the fission-fusion-fission process. Although the physical dimensions of ET.317 are not themselves declassified, publicly available information has allowed an estimate to be produced of the reentry vehicle of wide at the nose, wide before the skirt and from the nose to the start of the skirt. The yield of ET.317 was stated in official documents on many occasions to be 200 kt. When retired, the '' Reggie '' fusion secondary was recycled and reused in its successor, the

Chevaline Chevaline () was a system to improve the penetrability of the warheads used by the UK Polaris programme, British Polaris nuclear weapons system. Devised as an answer to the improved Soviet Union, Soviet A-35 anti-ballistic missile system, anti- ...

warhead, matched to a new boosted-fission trigger codenamed ''Harriet''. In that form, it is claimed to yield 225 kt, although there are as yet no official sources for that claim. Because the Chevaline warheads numbered two per missile, rather than the previous three ET.317 warheads, the surplus warheads were dismantled and while Jennie was no longer required, some Reggie secondaries were refurbished and installed as the secondary in WE.177C aircraft-delivered tactical nuclear bombs.Public Record Office, London. DEFE 19/208 e47/2

References

{{Strategic nuclear weapon systems of the United Kingdom Cold War weapons of the United Kingdom Nuclear weapons of the United Kingdom Polaris (UK nuclear programme)