Jeffrey LewisUK Trident Yield

Back in January 2008, Michael Bilton in the Sunday Times had a very interesting article on the UK Trident force that I somehow missed. Sort of a UK-version of Doug Waller’s excellent, Big Red.

The most intriguing part, to me, is the suggestion that the UK warheads have variable yields. Previously, UK officials had indicated that the UK “has some flexibility in the choice of yield for the warhead on its Trident missile.”

“Some flexibility,” according to Bilton, runs all the way down to 10-15 kt:

As a gesture to disarmament, in 1998 the Blair government dramatically cut the British nuclear stockpile – getting rid of all tactical weapons and limiting each submarine to a maximum of 48 warheads, weapons that can nevertheless cause terrible damage. They can strike anywhere on Earth and cause some countries to cease to exist. Britain’s post-cold-war Trident submarines go to sea with fewer missiles and warheads. Sometimes one or more of the missile tubes contains concrete ballast blocks to control buoyancy. Most British weapons have a yield of 80 to 100 kilotons – seven or eight times the destructive power dropped on Hiroshima and Nagasaki. But some are much smaller – 10 to 15 kilotons. Some missiles have multiple warheads and dummies; others contain only a small single device – probably a low-yield weapon, with limited destructive power. In some scenarios it doesn’t take a sledgehammer to crack a nut.

The key to deterrence-theory is to convince a potential adversary that we have not only the capability but also the will to fire a nuclear weapon if critically threatened. A British prime minister might feel constrained in giving the order to fire if the result was massively disproportionate to the threat from a rogue state or terrorist group. Smaller-yield single warheads could be used to demonstrate British resolve, coupled with a warning of the devastation that might follow if a potential enemy did not back off. Deliberate ambiguity is a crucial strategy. Would Britain make the first use of nuclear weapons? Ministers refuse to say. Keeping an enemy guessing is the name of the game.



  1. J House (History)

    Fascinating indeed.
    “The key to deterrence-theory is to convince a potential adversary that we have not only the capability but also the will to fire a nuclear weapon if critically threatened.”

    But to muster the will,the rub is in the definition of ‘critically threatened’.
    When one considers the future possibility of a sub-national group ‘critically threatening’ the US or the UK with a nuclear weapon, you have to wonder where on earth are they going to fire in response?
    The theory may still hold among nation-states, but is useless against groups like AQ that see no taboo in the use of NBC weapons and hold no territory or standing army, nor population or culture to defend.
    The taboo to bring mass destruction to the U.S. was broken on 9/11.
    The U.S. response could be argued as timid (compared to what may have been done to Tora Bora in say, 1946).
    So, defining ‘will’ and ‘critically threatened’ have important relevancy in the theory, which can be argued is an outdated cold-war mentality that fails to address non-state actors that are not deterred by annihilation.
    We should take them at their word when they say “We love death more than you love life”.

  2. blowback (History)

    They could just be lying and only really have a single type of warhead.

  3. Allen Thomson (History)

    Sounds like a two-stage device with a option to block the second stage.

  4. FSB

    What would you do differently if you knew your enemy had a 80kT nuke versus a 15kT (Hiroshima) yield? Do you think UK would be more cavalier about shooting off a 15kT weapon versus a 80 kT one? I doubt it.

    This is all crazy talk (your words from your Bulletin article some months back).

    When your enemy has a nuke you do not care what color it is, if it is a Pu or U weapon or if it is a trusted legacy weapon or an untested RRW. Your deterrence calculus is identical (i.e. you are either deterred or undeterred). Conversely, if you think your enemy may not launch a 80 kT nuke, you will also think that they won’t launch a 15 kT weapon.

    Maybe a 1 kT vs. 100 kT nuke option will have an effect on credibility of use of the weapon — in that case, just use a daisy cutter instead of the 1 kT nuke.

  5. Nick Black (History)

    Why wouldn’t they? Dial-a-yield is just a matter of controlling the volume of DT gas insertion at arming time, right? The feedback’s already there; you’re just giving it multiple states. It seems a pretty cheap exchange for the flexibility. No?

  6. John Bragg (History)

    Sigh. I am, as always, a rightwing nut, but does anyone seriously believe that a British PM or an American President will give a first-strike order to Hiroshimafy say, Natanz or Yongbyon?

    And if we’re not talking about a first strike, then we’re talking about retaliation. In which case, bigger may or may not be better (a 15kt detonation would still cripple a metro area while a 1Mt might erase it) but it certainly isn’t worse. So pointless.

  7. Yale Simkin (History)

    Most interesting is that they are revealing that their boosted primaries yield 10-15 kt (with then 70-80 kt from the secondary).

    It tells a bunch about efficiencies (unless some fission yield is from a compressed but non-fusion-fueled secondary – a sort of updated Zombie design)

    I don’t quite see the point of using 15kt vs 80kt.

    15 kt will incinerate exposed flesh over an area 6 kilometers in diameter, while 80 kilotons extends that out only 3 more kilometers.

    I don’t don’t think that I, as the attacked country, would see that as a great qualitative difference.

  8. George William Herbert (History)

    Nick –

    There are multiple possible options for “dialing” yield.

    You can partially or not inject boost gas, if it’s not sealed in the pit normally.

    You can vary timing or leave out the pulse neutron tube firing entirely.

    You can possibly fire a multipoint or two point implosion system intentionally “out of sync”, to make the implosion slightly less perfectly symmetrical.

    And you can shut off the secondary entirely, either by dropping primary yield below what’s needed to ignite the secondary or by blocking the radiation channel between them, or opening up the radiation case entirely, around the primary, so it just flows out and isn’t effectively contained.

  9. Nick Ritchie (History)

    By way of a brief history lesson for those that are interested, this goes back to the early 1990s when a decision was made to retire the WE177 free fall nuclear bomb that provided a ‘sub-strategic’ UK nuclear capability (they were all retired by 1998). UK plans originally envisaged procuring a new nuclear Tactical Air-Surface Missile (with the French no less!) but this was scrapped and the ‘sub-strategic’ mission was passed to the new Trident system in 1995. Some UK Trident missiles are therefore thought to be equipped with a single warhead that has a reduced yield of 10kt (100kt is the estimated full yield of the UK Trident warhead) to provide a ‘limited’ nuclear strike capability so that Blighty can fire a nuclear ‘shot across the bows’ of the enemy of the day to demonstrate its resolve to escalate a conflict to strategic use of nuclear weapons if necessary. All of this proved controversial and, as a footnote, the UK Ministry of Defence decided to formally end use of the term ‘sub-stratgeic’ in reference to Trident in 2007 in order to reinforce the ‘last resort’ nature of the deterrent threat (though it is assumed the 10kt option still remains operational, although for what specific purpose is anyone’s guess).

  10. Rwendland (History)

    Is that Times article really suggesting dialable (on the submarine) yield? I read it as some of the 16 missiles having (preset or factory built) lower yield warhead(s), exactly as announced back in 1996 or earlier.

    Here is a quote from parliament (22 Feb 1996 : Column 1137):

    The Parliamentary Under-Secretary of State, Ministry of Defence (Earl Howe): … The entry into service of HMS “Victorious” means that Trident now provides a continuously available sub-strategic capability. The UK sub-strategic capability is also provided by the RAF with its Tornados armed with WE177 free-fall bombs. The concept of sub-strategic capability has long been a vital part of our and NATO’s nuclear doctrine. Without it, a potential adversary could gamble on us not being prepared to use our full strategic capability in response to aggression. The sub-strategic capability removes the risk of him believing that we ourselves will be deterred from using nuclear weapons.

    … there is essentially no difference in the way that strategic and sub-strategic missiles are targeted.

    Lord Williams of Elvel: My Lords, are they the same missiles?

    Earl Howe: No, my Lords.

    I don’t know if this was the first official announcement of the sub-strategic Trident capability, but at 1996 it must be amongst the earliest.

    One wonkish detail is that HMS Victorious (S29) is the second Vanguard class SLBM, so a small puzzle is why the sub-strategic option was not available, as Earl Howe implies, with the first whose maiden patrol was about one year earlier.

  11. Distiller (History)

    Hmm. Tactical use of a strategic weapon system. Works only to strong-arm have-nots. Does that mean first use?
    Wouldn’t want to try a “shot across the bow” against a major power, giving away the position of one of my four precious SSBNs.

    Deterrence should be a no-nonsense affair. With a 15kT warhead the other guy might speculate he can ride it out. With 450kT times four or eight coming down he’ll think twice.

    In short: C.R.A.P. doctrine. Better put one or two nuclear Tomahawks on the SSNs.

  12. Josh (History)


    It says that Trident now provides a “continuously available” sub-strategic capability. So presumably it was available before, but not continuously.

    Now, are two submarines really enough for continuous operations?

  13. Gridlock (History)

    I thought it was dialable – read an article earlier in the year about PAL (and the British version, bike locks and a code set to 0000) which I’m sure mentioned selectable yield in the context of per-missile, but predictably I can’t now locate it…

  14. Gridlock (History)

    Ah, found it, it’s talking about the WE177 gravity bomb:

    “With the help of Brian Burnell – a researcher into the history of the British nuclear weapons programme who once designed bomb casings for atom bombs – Newsnight tracked down a training version of the WE 177 nuclear bomb at the Bristol Aero collection at Kemble.

    Tornado and earlier V-bomber crews trained with these, which were identical in every way to the live bombs except for the nuclear warhead.

    To arm the weapons you just open a panel held by two captive screws – like a battery cover on a radio – using a thumbnail or a coin.

    Inside are the arming switch and a series of dials which you can turn with an Allen key to select high yield or low yield, air burst or groundburst and other parameters.

    The Bomb is actually armed by inserting a bicycle lock key into the arming switch and turning it through 90 degrees. There is no code which needs to be entered or dual key system to prevent a rogue individual from arming the Bomb.”

  15. Rwendland (History)

    Josh, an interesting observation: “continuously available” – I’d missed that. The third Vanguard’s maiden patrol wan’t until June 1998, over two years after the statement. So this suggests the Royal Navy could maintain “continuously available” with just two Vangaurds. I thought the Navy claimed it needed three + one for refit to do that. I suppose for a two year burst it must be possible with just two young SSBNs.

  16. Rwendland (History)

    Hunting around the Parliament website, I find an interesting statement on the difference between the “sub-strategic role” and “tactical role”. UK Trident does not do “tactical role” – which puts the 10-15 kt capability the Times claims in a puzzling light.

    Here is what a retired SSBN Officer said to the Defence Committee in 2005:

    Commodore Hare: I think that there is a lot of misinformation about the so-called sub-strategic role which you yourself mentioned. Sometimes it is confused with a tactical role which is not what either our policy or the Trident system is about. This is not a system that is geared or operated to achieve military objectives, by which I mean taking out a town, city, territory or whatever. It is for strategic use only and is on the right hand of the deterrence equation to be used in extremis when the survival of the nation state is at stake. When the sub-strategic concept was introduced its role was described by Lord Robertson in his speech [ on 1 March 1999 in Aberdeen ]. It will be on the record. We use the term “sub-strategic”, not “tactical”, deliberately. It is a sub-strategic role. What it means is that it offers the government of the day an extra option in the escalatory process before it goes for an all-out strategic strike which would deliver unacceptable damage to a potential adversary. It gives it a lower level of strike with which to demonstrate will, intent or whatever. It does not have to be used at all but it gives the government of the day that extra option at the sub-strategic level. To my mind, that is a welcome option.

    Mr Havard: In terms of configuration that does not mean a great deal?

    Commodore Hare: Not a great deal.

    Reading that, the sub-strategic role comes across more like a one 80-100 kt warhead option rather than a down-graded 10-15 kt option.

    Another reason to doubt The Times 10-15 kt claim is that the tactical WE177.A (10 kT or 0.5 kT) was withdrawn in 1992, leaving the 200 kT and 450 kT versions of the WE177 to go onto 1998. So if a 10-15 kt Trident capability was introduced in 1996 that would be new capability rather than replacing the about to be withdrawn 1996 WE177 capability.

    This makes me suspect The Times 10-15 kt Trident option claim could very well be wrong. As the UK was pressing Russia and the U.S. to negotiate a START III to reduce tactical weapons it would be very odd for the UK to re-introduce something roughly matching that class of weapon.

    NB: The UK Trident sub-strategic role was actually announced back in 1993, so the Commodore claiming George Robertson introduced the concept in a 1999 speech is puzzling. Maybe the concept was revisited when the WE177 was withdrawn in 1998.

  17. Rwendland (History)

    To row back on my doubts above about a 10-15 kt Trident option, it must be acknowledged that some highly reputable people have given weight to that suggestion beyond the “some flexibility” statement.

    Michael Quinlan, in particular, wrote “Details of this [‘sub-strategic’] concept have not been disclosed, but it is widely conjectured and not officially denied that some missiles may carry only one live warhead, and that that one warhead may have an explosive yield — perhaps through the use of only the ‘primary’ detonation — well below that of the normal warhead (itself not disclosed, but generally assumed to be between 80 and 100 kilotons” in International Affairs 82:4, 2006. Quinlan left the MOD in 1992 so probably knows, and it is unlikely he floated incorrect notions here.

    And Paul Rogers wrote in 2006 that UK Trident “is deployed with two warheads … and a “sub-strategic” or tactical warhead that has around half the explosive power of the Hiroshima bomb.”

    Michael Bilton is also a distinguished journalist. So I am forced to concede it is very plausible that the UK now has a warhead with a yield associated with tactical use, despite the UK having given up that capability in 1992 and stating the UK “holds no tactical nuclear devices” in Parliament in 2002. Shame.

    This underlines that the difference between “sub-strategic role” and “tactical role” is more intent rather than yield or capability.

  18. Josh (History)


    This is fascinating. Thanks for digging up all this stuff. Just one more question: shouldn’t “tactical” be interpreted in terms of the range of the delivery vehicle, rather than the yield of the warhead?

  19. Yale Simkin (History)


    I don’t know whether UK Trident actually uses its version of the W76 as a variable yield weapon, but it is a fairly straightforward process.

    The UK version of the warhead apparently does dials-a-yield with a process similar to various mods of the US W61 did.

    By not feeding the boost gas into the core, the yield would be about 1/3 kiloton.

    With the boost gas, but with the secondary disabled, the primary yield goes nominal – say 10 kilotons.

    A fully boosted primary and enabled secondary gives the 1/10th megaton yield.

    Other options are possible by varying boost gas and by introducing or removing fissile rings to the secondary, but these are less likely in a standardized, tightly configured SLBM warhead.

  20. Rwendland (History)

    Josh, I only have an amateur interest in armscontrol and I’m sure some academic must have a good definition of “tactical nuclear weapon”. So I wouldn’t put much weight on my views, but I would have thought “tactical” should reflect intent as a battlefield nuclear weapon, rather than range.

    For example a nuclear depth bomb could be flown a very long way (eg on a Nimrod) before use against a submarine. But I think that is clearly a tactical role.

    Bombs in general do not have a clear maximum delivery range, so you cannot really use range to define if the weapon is tactical or not. It is how you use it.

    And of course there are gray areas. Part of the 200 kt WE177.C role was behind the battlefield supply route disruption such as rail marshalling yards, at 100+ miles range I guess. I think NATO called that tactical, but I guess it might have destroyed edges of Polish cities. But the intent was to stop battlefield resupply, so tactical is probably correct.

  21. bradley laing (History)

    Couldn’t a British Trident carry a one-of-a-kind, 10kt warhead instead of a W76?

  22. Yale Simkin (History)

    BL asked:
    Couldn’t a British Trident carry a one-of-a-kind, 10kt warhead instead of a W76?

    In principle yes, but designing, testing, and producing a new warhead and its reentry vehicle (or fitting an existing RV) and then making sure the whole package – from sub to missile to control to fuzing and firing, etc., would be a daunting task.

    As inefficient as “wasting” the fusion package in a W76 would seem, the alternative is also quite unattractive.

  23. George William Herbert (History)

    If one desired a 10kt only (not dialable) weapon, in an existing RV and with existing primary and fuzing and so forth, the “safe and easy” way is to replace the secondary with an inert mass but otherwise leave the weapon identical.

    That’s probably tripling the mass over a “bare” (weaponized packaged, but no radiation case) primary, but saving weight in the off the shelf RV is not useful. The entire point is to keep the interface and ballistic performance the same – so you make the absolute minimum change (replace secondary with inerts) and leave all the rest identical.