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Disarmament Diplomacy

Issue No. 28, July 1998

Nuclear Implications Explained
By Eric Grove

The SDR places considerable emphasis on changes in Britain's nuclear posture. Although all four Vanguard class ballistic-missile-armed nuclear-powered submarines (SSBNs) are to be built, the review announces that the warhead stockpile is to be reduced to "less than 200 operationally available warheads...a reduction of a third from the maximum of 300 announced by the previous government and a reduction of more than 70% in the potential explosive power of the deterrent since the end of the Cold War." It goes on to say that although deterrent patrols will continue the "day to day alert state" of the SSBN force will be reduced with the missiles placed on several days' notice to fire. Each submarine will carry no more than 48 warheads, a "reduction in potential destructive power" compared to a Chevaline-equipped Polaris boat of a third. The second volume of the review gives more detail, which together with other sources, allows the situation to be explained with some, if not complete, clarity.

The welcome increase in published information allows both suppositions about the nature of both the UK Polaris force and the Trident force to be confirmed and the welcome reductions to be put in a fuller perspective. An assumption of 100 kiloton (kt) yield for the US W-76 based Trident warheads gives a total yield per 48 warhead submarine of 4.8 megatons (mt). The total yield for a Chevaline-equipped Polaris boat was 6.4 mt, assuming the confirmed total of 32 warheads per submarine and a likely yield of 200 kt per warhead (the oft quoted lower yields for Chevaline never seemed very likely). This fits the Government's published figures exactly.

Of course things are not quite so simple as this. Given that the true destructive power of nuclear weapons (the so called equivalent megatonnage or EMT) reflects the 0.67 power of the TNT equivalent yield rather than the yield directly, the reduction in destructive power is actually less than the third it appears to be. Moreover the ability of Trident D-5 to fire at a large number of highly separated targets with impressive accuracy makes the destructive military capability of HMS Vanguard's 48 warheads much greater than HMS Repulse's 32, especially as all the latter were intended for the same target area, metropolitan Moscow. A Trident submarine can take out up to 48 targets, each with a strike equivalent to about three and a half Hiroshimas; a Polaris boat could just obliterate one area target with a multi-megaton blow.

It is significant that estimates of Trident warheads published by Norris, Burrows and Fieldhouse in Nuclear Weapons Data Book Volume 5 (Westview, 1994) are 192 for 1998-2000. There is probably thus little or no reduction in the planned current Trident stockpile, rather abandonment of further growth in the next century to fit out Trident with the full panoply of multiple warhead capability deemed necessary to penetrate expectedly improved Moscow anti-ballistic missile (ABM) defences. The previous government, we are told, intended to acquire about 300 operationally available Trident warheads. Assuming 100 kt per warhead this gives a yield of 30 megatons for the Major government's force and 20 mt for Blair's.

Rather frustratingly, three of the tables on p 5-3 of the Supporting Essays SDR volume deal in percentages rather than actual numbers but the figures of approximate numbers of operationally available warheads allow some informed guesswork about warhead numbers and yield. The 30 mt of the Major Administration's plans appear to be about 40 per cent that of the 1970s giving a figure for the latter of about 75 mt. Given an operationally available total of about 450 warheads (substantially more than previous estimates of total stockpile of about 350 for this period), the average yield of all operationally warheads comes out at about 170 kt. This demonstrates that most warheads were relatively high yield devices. In fact the latest model of WE 177, the C version, was a high yield 200 kt device, not the low yield nuclear depth bomb as often reported. The withdrawal of WE 177C as well as of the Polaris warheads has accounted for the substantial reductions in yield of which the Government is justly proud.

The comparatively high yield of the WE177 stockpile makes more credible its replacement by lower yield Trident warheads in the "sub-strategic" role (defined as the "option for a limited strike that would not automatically lead to a full scale nuclear exchange"). Trident warheads in the 100 kt range are actually less (two thirds as) destructive, than most WE177s. It is not clear if the potentially "sub-strategic" warheads deployed - presumably singly - on a proportion of Trident missiles can be "tuned" down before the SSBN sails to lower their yield further but this may not be necessary. If one is "going nuclear" the difference between 10 kt and 100 kt, an EMT difference of four not ten, may be relatively unimportant. The key fact is the number of warheads used. Indeed, Trident with its combination of range and accuracy gives the UK perhaps the most discriminating and capable nuclear force she has ever possessed.

Trident is now one end of a spectrum of power projection capabilities deployed by the Royal Navy rather than being a unique means of threatening mutual suicide. It is entirely appropriate therefore, in the contemporary strategic climate, that the operations of Trident submarines should be more like those of other warships. As someone who has been arguing this case for some time now, this author was particularly gratified by the measures to reduce the readiness of the Trident force. It is no longer appropriate to have SSBNs at instant readiness to fire at a predetermined target. Britain's Trident D-5s are thus now officially "de-targeted" and the submarines are officially at some days' notice to fire. This means that they have much more flexibility to do other things; "a variety of secondary tasks, without compromising their security, including hydrographic data collection, equipment trials and exercises with other vessels." Although the SSBNs can physically fire their missiles at very short notice on receipt of targeting instructions they no longer need to be able to fire instantly at a particular target. This, coupled with D-5's truly intercontinental range, gives much more flexibility in drawing up their operational programmes.

Although it is planned to maintain a Vanguard class SSBN at sea at all times, because of "risks of crisis escalation if it proved necessary to sail a Trident submarine in a period of rising tension or crisis", this will no longer require two crews per SSBN, a considerable and most welcome saving in highly expensive manpower. Previously, maintaining a Polaris SSBN at instant readiness on a particular station required a "port" and "starboard" crew for each boat.

It is one of the paradoxes of the post-Cold War period that Trident D-5, grossly over-designed as it was for the original requirement and only procured to obtain certain commonality with the Americans into the next Century, has come into its own as a new kind of flexible, minimum deterrent. It has allowed the withdrawal of hundreds of higher yield bombs and missile warheads as well as creating a more cost-effectively operated nuclear force that allows money to be spent on enhancing more useful conventional capabilities. "New Labour's New Trident" - although less of a diminution in actual destructive power than the government would like people to think - is a much more appropriate, discriminating and cost effective capability than the WE177/Polaris combination of old. Perhaps, in Sir Humphrey's immortal words, "buying at Harrods" has been vindicated after all.

Eric Grove is Deputy Director, Centre for Security Studies, University of Hull, UK.

© 1998 The Acronym Institute.

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