Ukraine, nuclear risks and the UK’s lower-yield capability

President Biden’s citing of the Cuban Missile Crisis in early October, along with its 60th anniversary shortly afterwards, has again raised fears that the Russian invasion of Ukraine could lead to the first use of nuclear weapons in war since 1945. While official sources in the US quickly rushed to emphasise that Biden’s comments did not reflect any change in official assessments of the likelihood of a Russian nuclear strike, there is no doubt that the material support of Ukraine by NATO brings the two sides closer to conflict than at any time since the Cold War. Much has been written about the likelihood and potential risks of escalation from the conflict, but the links of these potential scenarios with the UK’s lower-yield nuclear capability remain relatively unexplored.

The nuclear use scenario widely regarded as most likely is the use of so called ‘non-strategic’ or ‘tactical’ nuclear weapons by Russia, to try and force compliance or negotiations after it had suffered military setbacks. Such an act would be in keeping with the ‘escalate to de-escalate’ strategy that many Western analysts have claimed is part of Russian doctrine. That claim is contested, not least by Russia.

In practice, the much-debated contents of Russian nuclear doctrine are unlikely to significantly constrain decision-making in the Kremlin, and the likelihood of Russia using a lower-yield weapon in those circumstances rests on the fundamentally unknowable intentions of President Putin. The debate, in truth, boils down to a character assessment of Putin and the prior assumptions that are brought to bear on that assessment.

Russian lower-yield weapons

What can be stated with certainty is that, according to estimates from the Stockholm International Peace Research Institute, in January 2022 Russia had 1,912 of these lower-yield weapons. They include cruise and ballistic missiles, air-launched weapons, rockets, depth bombs, torpedoes and air defence systems. For the weapons where the yield is known most fall between 10kt and 100kt, with a handful yielding 350kt

Both of the common terms for these weapons are problematic, given that all nuclear weapons use can be regarded as strategic, and in the scenario under discussion they would not be deployed to give a tactical military advantage, but for a strategic purpose. Aside from generally having lower-yields than the average yields of strategic weapons, what tends to distinguish them is the shorter range of their delivery systems. As the yields are still orders of magnitude higher than conventional weapons, the term ‘low-yield’ is also misleading. Many of them are also more powerful than the 15kt bomb dropped on Hiroshima. As such, it is best perhaps to term them ‘lower-yield’ nuclear weapons. The P5 states have repeatedly committed to unilateral reductions of ‘non-strategic’ nuclear weapons at Non-Proliferation Treaty review conferences.

In Russia, these weapons are known to be stored in centralised locations, separately from their delivery systems, and several observable steps would need to be completed before they were used. The assurances from US intelligence sources that there are no indications of Russian preparations for nuclear weapon use is presumably based on surveillance to check whether these steps were being taken, alongside other indicators.

NATO responses

Although the threats emanating from senior figures in Moscow appear to only countenance nuclear use in a retaliatory scenario, and despite there being no evidence of any practical preparations for a Russian strike, speculation in the press has revolved around the ‘escalate to de-escalate’ scenario and a possible NATO response. The position of the Biden administration has been to warn of the ‘catastrophic’ consequences of nuclear use, while refusing to commit to any specific response, in order to keep all their options open.

While this desire to keep the least escalatory responses on the table is laudable, the longstanding debate over the presumed Russian ‘escalate to de-escalate’ strategy has contributed to far less benign developments within the US nuclear programme. The lower-yield US submarine-launched W76-2 and the planned US nuclear-armed sea-launched cruise missile were justified in large part in the Trump administration’s 2018 Nuclear Posture Review (NPR) as necessary responses to Russia’s lower-yield warhead arsenal and an ‘escalate to de-escalate’ scenario. While the Biden administration’s own NPR confirms an ambition to cancel the cruise missile, it retains the W76-2 and there are moves in the US Congress to continue funding the cruise missile against the wishes of the administration.

The rationale for these weapons is the fear that an adversary might believe its own use of lower-yield nuclear weapons would not be met with a full strategic nuclear strike. When discussing the UK’s lower-yield nuclear capability in 1996, a government minister raised the spectre that “a potential adversary could gamble on us not being prepared to use our full strategic capability in response to aggression”. The idea was that there could be no doubt about the willingness to use lower-yield weapons in such a scenario. Both the US 2018 NPR and justifications for the UK’s lower-yield capability, claim that these weapons are necessary for their country’s nuclear posture to be ‘credible’.

The UK’s lower-yield capability

The recently-released NIS report, ‘Extreme Circumstances: The UK’s new nuclear warhead in context’, examined the history and current status of this lower-yield capability. The evidence strongly suggests this took the form of a lower-yield variant of the UK’s main warhead with the secondary stage of the warhead having been replaced with a ‘dud’ component which had an identical size and weight distribution but did not detonate with an explosive yield. It would have been very close in design to the US W76-2, with a yield of 5kt or 8kt.

The original intention appears to have been for the UK’s Vanguard submarines to go to sea carrying missiles with a range of warhead loads, including some loaded with the lower-yield warhead variant. This practice may have ceased by the 2010s, as the government published official figures for the numbers of warheads and operational missiles carried on each submarine. This would have limited the scope for carrying the extra missiles for a range of strikes, and at the time government policy appears to have been guided by a genuine desire to reduce both overall and deployed warhead numbers.

Official statements ceased using the term ‘sub-strategic’ in 2006 under direction from the then Defence Secretary Des Browne, but in a response to the Defence Select Committee the following year the MOD stated that it intended to retain the lower-yield capability. This does not necessarily mean that the stock of lower-yield warheads would have been retained. The short timescale within which a number of US warheads were converted to the W76-2 variant demonstrates the relative ease of replacing the secondary of a warhead with a dud, or vice versa. The UK warhead may even have been designed with some adaptations to simplify the process.

The Government’s 2021 Integrated Review (IR) scrapped the disclosure of the number of warheads in the UK’s operational stockpile as well as the numbers of warheads and operational missiles carried on each submarine. While announcing the first increase to the UK’s warhead stockpile in decades, it claimed the government would maintain the “minimum destructive power needed to guarantee that the UK’s nuclear deterrent remains credible and effective against the full range of state nuclear threats”.

The increased warhead stockpile will allow for a greater number of warheads to be carried on submarines while keeping the same proportion of warheads in reserve. One of the possible reasons for doing so would be to recommence the practice of going on patrol with a range of nuclear strike options. The language in the IR about remaining ‘credible’ against a ‘range of state nuclear threats’ may indicate the intention to field the lower-yield variant of the UK’s warhead. If so, and references to ‘doctrinal threats’ elsewhere in the document suggest this would be a potential response to a Russian ‘escalate to de-escalate’ scenario. While we do not know for certain, it seems more likely than not that this language is re-emerging in UK nuclear doctrine in tandem with the nuclear posture that was associated with it in the past.

Doctrine vs reality

In practice, looking at the scenario of potential nuclear usage connected to the Ukraine war, these lower-yield weapons represent a dangerous and unnecessary escalatory option for the US, and potentially also for the UK. Compared to a diplomatic or conventional military response to Russian nuclear use in the ‘escalate to de-escalate’ scenario, it is by far the most dangerous and least effective option. It is impossible to know how likely that scenario is, other than to note that the invasion itself was a reckless and self-defeating war crime. As such, we cannot completely rule out the possibility that Russia will carry out another in the form of a lower-yield nuclear detonation.

Unless that unthinkable event occurs, we also cannot know to what extent NATO’s lower-yield nuclear weapons have deterred or constrain Russian actions, as their defenders would claim. We do know for certain that their presence did not deter Putin from launching the invasion in the first place, however, and it seems inarguable that the unstable balance of collective nuclear insecurity was a causal factor in this conflict. As we contemplate the exact scenario that these weapons were designed for, they offer no meaningful reassurance of a safe resolution, only a horrifying step in an escalatory ladder that should be avoided at all costs. Given the additional risks of miscalculation or a misunderstanding on either side, the only responsible course of action is to put them immediately beyond use, followed by their speedy elimination.

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