The last of the most powerful thermonuclear bombs in the United States arsenal - the B53 - was dismantled in Texas on 25 October 2011. Is this a significant moment, or is it scarcely relevant in a world of slow nuclear proliferation?
It is arguable that in a context where the full realisation of the power of nuclear weapons is fast receding from collective memory, this landmark is indeed significant - perhaps especially for the generation under 30 years old with no memory of how acute and immediate the nuclear threat appeared as late as the 1980s.
In August 1945, the Hiroshima bomb killed over 100,000 people. Yet many later tactical nuclear weapons were to be far bigger than the bomb that exploded over the Japanese city. The B53 was one. Its destructive power was truly massive: measured in nine megatons (millions of tons of TNT equivalent), and thus 600 times more powerful as that dropped on Hiroshima.
Even the B53, however, was far from the world's biggest: the Soviet Union had a twenty-five-megaton warhead on its SS-9 intercontinental ballistic missile (which was given the appropriate Nato code name of "Satan").
The B53 was basically designed to destroy underground bunkers; as such, it was planned to detonate at ground level, producing vast amounts of radioactive fallout. The fireball created by a single detonation of the B53 - of which 400 were built - would (depending on local conditions) be up to three miles in diameter; but the blast wave would destroy just about every building on the surface over more than eighteen miles.
The B53 would also produce a hugely destructive radiant-heat effect, capable of burning to death anyone caught out in the open in an area with a diameter of thirty-five miles. The process would in effect suck rock, soil, buildings and people into an immense radiated fireball. This means that the majority of the inhabitants of any one of the world's great cities would be killed by a single B53, as they would also by the Soviet equivalent.
The design of the B53 was fairly typical of larger thermonuclear weapons. It involved a three-stage fission-fusion-fission reaction:
* conventional explosives would be used in an implosion mode to compress a form of highly enriched Uranium-235 known as oralloy into a critical mass that would set off a fission reaction
* the temperatures created would then start a more powerful fusion reaction in a mix of two hydrogen isotopes, lithium and deuterium, not unlike the fusion processes that fuel the sun and other stars
* this massively powerful fusion (or thermonuclear) reaction would be so intense as to cause a relatively inert (and common) version of uranium, U-238, to undergo fission; and thus would be used to produce far more fission energy.
The announcement of the disassembly of the last B53 may seem to imply that everything has been taken apart and made safe. This is correct as far as the direct risk of a nuclear explosion is concerned; but the reality is that the highly radioactive cores of B53 and other dismantled weapons, known as "pits", are simply being put into protected storage.
The numbers involved in these long-term deposits are not small. At the peak of the cold war, the combined US and Soviet nuclear arsenals ran to over 64,000 warheads. Most are no longer deployed and many have been dismantled, but the pits are still there and will remain so for many years to come.
The blind spot
From the perspective of the present, the degree of overkill in the the cold-war period seems even more utterly absurd than it did at the time. The US stockpile of B53 bombs alone could have destroyed every major city in the world, as could their Soviet equivalent - and these bombs were but a tiny proportion of the total.
For decades the two sides were locked into a nuclear embrace from which they did not have the wisdom to escape, and were dependent on the supposed stability of deterrence through "mutually assured destruction".
There were two major flaws in this argument. First, it did not envisage untoward and uncontrolled crisis-escalation of the kind that happened and led to several very dangerous near-catastrophes - including the Cuban missile crisis in 1962 and the Able Archer crisis in 1983.
Second, it did not anticpiate nuclear accidents, of which there were many. There is little known about Soviet accidents on land, but more than a score of Soviet nuclear weapons were lost at sea. There are more records for the United States, which also lost nuclear weapons at sea and was involved in land-based accidents that caused serious problems of radioactive contamination.
The history of these extraordinary times is steadily receding for new generations, but it remains vital for one reason above all.
This is that rational individuals, mostly men, were willing or persuaded to engage in an arms race that entailed great peril for the majority of the world's population - yet they were unable to recognise and rise above what they were doing. Many of them, much later and in retirement, changed their minds and acknowledged the fundamental irrationality of what they were doing, but only rarely did they do so at the time. Their motives - in part misplaced patriotism, in part fear of "the other" - offer little justification in relation to the actions which every day they were prepared to undertake.
Perhaps the key point is that in certain circumstances, people may behave in a manner that is potentially catastrophic yet they cannot gain the vision to recognise the consequences. It has happened before and it could happen again. The dismantling of the last B53 thermonuclear bomb is good news - but learning the lessons of the cold war is a matter as much for the future as for the past.