That notion such weapons might exist and that examples of them may be unaccounted for is a worrying thought to say the least! The claim, hotly denied by Russian authorities at the time, generated fears that the bombs may have fallen into the hands of terrorists. Republican Congressman Curt Weldon headed a public inquiry into the perceived risks of these bombs, and was known to carry a mock-up of one to emphasize his points.
Examples of "suitcase nukes" abound in popular fiction, but is it even possible to fabricate a nuclear weapon so small? If so, is it likely that such devices exist and are even missing?
To get to the bottom of this it is necessary to consider what makes a nuclear weapon function.
The material needed must be 'fissile', which means it must be able to undergo a self-sustained fission chain reaction. Examples of such materials are certain isotopes of the elements uranium and plutonium. Put simply, fission is the process by which atoms are split, yielding energy, atoms of other elements, and particles called neutrons. As neutrons have no charge, they are not repelled away when they speed toward an atom (in the same way the two like-charged poles of two magnets will push away). They can strike the nucleus of a fissile atom and split it, yielding, again, energy and more neutrons. This process is repeated a huge number of times within an atomic explosion, all in an extremely short period. If the mass of fissile material reaches the condition where there are the same number of neutrons present than before the previous 'generation', then the mass can be said to be "critical". Any condition where there are more neutrons present than during the previous fission generation can be said to be "supercritical" and this is what is required for a nuclear detonation.
Another method, which works using plutonium, uranium, or a composite of the two, is to compress a mass of fissile material using explosives. In this case, the explosive charges are shaped to focus their energy inwards, in the same way that a glass lens will focus a beam of light. For this reason, the charges are known as explosive lenses. This "implosion assembly" will not actually increase the mass of fissile material present, but will increase its density considerably, allowing it to become supercritical. To aid this, at the center of the fissile mass is a device known as an initiator. The converging shockwaves crush the initiator, bringing quantities of polonium into contact with beryllium. Alpha particles emitted from the polonium liberate a flood of neutrons from the beryllium, helping to initiate the chain reaction. This is how the first nuclear device ever tested worked, and also the device which destroyed the Japanese city of Nagasaki on August 9,1945.
Early examples of both these types of bomb were bulky, though the second type requires less fissile material and with technological progress through the decades, examples have gotten far smaller. The first implosion bombs required a large mechanism to use a discharge of high voltage todetonate 32 or more lenses at exactly the same time. The electronics required to do this, for instance, are far smaller in 2011, or even just prior to 1997 when Lebed's allegations took place, than they were in 1945! Even still, a large quantity of explosives is needed to implode the fissile "core" of a bomb.
In this article from the Nuclear Weapon Archive, Carey Sublette outlined how small these kind of devices may be. He suggested that although it would add to the size of the device, a thin reflector of beryllium would reduce the mass of fissile material needed to produce an explosion, and thus the overall weight. A reflector surrounds the bomb and serves to reflect neutrons back towards its center. Sublette suggests that a fissile mass of around 10.1 kilograms could bring about a nuclear explosion without bulky explosives. The yield from such a bomb would be small; about the same as a few tens of tons of conventional explosive. This is a far cry from the sort of energy which could be liberated from a similar mass of fissile material if there were no size constraints - the device employed against Nagasaki used about 6.2 kilograms of plutonium to yield the equivalent of 22,000 tons of TNT. Such a small yield does not mean that the dangers of this weapon would be trivial as its release of so called "initial" or "prompt" radiation would present a tremendous hazard.
This method of assembling a supercritical mass is known as "two point linear implosion". Using this principle, the United States did develop a device that would fit within a 155 mm artillery shell. This W-48 shell was a cylinder 155mm across and by 846mm long (6.1 x 33.3-inches). Its explosion would have been equal to around 72 tons of TNT, and with it a very dangerous release of initial radiation. If its non-essential bullet-shaped nose cone was not present, and the fusing system was mounted alongside the device, this or similar shells could fit within the 24 x 16 x 8 inch space alleged by Lebed. To bring the device into the kiloton range would require fusion boosting. Here, the tremendous heat and temperature from the nuclear explosion can enable like-charged nuclei of heavy hydrogen isotopes (deuterium and tritium) to fuse together where they would normally push each other part. The result is, again, a release of energy and very high energy neutrons, which go on to strike, and split, fissile atoms. This can be achieved by injecting deuterium and or tritium gas into the fissile core just before the device is detonated, though this gas supply must be replenished and maintained.
Thankfully, the claims of Aleksander Lebed and Stanislav Lunev seem rather exaggerated and are likely to be in the realm of myth. Sometimes people exaggerate, or are genuinely mistaken, but the claims of these two men appear to be the only "evidence" supporting the notion of missing suitcase nukes. In a world where you can't trust former members high ranking members of the Soviet military and GRU defectors, who can you trust? One thing is certain; we cannot ask Lebed, who died when a Russian helicopter in which he was flying as a passenger crashed in 2002.
(Original suitcase image: Linda Bailey)
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