Are 'suitcase nukes' a genuine concern?

The allegation

On the 7th September 1997, 60 Minutes broadcast an alarming news item featuring the allegations of former Russian National Security Advisor, General Aleksander Lebed. Lebed claimed that the former Soviet Union had not only manufactured but had lost track of perhaps 100 of a very frightening weapon: a nuclear bomb in a casing which made it appear to be a small suitcase, designed to be detonated by a single operator with as little as a single half-hours notice. Lebed claimed that the devices had a yield of 1 kiloton (equivalent to 1000 tons of TNT), measured 60 x 40 x 20 centimetres (24 x 16 x 8 inches) and, prior to the dissolution of the USSR in 1991, had been distributed to members of the GRU (foreign military intelligence directorate).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.

How a nuclear device works

Nuclear weapons function by assembling the right amount, of the right material, under the right conditions, at the right speed. Thankfully for humanity, given that these weapons are our most destructive innovation, meeting these conditions is easier said than done, and the required materials are very hard to come by. 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.

Critical Assembly

This supercritical mass must be brought together very quickly, otherwise it will be simply blown apart before there have been enough atoms fissioned and before there is any significant release of energy. One method is to fire one piece of material into another. The very first, and unsuccessful, prototype for a nuclear weapon intended to implement this method using plutonium. It was soon realized, however, that this method would only be successful using very highly enriched uranium ... and quite a lot of it. Thus this so called "gun assembly", though simple, is bulky. It was a weapon of this type which destroyed the Japanese city of Hiroshima on August 6, 1945. Smaller, but still bulky gun assembly warheads were in the past tested for use in large US artillery shells, certainly much larger that a suitcase. In regard to suitcase bomb, images that abound on the web showing a gun-type weapon mounted in a suitcase do not accurately reflect just how large such a device would have to be to functionAnother 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.

The W54

As it happens, the theoretical device Subltette describes has physical dimensions closely resembling that of a weapon tested by the United States. This 'W-54"' warhead, in the form of the M388 projectile, formed the heart of a strange weapons system known as the Davy Crocket which was a nuclear recoilless rifle. This man-portable rocket weapon enabled the user to deliver a small nuclear warhead against his enemy. The problem being that the explosion could also potentially deliver a lethal dose of radiation, not only to the enemy but to themselves and any comrades who may be close by! These weapons were actually deployed by US soldiers in the field in Europe during the Cold War, which thankfully never turned hot. The warhead itself was a cylinder of 10.7 x 15.7-inches (27.3 x 40cm).

The SADM

The W-54 device was also made into another form of weapon; the Specialized Atomic Demolitions Munition or SADM. This man portable weapon was intended to be used to destroy structures such as bridges. It was also cylindrical in shape and at 15.7 x 23.6-inches (40 cm x 60 cm), with a weight of 150 lbs (68 kg); it would need to be kept in a rather large suitcase. Details from the former Soviet Union surrounding the type and designation of their nuclear weapons are not readily available in the public domain, though it has been suggested there may have been a similar Soviet device designated as the RA-155. An even more difficult claim to establish is that of Soviet defector Colonel Stanislav Lunev, formally of the GRU, who referred to the alleged missing "suitcase nukes" as being a small nuclear demolitions bomb called the RA-115.

How small can a nuclear device be?

Implosion devices do feature a subtype - those where the fissile mass is not crushed to many times its normal density as it is surrounded by bulky explosive lenses, but reshaped and compressed as it is imbedded in a cylindrical mass of explosives detonated at each end. A football shaped fissile material employed is an alloy of plutonium and gallium which is stable at normal density but needs only a moderate change in density to bring about a shift in its "phase". The amount of fissile material present is in excess of a critical mass when a spherical configuration is achieved and when hollow spaces within the core are collapsed. 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.

However...

These two point linear implosion devices are both very heavy and expensive. The reason is the large quantities of fissile material needed; about 13 kilograms. Various estimates suggest that weapons-grade plutonium costs around $4000 a gram. Admittedly, the price has risen drastically since the end of the Cold War but in any case, any missing nuclear device using the two point linear implosion assembly probably has a salvage price high enough to make it very unlikely that such a weapon would remain intact for terrorist use. Also even taking into consideration the rise in price of fissile material, it seems difficult to believe the Russian government, in the Soviet era or afterwards, would lose track of something not only so dangerous but so valuable!

Hard to believe, harder to prove

The closest actual weapon to a suitcase bomb, U.S SADM, at 68kg, weighed as much as a small adult. Though even smaller devices have been developed using the two point linear explosion principle, the sheer cost of the fissile material required likely rules out that such devices would be allowed to go missing. In any case, a nuclear bomb could not just be hidden for many years until used; they require continual maintenance and upkeep. Even if these devices exist outside of governmental control, they are unlikely to have remained serviceable, though the material they contained could perhaps be put to ill-use. 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)