Are Suitcase Bombs Possible?
By Carey Sublette Last changed 18 May 2002
It is impossible to verify at the time of this writing whether nuclear devices sized to fit in side a suitcase were actually manufactured by the former Soviet Union, as alleged by Alexander Lebed in September 1997. It is certainly possibel to assess the technicial plausibility of such a claim and to provide a analysis of the likely characteristics of the weapons Lebed described.
A suitcase bomb with dimensions of 60 x 40 x 20 centimeters is by any standard a very compact nuclear weapon. Information is lacking on compact Soviet weapons, but a fair amount of information is available on compact US designs which provides a good basis for comparison.
The smallest possible bomb-like object would be a single critical mass of plutonium (or U-233) at maximum density under normal conditions. An unreflected spherical alpha-phase critical mass of Pu-239 weighs 10.5 kg and is 10.1 cm across.
A single critical mass cannot cause an explosion however since it does not cause fission multiplication, somewhat more than a critical mass is required for that. But it does not take much more than a single critical mass to cause significant explosions. As little an excess as 10% (1.1 critical masses) can produce explosions of 10-20 tons. This low yield seems trivial compared to weapons with yields in the kilotons or megatons, but it is actually far more dangerous than conventional explosives of equivalent yield due to the intense radiation emitted. A 20 ton fission explosion, for example, produces a very dangerous 500 rem radiation exposure at 400 meters from burst point, and a 100% lethal 1350 rem exposure at 300 meters. A yield of 10-20 tons is also equal to the yield of the lowest yield nuclear warhead ever deployed by the US — the W-54 used in the Davy Crockett recoilless rifle.
A mere 1.2 critical masses can produce explosive yield of 100 tons, and 1.35 critical masses can reach 250 tons. At this point a nation with sophisticated weapons technology can employ fusion boosting to raise the yield well into the kiloton range without requiring additional fissile material.
The amount of fissile material that constitutes a “critical mass” varies with the material density and the type of neutron reflector present (if any). A high explosive implosion can compress fissile material to greater than normal density, thus reducing the critical mass. A neutron reflector reduces neutron loss and reduces the critical mass at a constant density. However generally speaking, adding explosives or neutron reflectors to a core adds considerably more mass to the whole system than it saves.
A limited exception to this is that a thin beryllium reflector (thickness no more than the core radius) can actually reduce the total mass of the system, although it increases its overall diameter. For beryllium thicknesses of a few centimeters, the radius of a plutonium core is reduced by 40-60% of the reflector thickness. Since the density difference between these materials is on the order of 10:1, substantial mass savings (a couple of kilograms) can be achieved. At some point though increasing the thickness of the reflector begins to add more mass than it saves since volume increases with the cube of the radius. This marks the point of minimum total mass for the reflector/core system.
A low yield minimum mass or minimum volume weapon would thus use an efficient fissile material (plutonium or U-233), a limited amount of high explosives (sufficient only to assembly the core, not to compress it to greater than normal density), and a thin beryllium reflector.
We can now try to estimated the absolute minimum possible mass for a bomb with a significant yield. Since the critical mass for alpha-phase plutonium is 10.5 kg, and an additional 20-30% of mass is needed to make a significant explosion, this implies 13 kg or so. A thin beryllium reflector can reduce this by a couple of kilograms, but the necessary high explosive, packaging, triggering system, etc. will add mass, so the true absolute minimum probably lies in the range of 11-15 kg (and is probably closer to 15 than 11).
This is probably a fair description of the W-54 Davy Crockett warhead. This warhead was the lightest ever deployed by the US, with a minimum mass of about 23 kg (it also came in heavier packages) and had yields ranging from 10 tons up to 1 Kt in various versions. The warhead was basically egg-shaped with the minor axis of 27.3 cm and a major axis of 40 cm. The test devices for this design fired in Hardtack Phase II (shots Hamilton and Humboldt on 15 October and 29 October 1958) weighed only 16 kg, impressively close to the minimum mass estimated above. These devices were 28 cm by 30 cm…