Nuclear Detonation

The threat of the detonation of a hydrogen nuclear warhead by striking one with another object such as a missile or the shrapnel from a nearby explosion lurks in a scriptwriter’s creative mind for many war and adventure films. Suppose the nuclear warhead is aboard an ICBM and is struck by an interceptor missile or the warhead itself is penetrated by fast-moving BBs. What will happen?

Answer

We know of no nuclear explosive device that does not require at least very good spherical symmetry to be detonated. The simpler atomic devices have either two hemispheres that must be rapidly moved together into a sphere, or two spherical sections held apart until a slab of nuclear material is shot into the gap to start the fission reaction. The more complicated hydrogen devices require a strong, spherically symmetrical implosion from the perimeter shell to initiate the fusion reaction.

Dropping the weapon from any height would damage the casing asymmetrically. Even shooting pellets or bullets, etc., through the casing into the warhead would create an asymmetrical result but no explosion. Initiating the nuclear reaction and keeping the reaction going are not easy. The process is certainly not worth worrying about to the extent portrayed in the movies. One should be more concerned about whether the proper safety precautions are being practiced against accidentally dropping the thing on one’s toes.

By the way, the smallest practical nuclear weapon tips the scales at only about 9 kilograms, about 20 pounds, and is small enough to fit into a bulky attach case. For the physics details and an estimate of the smallest device, see the reference below.