Fresh on the heels of news that two satellites have collided in outer-space, comes news that two-weeks ago a French and British nuclear submarines collided.

The submarines, which each carried dozens of nuclear warheads grazed each other while their sonars were switched off.

The press, and politicians, have had a field day, the BBC cites Liberal Democrats Shadow Defense Minister as saying “If there were ever to be a bang it would be a mighty big one”.

He is not the only person to either explicity state or imply that the collision could have resulted in a nuclear explosion. He is also wildly wrong.

It should be pointed out, forcefully, that it is physically impossible to detonate a nuclear bomb by bumping it.

Atom bombs work by a runaway nuclear chain reaction. To put it simply: neutrons emitted by the nuclear decay of atomically unstable nuclei (such as those of Plutonium and artificially enriched Uranium atoms) strike other nuclei, provoking their decay (fission) and the emission of more neutrons, ad infinitum. This process releases immense amounts of energy, hence the explosion.

(It should be noted that nuclear chain reactions in non-highly-enriched materials, such as fuel grade Uranium are cannot produce an explosion. Although a self-sustaining reaction is occurring in your local nuclear power-station’s reactors, they cannot ever explode, there simply is not enough available power*.)

To cause the explosive event, it is necessary to have a “supercritical” density of highly enriched fissile material. This can be done by ramming a smaller mass of the material into a larger mass, or by crushing the mass with shaped conventional explosives.

Fusion bombs contain atomic bombs as the trigger so the above still applies.

I repeat: bumping The Bomb won’t set it off. This is high-school physics folks.

In 1972, the U.S. Government sought to raise the sunken Soviet nuclear submarine K-129. Details of this incredible enterprise are outside the scope of this article, however in the course of this partially successful mission, K-129 broke into pieces as it neared the surface, most of it then fell back down to the ocean floor, including its nuclear missiles. No detonation took place.

The real risk is water intake resulting in sinking and loss of crew. Less probably, had a breach of the nuclear power plants on either of the vessels occurred, vast amounts of radioactive particles could have been released into the environment, devastating regional biota. A steam explosion, as cold sea-water contacted the exceedingly hot reactor core is also a possibility in that eventuality.

The likelihood of this ever happening again is almost zero, however I do not expect that will stop the anti-atom lobby from going bonkers over the incident.

*This does not mean that they cannot be exceedingly dangerous if mishandled.