Samizdata

Every time I see or hear the media talking about “dirty bombs” my teeth grate. Dirty bombs are nuclear weapons that are either too inefficient to “burn” most of their fissionables in the explosion or else are built intentionally to spread fallout. The media almost always means what is now being called a radiological weapon, something like a terrorist’s standard anti-personnel bomb, but packed with bits of radioactives lifted from medical equipment, old watches or what have you, instead of rusty nuts, bolts and nails.

Any idiot can build one, but they will probably fail to get more than local dispersal. Bad enough in fact; but by the time the SafetyNuts are done you’d have an entire city evacuated for years over the risk of a handful of extra cancers per year. So this is more mass disruption than mass destruction unless the enemy acquire some really nasty radionucleotides and manage a good dispersal within the desired target footprint.

The first type of real “dirty bomb” is what we will almost certainly get from the enemy. It is dirty because they don’t know how to make their fissionables fission before the plasma blows itself apart. When the fission process occurs there is a chain reaction in which a neutron splits one nuclei, which releases at least one more neutron and on average “a bit” that in turn cause another one or two atoms to do the same. This exponential growth happens so quickly it has no meaning on a human scale.

But on the bomb level it does. Each atom releases a great deal of energy; that radiated energy turns the fissionables and the bomb casing into a plasma racing outwards at an enormous speed… which is still slow relative to the neutron cascade.

If the cascade happens fast enough, virtually all of the Uranium 235 or Plutonium fissions and releases energy before the wisp of plasma blows the ghost of the bomb matter outwards. If the cascade is a bit slow the “flame” is extinquished and the cascade stopped because the remaining fissionables are too far apart to have much chance of a neutron hit.

Those unfissioned materials become a nasty bit of the fallout. It is highly unlikely that an “amateur” bomb will get things right the first time, so one expects 5% or less of the material will “burn”. The other 95% gets dispersed in the mushroom cloud. This is the scenario all of us “in the know” fear: a couple kiloton fizzle with a lot of very hot debris and a deadly cloud drifting down wind.

The second type of dirty bomb is even uglier. If you re-read your ’50’s literature, you’ll run across the “Cobalt Bomb”. This is a thermonuclear weapon packed with Cobalt or other materials that will generate nasty and long lived fallout. The Doomsday bombs of yore.

To my knowledge, no one ever built one. It’s not a terrorist likelihood as H-Bombs are out of range of the technology of anyone but a major state at present… without help that is.

A third type, which falls between the cracks of the definition, is the Neutron Bomb. This is an extraordinarily “clean” bomb with very low yield. It causes limited blast damage but it gives off an extremely intense pulse of neutrons. These kill everything (except roaches and Radiodurans bacteria) in a wide radius. The buildings are untouched and the people are dead.

These were tested but not deployed¹. It is doubtful anyone could build one without extensive experience or testing.

Let’s pray none of us ever learn about these things first hand.

¹= A clarification for those who want precision: a large number were built, but after a big political battle over deployment in the Reagan years, they were put into storage. They were perhaps disassembled during Bush Sr’s Presidency. I do not know if any of these tactical “Enhanced Radiation Weapons” remain in the US inventory.