What is the most likely radiological device that a terrorist might use in a Weapons of Mass Destruction incident?

Answer 1

A radiological device (or "dirty bomb") consists of a normal explosive device placed next to (or encased inside of) some sort of radioactive material. Generally speaking, the most likely candidates for this radioactive material are:

Cobalt-60 : obtainable from many medical radiation devices (old NMRI or X-ray machines of various types). While only a few ounces of Co60 would be obtained from any one device, it is rather pure, and finding older devices in junkyards isn't terribly difficult.

Thorium : obtainable from any "rare-earth" mine, as a waste product. It would require some refinement to remove much of the "junk" inside the mine tailings. But this is not terribly difficult. The amount of reprocessing would likely be substantial, however (that is, to get a good amount of Thorium, you'd have to likely reprocess several tons of waste).

Spent Reactor Fuel : this is perhaps ideal. It contains all sorts of various radioactive materials, and in a concentrated form. It is also obtainable with some modest effort.

Uranium tailings : obtained from any uranium mine, the waste tailings are generally sufficiently radioactive to be useful in a radiological bomb. Minor reprocessing to remove non-Uranium (i.e. ordinary rock) from the tailings would significantly improve the purity.

Cesium (particularly Cs-137) : is found in a variety of industrial and medical devices. However, it generally is only found in very very small quantities, or else in some form of compound requiring significant processing to remove the pure Cs. This is probably the least likely source.

Overall, for a well-financed and well-organized terrorist organization, I would think that hijacking or stealing spent reactor fuel is the best source for creating a dirty bomb. If they can set up a small reprocessing plant (which isn't difficult to do) in some remote location, Thorium tailings are the likely choice, since they're much easier to steal and no-one is keeping track of Thorium thefts.