Nuclear terrorism forms an important part of the armamentarium of one of the Bush administration's favourite pastimes- threat inflation. While it is true that the potential damage that terrorists could cause with even a 1 kT nuclear weapon is tremendous (Times Square NYC, noon on a weekday), there are many very realistic obstacles they need to overcome before they can acquire, process, build, transport and use any kind of a nuclear weapon.
The more realistic fear that governments and the public have is about dirty bombs, explosives packaged together with low-tech dispersive radioactive material that would largely circumvent the need to achieve the myriad steps needed to be in charge of a bonafide atomic device.
Writing in the Bulletin of the Atomic Scientists, Sonia Ben Ouagrham-Gormley challenges two assumptions made by proponents of a nuclear terrorist attack scenario: access to knowledge and the existence of a nuclear black market (exemplified by black market czar Pakistani scientist A. Q. Khan). Gormley correctly tackles the myth of easy access to nuclear material and knowledge and identifies the slip between the cup and the lip- from knowledge to working product.
She also questions the ease of facilitation of trade in the nuclear black market and doubts the existence of a dedicated clientele, an essential feature of any black market. The clientele should also have the understanding and sophistication to purchase and process nuclear material (In the early days of Al Qaeda, Bin Laden was had when someone sold him mercuric oxide as yellowcake).
Lastly, she questions the nature of materials that have been implicated in nuclear smuggling until now, most of which included depleted uranium and isotopes like Osmium 167, too ineffectual in a dirty bomb, let alone a weapon.
But I think she is missing out on three other important isotopes which are widespread products of research reactors, large scale reactors as well as medical research reactors- Iodine 131, Cesium 137 and Strontium 90. Out of these, Iodine 131 can be absorbed by the thyroid gland and leads to thyroid cancer, but its effects can be thwarted rather easily by ingesting tablets made of normal non-radioactive iodine, provided such tablets are easily available (the slow dissemination of these tablets was partly responsible for the large number of deaths from Chernobyl). Cs 137 and Sr 90 pose more serious problems, and I would think that more than anything else they would be choice materials for a dirty bomb. Both isotopes seem to strike the golden mean for radioactive lethality, possessing half-lives of 30 years and 28 years respectively; long enough to compare to a human life span, and short enough to be intensely radioactive. Moreoever, both elements chemically resemble two key elements in the human body. Cs 137 behaves somewhat like potassium and distributes throughout body fluids and compartments, whereas Sr 90 resembles calcium and deposits in bones, greatly increasing the risk of bone cancer. Both elements if ingested in reasonable amounts will pose almost irreparable risk and cause permanent damage.
I certainly don't think one should be immediately paranoid about these isotopes, but it is clear that if they wanted to, terrorists could steal them from multiple sources. I would think that any perceived scenario involving terrorists and dirty bombs should include discussion of these three isotopes, which because of their ease of access and purity are in some ways much more lethal than uranium or plutonium.