I refer to this article:

"http://www.strategypage.com/dls/articles2004/2004113.asp"

 

 

Technically the answer is "Yes, but ..."

 

Uranium, even highly enriched uranium, has a VERY long half life (703,800,000 years for U-235, 4.4 billion years for U-238) so it is only mildly radioactive.  It is also an alpha emitter (will not penetrate skin) and is not taken up into the body if ingested, so it has to be inhaled as a dust or implanted in the body (as shrapnel) to have even limited effect.

 

For an effective dirty bomb you want a short half life isotope that is a beta or gamma emitter and biologically active, such as Co-60, Cs-134, Cs-137, or I-131.  These isotopes are over 100,000 x more effective per unit weight than pure U-235 for a dirty bomb. 

 

The catch (usually overlooked), of course, is in the producing, extracting, and storage of the radioactive materials, and preparation of the warhead.  Large scale production usually requires at least one large nuclear reactor and a fuel recycling facility to extract the materials from spent fuel and irradiated materials.  The material then has to be processed to either a soluble liquid or micro pulverized solid for proper dispersal.

 

Storage is another problem.  Half life is just that, after the period of time specified half of the material you started with has converted into something else, probably something stable (i.e. not radioactive).  Simple math shows that your stockpile cannot exceed 2 * (half life in years) * (annual production rate).  Short half life materials can also be hot  both physically as well as radioactively, and large gamma sources require either heavy shielding and remote manipulators, or a huge supply of suicidal idiots.

 

Given the massive radiation emissions, assembling a radiological warhead, attaching it to a missile, and launching it will have to be done remotely.  This will have to be done at the last moment to minimize potential radiation damage to the guidance package.