Hi, I know LANL (Los Alamos National Laboratories), Lawrence Livermore and UC Berkeley maintain the most sophisticated super-computer models for predicting radioactive dispersal (in addition to blast effects). As you might imagine, there are hundreds and thousands of variables that enter in and influence the results.
Where I live, next to the Hanford Reservation, they utilized these models while Hanford was actively involved in special materials production, mainly in case of accident. We also have the favor of living 30 miles from the Unmatilla Chemical Weapons Depot, where they are getting close to destroying the sarin stockpiles and are also interested in dispersal patterns.
As far as detection technologies, there are numerous sorts of signatures that special materials like plutonium possess, including the emission of particles and rays. If you wanted to find radioactive material hidden in a city, you might want to use something like a whole-city-block CT Scanner (speculation; don't know if such a thing exists).