Highly enriched Uranium 235 is the easiest material from which to build a nuclear bomb because you can use the extremely simple (if wasteful) "gun device", Hiroshima-type design instead of a highly complicated implosion design. Using weapons grade Uranium to build a relatively harmless "dirty bomb" is like hitting an enemy over the head with your gun instead of shooting him. It only seems plausible here because the radiological dangers of U235 have been blown out of proportion in the subplot with Bazhaev's son suffering from radiation poisoning.