Radioactive material does not "disperse", at least to the extent that it becomes not radioactive by dilution. UF6 is a solid material at normal temp/pressure, though it's a vapor at 135ºF, so likely to sublimate in the Middle East. It's likely to form a dense radioactive dust that has a radiological half-life of 700M years. The main danger is from inhalation of dust or gas and results in lung and other cancers. The main product of U-235 fission is alpha particles which are easily stopped by thin materials. It's those alpha particles, basically helium nuclei, that are a danger for inhalation. They are like a short-range cannon that does damage by kinetic energy transfer, punching through the living cell walls in lung tissue and damaging DNA. The main hope (?) would be that the intensity of the radiation from spread of material would lower its effects below the normal background radiation.
The biggest limit to transport is to keep the concentrated mass below the critical mass; this mass depends on the amount of enrichment, but it looks like a 100 pound sphere is the limit for pure U-235; as UF6 that mass should be much larger with an atomic mass of around 350 vs plain U-235 of, yup, around 235 grams/mole. I don't know if fluorine is a neutron moderator.
At no point is U-235 "glowing." It's unlikely to even be warm. In a cloud chamber a reasonably small chunk will look interesting, but far below the activity of Americium that is used in home ionization detectors for fire alerting.
I recall reading a story of a physicist called in to look over a reactor complex storage facility who noted that while the individual buckets of material were sub-critical, the orderliness and packing on the facility floor were just about at the limit. Being really tidy can be a real problem.
If Iran was interested in dirty bombs they would have been packing this material onto warheads aimed at the Israeli defense manufacturing plants. Even "shot down" the material would continue on a ballistic path to spread on impact and leave the Israelis with an expensive cleanup. They would need dust filter masks and a decent washdown every working day for 1000 years if no other measures were taken to scoop all the dirt up and bury elsewhere.