the words "some" and "somewhat" are doing an impractical degree of heavy lifting. Pre-activation material is just not very radioactive and is thus not a terrorist risk. Post-activation material is basically impossible for a non-nation state actor to store and transport. If you just want to dirty up an area by, e.g., spreading radioactive material on a subway, just buy a lot of smoke detectors or do a midnight raid on a hospital or factory radiation source (not advice!). The point is, these are not going to be practical targets.