Airplanes killed three thousand people on 9/11, and could have killed a lot more if the towers had fallen sooner before most people were evacuated. The effects of the 9/11 attacks were likened in their consequences to (very small) tactical nukes. But I don't class airplanes as Weapons of Mass Destruction, nor do I consider the fact that Saddam possessed airplanes as proof that he possessed WMDs.
Unless a radiological bomb contains large quantities of highly radioactive substances (which means they have very short half-lives and can't be stored for long periods after their production and prior to their use), and unless the wind patterns and weather conditions are just right, and unless countermeasures are not promptly undertaken, very few if any people will die immediately from the radioactivity. And by the time anyone gets around to developing cancer many years from now, we could easily have a cure for cancer.
I'd be far more afraid of an ammonia nitrate truck bomb set off in front of a crowded office building to kill a couple of hundred people than I would a "dirty bomb". Just because a weapon contains some radioactive substances doesn't automatically make it a WMD. And trying to pretend otherwise is not a very persuasive method of convincing people that Saddam really, really did have WMDs.