Could an asteroid impact cause radioactive fallout?
Could a (regional or global) fallout of radioactive material be a "bonus" disaster effect of an asteroid impact? My reasoning for how such a scenario maybe could be possible:
A) Some asteroids maybe consist of lots of radioactive heavy metals because they were initially formed by ejecta from the cores of planets during planetary collisions. Maybe even an airburst of a rather small such an asteroid could cause widespread dangerous radiak fallout?
As I understand it, Earth's initial uranium has fallen into the core. The uranium mines dig into concentrated deposits of uranium in the crust which came from uranium rich asteroids.
B) If any kind of asteroid large enough hits a uranium rich part of the Earth crust (making it the second one on the same spot), it could maybe eject much of Earth's own uranium and thus cause a dangerous fallout?
I'm not aware of any relevant uranium ore deposit, which is related to meteoritic material.
Dangerous fallout is by far the most caused by short-lived radioactive isotopes. Those isotopes are rare in meteorites as well as in rock on Earth.
Natural nuclear reactors, which would produce short-lived radioactive isotopes, don't occur any more on Earth, since the natural U-235/U-238 isotope ratio is too low, now. This is due to the half-life of U-235 of about 713 million years in contrast to 4,468 million years for U-238.
If an asteroid would impact into a uranium ore deposit, most of the molten ejected material would fall back to Earth as tektites.
Evaporated ejecta and aerosols (dust) would probably increase health risk for some time, until it's washed out from the atmosphere. But other, less-radioactive aerosols would be a health risk, too.
Great answer! Do you have any comment about whether uranium ores mined today really have come to be concentrated because of uranium rich asteroid impacts? I have heard this claim but I can't easily find confirmation. Are there asteroids which are much richer in heavy metals, for example those asteroids which come from planetary cores after planetary collisions (if such ejecta is feasible)?
Thanks! Iron-nickel meteorites (http://en.wikipedia.org/wiki/Iron_meteorite) probably come from asteroid/protoplanetary cores. Iridium is probably most famous for its abundance in meteorites. Uranium is used for age estimates of meteorites. But for mining, deposits formed by geologic processes, usually are the first choice. Might be, the claim is related to the georeactor hypotheses (http://www.geoneutrino.nl/Publications/RadPhysChem-georeactor.pdf).
@LocalFluff More on the origin of uranium: http://www.world-nuclear.org/info/Nuclear-Fuel-Cycle/Uranium-Resources/The-Cosmic-Origins-of-Uranium/
A far more likely source of fallout is from humans with itchy trigger fingers launching nuclear weapons when a city suddenly disappears in a cloud of plasma with no warning.