I assume it is for the same reason as with steel[1] - the refinement process involves atmospheric air, and after we started detonating nuclear bombs, we raised

I assume it is for the same reason as with steel[1] - the refinement process involves atmospheric air, and after we started detonating nuclear bombs, we raised the background radiation level of the atmosphere, and thus of the metals we produce. [1] https://en.wikipedia.org/wiki/Low-background_steel https://en.wikipedia.org/wiki/Low-background_steel