As I know, uranium and iron simply aren't mixable, which means there are no U-Fe alloys. The Google says, uranium is insoluble in molten iron.
But, if something is "insoluble", it actually means only that there is a very low maximal concentration.
What if we want to mix only 1ppm of U in molten Fe? What if 0.01 ppm? What is the maximal concentration?