They can burn. Think of a magnesium fire with lethal smoke. The only 'good' thing is the oxides are so heavy that they rapidly settle out.
.....Think of a magnesium fire ...
I have in mind a uranium bullet that penetrates tank armour and becomes incandescant but remains intact. they are said to rattle around in the tank.
If there was a moab, I don’t think the explosion would be capable of disintegrating the uranium mass. It might be blown away, but is intact where it landed. If intact, it is recoverable.
It was to contain the plutonium that they worked so long and hard to extract and refine, instead of scattering it all over the site.
The confidence in the bomb was such, that it wasn't used, and is still rusting away at the site.
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