Phase Diagrams at Very Low Concentration of one Component

[doollas]

What happens to one of the components when it is at VERY low concentration in a two component alloy? My problem: Uranium decays to thorium. The thorium isotopes it decays to typically have pretty short half lives. During casting, it is well known that thorium fractionates to the slag, crucible, or pretty much anywhere else besides the uranium melt. However, the thorium is at VERY low concentrations (radiogenic Thorium at parts per billion, typically natural thorium isn't there but if it is, we are still talking at maybe a part per million). When talking with a material scientists thus far, they claim that anything at that low of concentration is too dilute to form any separate phases, so what mechanisms could be driving this fractionation? The phase diagram for the two is below. Maybe I'm reading it wrong and this is obvious to some of you, any help would be greatly appreciated!

 

[mjc123]

"Slag" isn't part of the phase diagram, so it's not exactly a phase diagram question, more of a partition coefficient one - if Th has a higher affinity for "slag" than uranium, that's where it will go, even if on its own it would not phase-separate from U.

 

[doollas]

So that would happen just through random contact due to convection/induction mixing the melt? This fractionation happens in all kinds of scenarios whether it is a bomb reduction, simple casting where there is just an oxide slag, etc. The Th tends to concentrate on just about everything BUT the U, so it sounds like Th has a very high partition coefficient for everything in these scenarios but U, with respect to U (K=>>>>1 [slag,crucible]/[U melt])? Thanks!

 

[mjc123]

Well, don't quote me, 'cos I'm no expert on uranium casting; but "sticking to something else" is conceptually different from "phase-separating from uranium", and in principle, if favourable, will still happen at very low concentrations where phase separation wouldn't be expected to. That's what I thought anyway.