Since we're doing
crazy-but-fun ideas this week, here's another one.
From Wikipedia's
fluoride volatility article, the table of fluoride boiling points says
LiF 1676 C
ThF4 1680 C
A
1958 report gives an equation for the vapour pressure: Log(P/atm) = 7.940-15,270/T (P in atmospheres, T in Kelvin)
equivalently: Ln(P/Torr) = 24.916 -
35,160/T (P in Torr=mmHg, T in Kelvin)
The CRC handbook (75th ed) gives a table for LiF vapor pressures, so we can compare:-
Code:
Pres/mmHg BPt LiF BPt ThF4
400 1591 1585
100 1425 1458
10 1211 1282
1 1047 1139
ThF4 is higher boiling at low pressures, but that is less of a problem than its melting point of 1110 C. A ThF4 still would have to run above 1200 C to give some margin against freezing, so ORNL could never have considered it for Hastelloy equipment. However, we've had lots of discussions about C-C composites or tungsten for the reactor itself. The still is not subjected to neutron damage or the regs for critical systems, so it should be easier. The best material would probably be a B4C/boron fibre-carbon composite, for criticality reasons. If it could be constructed, a ThF4 still would enable the following reprocessing scheme for a 1 or 1.5 fluid reactor:-
1)Fluorinate out the uranium, and hopefully some of the plutonium as well.
2)If there's much TRU's left, electrolyse them out as in the IFR scheme. Some Th will probably get deposited with the TRUs, but it's all going back to the core, so it doesn't matter. This step is to avoid a criticality risk in the still, so if that could be avoided another way, it is not necessary.
3)Distill the LiF/BeF2 and ThF4 away from the fission products. Some cadmium (but the yield is low) and cerium (but the Xsec isn't too bad) may distill over too, but not much.
4)Now the TRUs are more concentrated, take another pass at them in the electrolyser, using some cheap salt mix like NaF/KF as solvent. Dispose of the fission product mix, together with some residual Th. In 500 years time it will be just another mixed Th/rare earths ore, if anybody want to bother digging it up again. (OK, not quite, but sorta...)
Even crazier scheme
UF4, Bpt 1417 MPt 1036, is actually easier to deal with than ThF4, on a physical properties basis. The difficulty, of course, is that the distillate is pure 233UF4, so you don't want more than a few litres of it in one place. The vapour line/condenser/liquid take off pipework would have to be made to be definitely sub critical even if it got plugged full of frozen UF4 from end to end - hence wanting to make it out of boron composites. Carbon-carbon on its own would be unhelpful!
If a UF4/ThF4 still were possible, you could do without the fluorinator/reducer kit, and its possible corrosion problems, as well as the bismuth liquid extraction system. There are some fission products that might be hard to get rid of with just distillation, but the severe neutron poisons like Sm and other heavier lanthanides will all go, and the rest might be tolerable, or could be removed on a very slow schedule by smaller and so cheaper equipment. Distillation is well understood, simple, reliable, quick, and CHEAP, compared to any reactive process. For a CR=1.0 reactor, it might be enough, 1 fluid or two.
Now, what makes this impossible?
(edited to mark the table as 'code' to force monospace font)
(and again to fix typo in ThF4 vapour pressure equation, 25,160 fixed to
35,160)