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10.1 OXIDE CHEMISTRY OF ThF,-UF, MELTS
B. F. Witch C. E. L. Bambcrger
C. F. Baes, Jr.
The precipitation of protactinium and uranium
from LiF-BeF ,-ThF , mixtures by addition either
of Be0 or Tho, was demonstrated several years
ago by Shaffer et al. '-' as a possible method for
processing an MSBR blanket salt. The purpose
of the present investigation is (1) to verify that
the oxide solid phase formed at equilibrium with
UF ,-ThF ,-containing melts is the expected solid
solution of UO, and Tho, and (2) to determine the
distribution behavior of Th4' and U4' between the
oxide and the fluoride solutions:
u '(f) + Th '(0) U4 '(0) + 'I'h '(f) (1)
(where f and o denote the fluoride and the oxide
phases).
The experimental technique is similar to that
used in a U0,-Zr02 phase study. The Tho, and
1J0, were contacted with 2LiF * BeF , containing
UF, and ThF, within nickel capsules under a
hydrogen atmosphere in a rocking furnace. The
equilibrated oxide solids were allowed to settle
before the samples were frozen. Provided sufficient
fluoride phase had heen added originally,
good phase separation was thus obtained.
A (U-Th)@, solid solution has been found in
every sample examined thus far, and, moreover,
'J. H. Shaffer et 31., Nucf. Sci. Eng. 18(2), 177 (1964).
'J. H. Shaffcr, G. M. Watson, and W. R. Crimes, Ke-
actor Chem. Div. Ann. Progr. Rept. Jan. 31, 1960,
ORNL-2931, pp. 84-90.
3J. H. Shaffer et af., Reactor Chem. Div. Ann. Progr.
Rept. Jan. 31, 1961, ORNL-3127, pp. 8---11.
,K. A. Romberger, C. F. Uaes, Jr., and H. H. Stone,
J. Inorg. Nucl. Chem. 29, 1619 (1967).
with LiF- 2
136
the lattice parameter determined by x-ray diffrac-
tion was consistent with the coinposition calcu-
lated for such an oxide phase. 'The equilibrium
quotient for the above metathetic reaction was
determined by analysis of the fluoride phase for
the small amount of uranium which it contained.
The results obtained thus far give
Xu(o)
Q - -T!!.('> ~
'u(f) 'Th(o)
1000
to 2000
at 600OC. The mole fraction of uranium in the
oxide phase [Xu(o)] was varied from 0.2 to 0.9,
while the mole fraction of thorium in the fluoride
phase was in the range 0.01 to 0.07.
It thus appears that relative to Th4+, the U4'
present is strongly extracted from the fluoride
phase by the oxide solid solution formed at equi-
librium. There is every reason to expect that *
Pa4' will distribute even more strongly to the
oxide phase. This is in agreement with the ef-
fective precipitation of U4' and Pa4' by oxide
first reported by Shaffer et al. The formation of
a single oxide solution phase to which 'i'h4+,
U4 ', and Fga4'all could distribute offers a much
more flexible and effective oxide separation
method for a single-region breeder reactor fuel
than would he the case if only the separate oxides
('Tho, and IJO,) were formed. For example, a
possible processing scheme for an MSHM might
involve some sort of a countercurreut contactor
(Fig. 10.1) in which a (U-Th)O, solid solution of
5,.. lhese x-ray examinations were performed by G. D.
Bruriton and D. R. Sears of the Reactor Chemistry Di-
vision. The mole fraction of Tho2 in the oxide solid
solutions was calculated from the lattice parameter,
using the equation given by L, 0. Gilpatrick and C. H.
Secoy, Reactor Chem. Div. Ann. Progr. Rept. Jan. 31,
1965, ORNL-3789, p. 241.