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c
principal phase in mixtures containing 75 mole %
KF was believed to be K,UF, while a blue phase
with optical properties similar to those of the blue
NaF-UF, complex was assigned the formula
K,U,F,. It was also previously reported6 that all
KF-bearing reduced-uranium mixtures that were
analyzed chemically contained varying quantities
of tetravalent uranium, usually in a form that could
not be recognized as such either petrographically
or by x-ray diffraction analysis. Some progress
was made during the past quarter in gaining a
better understanding of phase relationships in this
KF system. Investigation of the KF-UF, system
has been considerably aided by beginning the
study of the KF-LaF, system. The compound
LaF, is isomorphous with UF, has almost identi-
cal lattice dimensions, and, furthermore, has only
one stable valence state (34, whereas UF, is
maintained with difficulty. Dergunov7 published
partial pha s for the alkali fluoride-LaF,
systems, and Zachariasen’ has studied mixtures
of KF and NaF with LaF, by an x-ray diffraction
technique, Both workers indicated only a single
compound, KLaF,, formed by these two compo-
nents. According to Dergunov, it melts incongru-
ently at 770OC. It seems probable, therefore, that
the compound that was called K,U,F9 was actually
KUF, with an incongruent melting point of
820 * 10°C. Tests performed during the past
quarter demonstrated that the red crystal line phase
formerly designated K,UF6 can have as much as
75% of the uranium in the tetravalent state. Since
this phase is usually cubic, as is K,UF, and
PERIOD ENDING DECEMBER 70, 7954
phases or a recognizable UF, complex in addition
to the KUF,.
NaF-UF, Systems. A partial phase diagram for
the NaF-UF, system was presented previ~usly.~
No additional data have been obtained sirice that
time, but available information on the NaF-LaF,
systemlo*” indicates that the incongruently melt-
ing compound in this UF, binary system is probably
NaUF, rather than Na,U,F,. Since UF, is the
primary phase that separates from the melt at the
50-50 composition and since UF, has a higher
density than the liquid, it is likely that the earlier
identification of compound composition was in
error because of lack of equilibrium conditions on
cooling.
Solubility of UF, in NaF-KIF-LiF Eutectic
R. J. Sheil
Materials Chemistry Division
Thermal analysis data for UF, dissolved in the
NaF-KF-LiF eutectic composition (1 1.5-46.5-42.0
mole %) were reported earlier.’, In view of current
interest in alkali fluoride fuels, it appeared desirable
to check the earlier data with newer techniques.
A purified mixture containing 4 mole %
UF, was heated in a plastic inert atmosphere box
similar to the one described previ~usly.’~ The
liquidus temperature for this mixture was determined
by visual observation to be 560 + 10c’C. The
mixture was cooled to 5OO0C, and a sample of
liquid was filtered through a nickel filter stick.
Chemical analysis showed that 17.6 wt % uranium
was in solution at 5OOOC as compared with 18.3
wt % in the unfiltered material. This shows that
only 0.2 mole % UF, precipitated in cooling from
‘ 560 to 50OOC a helps to explain why it is difficult
to detect liquidus temperatures in mixtures
s. It is probable
the NaF-KF-LiF
ide a usable fuel
d circulati ng-fuel
Ibid.. p 59, Fig. 5.4.
59