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ANP QUARTERLY PROGRESS REPORT
UF, Stability
W. C. Whitley R. J. Sheil
Materials Chemistry Division
During the past quarter studies have been under
way on the stability of UF, separately and mixed
with KF. Disproportionation of UF, in KF in
Inconel at 1000°C has been established. When
contained alone in Inconel, UF, does not detect-
ably disproportionate below 125OOC.
Stability of UF, at Elevated Temperatures. The
disproportionation of UF, as a function of temper-
ature was studied earlier under high vacuum.
Recent availability of high-purity UF, made de-
sirable a study of the high-temperature stability
of UF, in a helium atmosphere. For these experi-
ments 25-9 samples of UF, that contained 99.4%
of the uranium in the trivalent form were sealed
by welding in lnconel tubes containing 4 atm of
helium. After being heated at the desired temper-
ature for 1 hr, the tubes were removed from the
furnace and were air quenched to minimize re-
combination of UF, and uranium metal. After the
UF, samples were cooled to room temperature and
ground in an inert atmosphere, they were examined
petrographically, by x-ray diffraction, and by
chemical analysis. The sample heated to 120OOC
showed no evidence of disproportionation, the
sample heated to 125OOC showed very slight
evidence, and the sample heated to 130OOC showed
definite evidence. In addition to the increase in
UF, content noted in the petrographic examination,
an increase in U4' was shown by chemical analy-
sis (from 0.6 to 1.6%), and the presence of uranium
in the tube walls that had been in contact with the
UF3,was established. These data show that UF,
is more sfable at elevated temperatures than
estimates of its thermodynamic properties l5
had indicated. It is interesting to note that the
sample of UF, heated to 13OOOC was practically
insoluble in boiling 10 N HCI, whereas samples
heated to lower temperatures dissolved readily in
this solvent. This could indicate that either a
sintering action occurred, which takes place in
oxide systems well below the melting point, or
14L. Brewer et al., Thermodynamic Properties and
Equilibria at High Temperatures of Uranium Halides,
Oxides, Nitrides, and Carbides, MDDC-1543 (Sept. 20,
1945. rev. ADr, . 1. . 1947).
15A. Glassner, A Survey of the Free Energies of
Formation of the Fluorides, Chlorides. and Oxides of
the Elements to 2500°K, ANL-5107 (Oct. 22, 1953).
60
that the small amount of liquid UF, formed at the
high temperature exerted a cementing action of the
UF, crystals on solidifying.
Stability of UF, in KF. An attempt was made to
determine the stability of UF, in molten KF as a
function of temperature by starting with dry KF
P
and high purity UF, (99.4%). idixtures of KF and
UF, in the molar ratios of KF to UF, of 3.0, 1.5,
and 1.0 were mixed, sealed in lnconel capsules
containing ?$ atm of helium, and heated for 2 hr
at various temperatures before air quenching. A
period of two weeks elapsed between the time of
the experiment and the time that the samples were
submitted for chemical analysis for total uranium
and U3', and thus there is some doubt of the
significance of the U3' determinations. However,
both the total uranium and the trivalent uranium
values exhibited a downward trend with increasing
temperature in the temperature range 750 to 1050OC
for mixtures having a 3 to 1 KF-UF, molar ratio.
An average of 7 moles of UF, disappeared per
gram-atom of uranium lost, presumably by alloying * .
of the uranium metal with the lnconel capsule
walls. Mixtures having 1.5 to 1 and 1 to 1 KF-UF,
molar ratios were heated to 850 and 1050°C. Analyses
of these melts also showed the downward trend
in U3' and total uranium values with increasing
-
temperature. In these samples, however, the ratio
of moles of UF, (or gram-atoms of trivalent uranium)
to gram-atoms of uranium lost averaged about
5 as compared with the theoretical value of 4. The
capsules used to contain these materials were not
analyzed for metallic uranium; however, a similar
experiment was performed in which an 85 mole '%
KF-15 mole % UF, mixture was heated to 1000°C
in an lnconel capsule for 90 min. The melt was
sampled by drilling aiter the top and bottom of the
capsule had been cut off. The remainder of the
melt was dissolved out, and the walls were carefully
cleaned mechanically; then, approximately
7 mils of the tube wall that had been in contact
with the melt was removed by drilling. Flashes
of light observed during drilling gave a qualitative
indication of the presence of uranium in the tube
walls which was confirmed by chemical analysis,
*
which showed 17.9 wt % uranium. The total of
0.28 g of uranium that was recovered from the tube
walls represented 3.6% of the total uranium coni)
tained in the KF-UF, mixture. Analysis of the
melt showed 70% of the total uranium to be in
the trivalent state. It seems certain that dispro-