ORNL-1771 - Oak Ridge National Laboratory

Preparation of UF3-Bearing Fuels
C. M. Blood
H. A. Friedman
F. P. Boody
F. W. Miles
G. M. Watson
Materials Chemistry Division
Since preliminary results have shown that UF,-
bearing fuels are less corrosive to lnconel than
UF4-beariny fuels, considerable effort has been
devoted to studying methods for preparing these
materials. The experiments performed thus far
have given results that ore not as yet well under-
stood.
The tabulated values for standard free energy of
formation indicate that at 800OC the reaction
$U0 + %UF,+UF,, 4F0 - -16 kcal,
should proceed to essential completion if the com-
pounds are in their standard states. There is ample
evidence that UF, dissolved in NaZrF, is more
stable to reduction by chromium than would be
e~pected,~ but the difference between the expected
and the actual stability is hardly sufficient to
suggest that reduction of UF, by uranium metal
should be detectably incomplete.
The reduction of UF, to UF, with uranium or
zirconium metal has been attempted in a large
number of preparations by the following technique.
About 500 g of material containing the predetermined
quantity of UF, is treated with HF and H, in nickel
equipment in the usual fashion to render it essen-
tially free from contaminants. This purified mixture
is cooled below the melting point, and the desired
quantity of uranium or zirconium metal turnings is
added. The reaction is allowed to proceed while
the mixture in the reactor is stirred by a stream of
hydrogen bubbling through it, Samples of the melt
can be obtained by drawing a sample into a filter
stick of nickel containing a sintered-nickel filter
medium, The specimens removed from the filter
stick are available for petrographic, x-ray, and wet
chemical exam inati on.
The data obtained from several preparations are
presented in Table 5.13. It is obvious that com-
plete reduction was not obtained in any preparation
and that reduction in the NaF-KF-LiF system was
much less complete than in the other solvents.
While the number of experiments performed to date
may be too small to establish reliable trends, the
following pertinent observations cam be made on
the basis of examination of these experiments and
results.
PERIOD ENDlNG SEPTEMBER 70, 1954
UF, in ZrF, Systems. About 90% of the antici-
pated reduction was obtained when large quantities
of reducing agent were used to obtain UF, in
ZrF4-bearing mixtures. In view of he extreme
precautions which must be observed to prevent
oxidation of the material during preparation and
handling, it appears safe to state that about 90% of
the UF, can be reduced under optimum conditions.
No alkali metal was observed under the operating
conditions used. The extent of reduction in these
systems could be qualitatively distinguished by
petrographic examination.
UF, in Molten LiF. More than 90% of the UF, in
molten LiF was reduced in each case, but complete
reduction was not accomplished. It is not possible
to say whether the difference between 0.96 at
825OC and 0.91 at 850°C is real. No alkali metal
vapor was detected. Petrographic examination
detected the presence of small quontities of UF,.
UF, in Molten NaF-KF-LiF.
In five experiments
at 780 to 80OT the maximum reduction obtained in
NaF-KF-Li F was 48% of that expected. The extent
of reduction was only slightly affected by an in-
crease in the reducing agent from 92 to 121% and
by an increase in the reaction time from 3 to 52 hr.
At lower equilibration temperatures the extent of
reduction was increased. The highest value was
obtained at the shortest equilibration time and with
the largest excess of reducing agent.
In all the experiments at 780 to 8OO0C, alkali
metal vapor was evident; however, at 60OoC no
alkali vapor was observed. It is possible that a
part of the difficulty in obtaining complete reduction
is due to reactions of the type
U + 3KF;=--"UF, + 3K0 , AFO = t26.1 kcal,
and
UF, + KFeUF, + KO , L\Fo = t30.3 kcol,
where the metallic potassium being relatively
volatile is removed from the reaction by the hydro-
gen stream. It is also possible that the dispropor-
tionation reaction
UF,&$UF, + tu, AP = +16 kcal,
is responsible. However, it is especially difficult
to see why this reaction should be more important
in an NaF-KF-LiF solution than in an LiF solution.
Under petrographic examination the products of
the eight experiments with NaF-KF-LiF solution
appeared to be identical even though by chemical
analyses they showed widely different UF,-to=UF,
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