ORNL-1771 - Oak Ridge National Laboratory

NaF-ZrF, mixtures and in LiF appears to be 90%,
or more, complete, in such mixtures as NaF-KF-LiF
only about 50% of the UF, is reduced. Toward the
end of the quarter it became evident that several
of the materials such as ''3KF-2UF3" previously
believed to contain only U3' regularly contained
large and varying quantities of U4'. Furthermore,
the crystals containing variable quantities of U4'
are distinguishable as such only with great diffi-
culty, if at all, by petrographic and x-ray diffraction
examination, Con sequent l y , the previously reported
data on the solubility of UF, in various systems
must be reinterpreted.
The reasons for the incomplete reduction of the
tetravalent uranium are not yet completely under-
stood, and it is not possible at present to define
the extent to which the reduction will proceed at
various temperatures and in the various solvents.
Accordingly, the significance of much of the rna-
terial presented below is not completely known.
UF, in ZrF4-5earing Systems
The solubility of UF, in NaF-ZrF, mixtures was
previously shown* to increase with increasing
temperature and with increasing ZrF, concentration
of the solvent over the range 47 to 57 mole % ZrF,.
Since the publication of that information it has been
shown that the reduction of UF, in ZrF,-bearing
melts by excess uranium metal is slightly less than
90% complete at 800OC. The temperature de-
pendence of the reduction is not yet known for
this system, but it is likely that the UF, is more
completely reduced at lower temperatures,
An examination of the NaF-ZrF,-UF, system has
been attempted by thermal analysis, with petro-
graphic examination of the resulting solid phases.
In these studies, UF, and excess uranium metal
are added to the desired NaF-ZrF, mixture before
the sample is heated, and the sample is stirred
constantly while in the molten state. Therefore
PERIOD ENDING SEPTEMBER IO, 1954
less than 5 to 6 and that Na3U,F9 appears as the
primary phase iil systems in which the NaF-to-ZrF,
ratio is about 15. It also appears that Na3Zr2F,,
crystals may contain small quantities of UF, in
solid solution. The solubility data obtained from
these and previous studies give little reason to
expect that UF, can be dissolved in NaF-ZrF,
mixtures in sufficient amounts to provide fuel for
ref 1 ec to r-mo d er at ed reactors.
UF, in NaF-KF-LiF Mixtures
Thesolubility of UF, in the NaF-KF-LiF eutectic
was stated previously" to be equivalent to at
least 15 wt % at temperatures as low as 525°C.
Subsequent careful examination of this system ha5
revealed that when UF, and an excess of uranium
metal are added to the purified NaF-KF-LiF mixture
the dissolved uranium species aggregate at least
22% total uranium in the mixture. However, it is
obvious that only 40 to 45% of the soluble uranium
is present as UF, at 8OO0C, while 55 to 60% may be
trivalent at 600°C. Further study of the system will
be necessary before these values can be determined
more accurately.
Thermal analysis data have been obtained for
several mixtures which were prepared from UF, and
the NaF-KF-LiF eutectic and then treated with an
excess of uranium metal. The data obtained, as
shown in Table 5.2, are in agreement with the data
obtained from filtration studies which showed high
uranium concentrations at low temperatures. Petro-
graphic examination of these materialsrevealed that
at low uranium concentrations a red phase(refractive
index, about 1.44), which is probably K,UF, and
which may contain UF, is predominant. At high
"5. M. Watson and C. M. Blood, ANP Quu. Prog. Rep.
June 10, 1954, ORNL-1729, p 53.
TABLE 5.2. THERMAL ANALYSlS DATA FOR
UF3-BEARING NaF-KF-LiF MIXTURES
the reaction
d
%UF, t bUo---- UF,
might be expected to reach its equilibrium value at
any temperature above the melting point. Consequently,
in contrast to experiments in which the
metallic uranium is removed by filtration at high
temperatures, the solid products in slowly cooled
.........
Theoretical Composition
lJo Used
(mole X)*
(% of theory)
NoF KF LiF UF:,
.
10.8 39.5 43.7 6.0 200
10.6 38.6 42.8 8.0 110
Thermal Effects
(OC)
475,455
510,460,455
melts might be expected to be nearly completely
reduced.
These studies indicate that UF, is the primary
10.1
9,6
8.8
37.0
35.3
32.2
40.9
39.1
35.7
120
16-0
23.3
.
110
110
200
520,455,445
565,475
570,490,470
phase in systems in which the NaF-to-ZrF, ratio is *Based on complete reaction of UF4 with U".
57