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(up to one week) the transparency of the containers
was noticeably affected, and the base line of the
spectra rose. The net absorbance of the U4' peaks,
however, remained reproducible even though in
some experiments the temperature was cycled from
500 to 700°C. No spectral experiment was continued
for longer than a week. However, in other
studies, melts of 2LiF. BeF, were maintained as
long as one month at 5OOOC without noticeable
damage except loss of transparency (presumably
due to some process of devitrification) of the fused
silica.
The precision of these quantitative spectral measurements
of U4' was better than that possible with
windowless cells.
The molar absorptivity (A,)
of U4+had been observed" to be 14 i 10% and 7 i
10% at 1090 and 610 nm, respectively, in molten
LiF-BeF at S5OoC contained in windowless cells.
With the melts contained in the present silica cells
the A, of U4+ IS 14.4 k 0.5 and 7.3 i 0.3 at 1090
and 640 rim, respectively, in molten 2Lj.F . BeF
at 560OC. The effect of temperature on the Ala of
several absorption peaks of U4' in 2LiF . BeF
was studied. The results are summarized in Table
10.1.
Behavior of Cr3 '. - The solubility of CrF, in
LiF-BeF is being investigated spectrophotometrically
with the melts contained in SiO, cells.
A minimum solubility of 0.43 mole % Cr 3' was obtained
at a temperature of 550°C. This is not the
solubility limit, but the solutions are too opaque
to observe spectrophotometrically in a 1-cm path
length. Further studies will be carried out with
SiO, cells of a shorter path length. The spe-- ctrurn
of Cr 3' in LiF-ReF at 500°C consists of three
peaks at 302, 442, and 690 nm, all arising from
transitions within the 3d level. 'l'he molar absorptivity
of the 690-nm peak was calculated to
be 6.G. This value compares favorably with an
Table 10.1. Effect of Temperature on the Molar
Absorptivity of Several Peaks of U4'
in Molten 2LiF. BeF2
Temperature (OC)
4 98
56 0
650
698
Molar Absorptivity
1090 nm 640 nm
16.4 8.0
14.4 7.3
13.8 6.2
12.9 5.7
approximate value of 7 reported from a cursory
study of the system in windowless cells. l3 The
molar absorptivity of the two other peaks is some-
what higher, about 10, but further work will be
necessary to establish a more precise value.
10.3 E k E: CT RB C A L CON DUC TI VI TY
OF MOLTEN FLPlORlDES
AND FEUOROBQRATES
J, Sraunstein K. A. Romberger
Studies have been initiated to mea.sure the electrical
conductivity of molten fluorides and fluoroborates
of interest as fuel, blanket, and coolant
salts for the Molten-Salt Breeder Reactor. Preliminary
measurements were made to provide estimates
of the electrical conductivities for engineering
use in the design of electrical circuits for
instrumentation in the MSBE. Cell design has
been investigated, including the use of silica in
view of recent indications that dry fluorides may
be stable to silica in the presence of low concentrations
of SiF, (see Sect. 10.2).
Preliminary measurements were made of the conductivity
of 213'-BeF 2, of LiF-'ThF eutectic
(29 mole % ThF 4), and of NaBF by observing
current-voltage curves at 100 hertz and using platinized
platinum wire electrodes in a silica cell at
4000 hertz. l4 Potassium nitrate at 390OC was
used to determine the effective cell constant.
Temperature coefficients were estimated from data
on mixtures of alkaline-earth fluorides with alkali
fluorides or with cryolite l5 and from the data of
Greene
on FLINAK and NaF-ZrF4-UF, mixtures.
The cell used for the current-voltage measure-
ments was similar to that used by Greene l6 and
consisted of a pair of platinum wire electrodes of
0.07-cm diameter separated by about 2 cm and iin-
mersed to a measured depth in the melt contained
12J. P. Young, Anal. Chem. 36, 390 (1964).
'3htSR Program Semiann. Progr. Rept. Aug. 31, 1966,
OKNL-4037, p. 191.
14J. Braunstein to R. L. Moore, private comnunication,
Aug. 14, 1967.
"E. W. Yim and M. Feinleib, J. Electrochem. SOC.
104, 626 (1957); J. D. Edwards et at., J. Efectrochem.
SOC. 99, 527 (1952); hl. de Kay Thompson and A. L.
Kaye, Trans. Efectrochem. Soc. 67, 169 (1931).
16N. D. Grcene, Measurements of the Electrical Conductivity
of Molten Ffuorides, ORNLCF-54-8-64 (Aug.
16, 1954).