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NATIONAL BUREAU OF STANDARDS -1963
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CHEMICAL SEPARATIONS IN MOLTEN FLUORIDES
F. F. Blankenship

Reactor Chemistry Division AELEASED FOR ANNOUNCEMENT
Oak Ridge National Laboratory
Oak Ridge, Tennessee IN NUCLEAR SCIENCS ABSTRACIS
Among the advantages of fluid fuels is their amenability
to convenient reprocessing; expensive refabrication by remote
handling procedures is there by avoided. Several methods of
removing neutron poisons have been explored recently. The
feasibility of breeding depends on how well the poisons can
be removed.
The problem of poisoning by 135 Xe is a case in point.
If all the xenon produced is burned, the consequent poison
fraction rises to an intolerable 5%. The problem is accen-
tuated in thermal breeders that are moderated by graphite in
the core. The voids in the graphite, totaling in volume to
approximately the amount of fuel in the core, serve as
readily available sinks for the xenon. The effect is suf-
ficiently pronounced to virtually defy cure by stripping
unless a graphite of greatly decreased permeability is
developed. More promising is the possibility of removing
Research sponsored by U.S. Atomic Energy Commission
under contract with the Union Carbide Corporation.
LEGAL NOTICE
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This report was prepared as an account of Government sponsored work. Nelther the United
States, nor the Commission, nor any person acting on behalf of the Commission:
A. Makes any warranty or representation, expressed or implied, with respect to the actu-
racy, completeness, or usefulness of the information contained in this report, or that the use
of any information, apparams, method, or process disclosed in this report may not Infringe.
prinitely owned righto; or
B. Assumes any liabilities with respect to the use of, or for damages rosulting from the
use of any information, apparatus, method, or process disclosed in this report.
As used in the abovo, "porson acting on behalf of the Commission" Includes any eno-
ployee or contractor of the Comm:1ssion, or employee of such contractor, to the extent that
such omployee or contractor of the Commission, or employee of such contractor prepares,
disseminates, or provides acceso to, any information pursuant to his employment or contract
with the Commission, or his employmert with such contractor.
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1351, the precursor of 135 Xe, by sparging with HF as studied
by Baes and associates.'
The fuels are mixtures of fluorides such as LiF and BeFz
containing UF, and possibly other fluoride additives such as
ZrFe. Because of the mild reducing action of the container
metal, the fission product iodine occurs in the form of iodide
ion, i°. The reaction involved in removal of I“ is
HF + I" -- F
+ HIT
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A constant quotient,
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[HI]
THF] [15]
QF
holds under equilibrium conditions; Qc remains constant for
a given" solvent and temperature because the solute concentra-
tions are in the range where Henry's Law holds. For sparging
under equilibrium consitions, the fraction of iodide removal
is given by
:
on by
in ([1°]][1–)") = -Q(1Hp/W) .-
The terms [1"]° and (1") are, respectively, the concentrations
of iodide present initially and of iodide present after nye
moles of HF have been passed through Wkg of melt.
The appara tus employed in studying iodide removal was.
a simple nickel pot with a dip tube for inlet gas and a
thermocouple well. At the exit from the pot, where HI could
react with the nickel walls because the temperature was lower,
a gold lining was to prevent the reaction; similarly, Teflon
tubing carried the exit gas to the analyzer portion of the
assembly. To avoid corrosion, the inlet gas was a mixture
of HF in at least 90% hydrogen. A 10-cm bubble path was
adequate to achieve equilibrium.
Values of Q were determined over a temperature range
from 434 to 635°C; Q decreased with increasing temperature,
ranging from 50 to 15 kg/mole.
Since the half-time for decay of 1351 to 135 Xe is 6.7 hr,
iodide-removal half-time of about an hour might be appropriate.
With Q = 40 kg/mole (2LiF. BeF2 at 800°C), half the iodide
present in a reactor fuel could be removed in 1 hr by passing
a minimum of 388 cc of AF per kilogram of fuel per hour. This
is quite a modest requirement, but of course a final stripping
with H, or He will be required to remove the HF.
Rare earths are the next most important poisons among
the fission products. They constitute 22 at. % of the fis-
sion products but cause 40% of the poisoning. .
It appeared possible to take advantage of the fact that
the rare-earth fluorides have considerably lower vapor pres-
sures than the fuel constituents. This possibility has been
studied by Kelly,' and the prospects appear favorable for
removal of rare earths in a heel that remains after distilling
UL
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the fuel constituents.
Distillation in the most recent experiments was carried
out with a graphite pot and receiver enclosed in an inverted
U-tube. The pot had a capacity of about 17 g fuel and an.
exposed fuel surface area of 1 cm. Distillation was started
or stopped by evacuating or admitting an overpressure of
helium. Rates of distillation were of the order of a few
milligrams per square centimeter per second, wkich were ade.
quate from the standpoint of feasibility. The rate for Lif
was determined first, then successive portions of the solvent
for the Molten-Salt Reactor fuel were added and distilled.
Each repetition brought the proportions of constituents in
the doses to the composition that would yield fuel solvent
(LiF-BeF:-ZrF4, 65-3.-5 mole %) as product; this was LiF-
BeFz-ZrF, at 85.4-10.7-3.9 mole %.
Analyses of the residue and condensate obtained during
the distillation provided a measure of the relative volatility
of BeF2 and ZrF, compared to Lif. In the LiF-rich region
that was studied, the activity coefficient of LiF was near
unity, and accordingly estimates of the activity coefficients'
of BeF, and ZrF, could he obtainod. These were about 2 x 10-2
for BeF2 and 5 x 10-4 for ZrF4, in approximate agreement with
previous estimates based on other methods.
In the course of distillation NdF, was added to the pot;
de :ontamination factors of the order of 100 were observed. -
This was in line with expectations based on vapor pressures
and with other measurements as well.
Attention has also been paid to the possibility of
removal of rare earths by chemical reduction. In beryllium-
containing melts, such as those of current interest, there
is a difficulty; standard free energies of formation show
that Be Fz is reduced more easily than the rare-earth fluo-
rides. Thus, the desired reduction can supersede that of
beryllium only if some special sink is provided in which
the reduced rare-earth metals have an unusually low activity.
An obvious choice for trial as a reducing agent 18 beryl-
lium metal itself; in this case the oxidized product would
not constitute a new ingredient in the salt mixture. Actu-
ally beryllium proves to be very effective in reducing rare
earths from LIF-BeFz mixtures, as demonstrated by Cathers
and Schilling, who used both beryllium and other reducing i
agents strong enough to reduce beryllium. The reduction of
rare earths proceeded in either case with the formation of
extremely stable internetallic beryllides, having the for-
mula (R.E.)Be, s . In most of their experiments Cathers and
Schilling used liquid alloys of active metals such as alumi- ..
num or lithium as the reducing agent. The interme tallic
compounds that formed were not soluble in either the metal
layer or the salt layer. As far as removal of rare earths
from the salt was concerned, however, the decontamination fac-
tors were more than adequate. Among the rare earths, neodymium
is one of the worst poisons. A reprocessing decontamination
factor of about 20 is required for neodymium if the increase
in uranium 16:ventory to override the poisoning is to remain
under 0.1%. Actually, decontamination factors as high as 52
for neodymium have been obtained using beryllium as the reduc-
ing agent. The solids handling problem ansociated with the
formation af the insoluble beryllides is a disadvantage, but
if well-behaved slurries or "creams" couid be formed in
heavy liquid metals, then the same procedures might be used
as with true solutions in the metal layer, in which latter
case only liquids are handled. Since it is highly probable
that the beryllides are wet by liquid metal, the proposed
slurries should be formable. Cathers and Schilling also
found that zirconium was readily removable by reduction; this
is one of the fission products not removable by distillation.
Grimes and Shaffer took advantage of the very low activity
coefficient of metallic rare earths in liquid bismuth in devis-
ing a method of reducing and extracting rare-earth cations
from Liz BeF, and related melts.* Their experiments were car-
ried out with 2 or 3 kg of bismuth covered with about 2 kg of
salt in steel liners. Rare earths, spiked with radioactive
isotopes, were added to the salt at a concentration of 10-4
mole fraction. Lithium, that served as the reducing agent,
was added in increments directly to the bismuth. After each
addition both layers were sampled. A plor of the logarithm.
of the Liº concentration in the bismuth against the logarithm
of the ratio of rare earth in the metal to the rare earth in
the salt gave the expected straight lines. For reasons
that are not yet clear, some of these lines did not have
the expected slope. Nevertheless, it was found that a con-
centration of 0.02 mole fraction Liº in the bismuth was suf-
ficiently reducing to remove from the selt nearly all the
cerium, lanthanum, and neodymium, as well as substantial
quantities of samarium and europiun. At the low reduction
potentials to which the 0.02 mole fraction alloy corresponds,
no formation of the insoluble beryllides occurs and thus
only liquids need be handled. Regardless of which strong
reducing agent is employed, lithium alloys are the end
product. Current work on this method involves measuring
the activity coefficients of lithium and the rare ear the
in the bismuth. Preliminary results gave coefficients (based
on mole fractions) of the order of 10-s for Liº, and 10-14
for the rare earths.
Protactinium removal from blanket melts is an important
aspect of breeding from ThF. Several methods of accomplish-
ing this have been tried. Shaffer and McDuffie showed that, .
in simulated blanket mixtures in which Zroz was insoluble,
protactinium was removed from solution by deposition on Zroz.
Coprecipitation of protactinium with other insoluble oxides
had been demonstrated previously.
In view of the good results with reduction and extrac-.
tion of rare earths into bismuth, a" similar procedure, with
thorium as the reducing agent, was tried for protactinium
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removal. Shaffer and Grimes found that 233 Pa, at tracer
levels, would remain indefinitely in blanket melts (LiF-BeF2-
Thf,, 73-2-25 mole %), but that on treatment with thorium,
the 233 Pa left the melt and appeared on metal walis, whether
a layer of bismuth was present or not.' The fact that the
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protactinium showed a predilection for solid metals rather
than liquid bismuth was disadvantageous, but attempts to use
liquid bismuth alloys as transfer agents are in progress.
Additional experiments with 231 Pa at realistic concen-
trations of the order of 30. ppm were carried out by Barton
in a glove box facility. The substitution of 23 3 Pa for
231 Pa circumvents shielding problems associated with the
high gamma activity of 231 Pa and provides a convenient means
of analysis in the form of the alpha activity of 23 3 Pa. In
general the behavior encountered by Barton resembled that
manifested in the tracer-level experiments. Thorium appeared
to be a suitable reducing agent for protactinium. However,
the reduced product tended to remain suspended in the salt
phase in a form that would not pass through sintered metal
filters having a pore size of 0.0015 inches. Experiments to
obtain protactinium deposition dy electrolytic reduction are
in progress.
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References
1. Reactor Chem. Div. Ann. Progr. Rept. Dec. 31, 1965,
ORNL-3913, p. 38-40.
2. Ibid., p. 35-38.
3. G. I. Cathers and C. E. Schilling, Chem. Tech. Division
Ann. Progr. Rept. May 31, 1965, ORNL-3830, p. 303; also
in Chem. Tech. Div. Ann. Progr. Rept. 1966, in press.
4. MSRP Semiann. Progr. Rept. Feb. 28, 1966, ORNL-3936,
p. 141-5.
5. Ibid., p. 145.
6. J. H. Shaffer et al., Nuc. Sci. Eng. 18, 177 (1964).
7. MSRP Semiann. Progr. Rept. Feb. 28, 1966, ORNL-3936,
p. 147.
8. Ibid., p. 148.
Th1
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END
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DATE FILMED
12/ 23/ 66




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