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centration (97% removed). The distribution of prot-
actiriiurn was as follows: 72 2% in the steel wool
plus untransferred salt, 4.3% in the unfiltered salt
transferred away from the steel wool, 8.7% in the
steel liner and dip leg, and 12.0% in saiiiples, for
a total of 97.8% recovered. The data obtained in
Lhis experiment confirmed the resulis of previously
icprted7 experiments that indicate the Rrillo
process may warrant further examination.
Thorium Reduction Followed by Fiitratior!
We reported earlier6 that a large fraction of the
reduced protactinium that would not pass through a
sintered copper filter was found in samples. of unfiltered
salt. This suggested the possibility of collecting
the reduced protactinium on a rrietal filter
from which it could presumably be removed by dissolving
it in a molten salt after passing I-IF through
the filter. We performed several experiments to test
the efficiency of protactinium recovery by filtration.
The initial treatment of molten LiF-ThF4 (73-27
mole %) mixed with enough 231Pa to give a concentration
of 20 to 60 ppm was performed in unlined
nickel pots. The molten salt was treated first with
mixed hydrogen and HF, followed by a brief hydrogen
treatment before it was transferred through a nickel
filter into a graphite-lined pot equipped with a
graphite dip leg. Here the reduction of protactinium
was carried out in the usual fashion, either by exposure
to a solid thorium rod or to thorium turnings
suspended in a nickel basket, taking sainples of filtered
and unfiltered salt after each thorium treatment
of the melt. The reduced melt was then transferred
back into the nickel pot through the transfer filter.
In four experiments the amount of protactinium
found in the transfer filter varied from 10 to 30% of
the total amount present. This represented 40 to
95% of the amouiit of protactinium suspended in the
reduced salt (average, 69%). The amount of protactinium
in the graphite liner and dip leg varied
frorn 20 to 57% of the total (average, 33%). The
data show that a filtiation method will not catch
prntactinium on the filter, but nevertheless the removal
of protactinium from a melt does appear feosible.
One experiment was attempted with a niobium
liner and dip leg. The reduction of protactinium
with thorium proceeded normally (12% of the Pa remained
in the filtered salt after 2 hr of thorium
treatment), but transfer of the reduced salt through
the filter could not be effected because of a clogged
154
filter. A considcrable amount of grayish. material
was found in the bottom of the pot after it cooled to
room temperature. A sample of this material was
reported to contain only 0.35 mg of Nb per g, but
it is quite possible that this aruount of iinpuiity in
the molten salt wodd have been sufficient to clog
the filter.
The effect of iron on the behavior of protactinium
in thorium reduction experiments has noi been defined
imambiguously as yet, but we continue to find
reasonably good correlation between the distribution
of iron and protactinium in these experiiiients. Counting
of 233Pa and 59Fe in both solid samples and
solutions of samples provided a check on the accuracy
of 23 'Pa alpha pulse-height analyses and
colorimetric iron determinations.
Concl us ion
- 1 horium metal is an effective agent for reducing
protactinium in molten fluoride breeder blanket
mixtures, but further study will be required to determine
the best method of separating the reduced
protactinium from the salt mix.
R FUEL REPROCESSING BY
REDUCTIVE EXTWACTaCaN INTO
MOLTEN B!SMUTH
I). M. Moulton vir. R. Grimes
F. F. Blankenship J. H. Shaffer
An electromotive series for the extraction of
fission products frorn 21,jF.BeF2 into liquid bis-
miiih has been constructed in the way described for
the MSBR blanket materials in the preceding ieport
in this series. Briefly, standard half-cell reduc-
tion potentials are calculated for each metal, using
as the standard states the ideal solutions it1 salt
and bismuth extiapolated from infinite dilution to
unit mole fraction. 'The exception is lithium, for
which the standard state in sa1.t is 2LiF.BeF2.
(This standard state is also used for beryllium, but
it is not assigned a standard state in bismuth be-
cause of its very low solubility.) This choice of
standard states is the normal practice when deal-
ing with dilute solutions. These standard poten-
3~ i
tials are called 'P d to distinguish thein from yo,
9.VSH Program Serniann Pro&. Rept. Feb. 28, 1367,
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p. is(?.