HOT-CELL EVALUATION OF THE GRIND--LEACH PROCESS. I. IRRADIATED HTGR CANDIDATE FUELS: PYROCARBON-COATED (Th,U)C$sub 2$ PARTICLES DISPERSED IN GRAPHITE. Page: 17 of 32
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retention of heavy metals by the leached graphite was the highest
encountered in our hot-cell studies.) Retention of principal fission
products was comparable to that observed in previous experiments; as
much as 42% of the 144Ce, 8.3% of the 137Cs, and 3.6% of the 106Ru
remained in the graphite.
The complete data (Table A-8 in Appendix) show variable results for
the leaching of uranium and thorium. Somewhat better reproducibility was
demonstrated in leaching '44Ce and 137Cs than for the uranium and thorium,
probably because of the refined leaching procedure and the greater pre-
cision provided by gamma scanning. Although adequate techniques were
developed for analyzing uranium and thorium in unirradiated graphite
residues, these techniques are undoubtedly subject to error when applied
to the "hot" analytical facility. Also, it is possible that the fraction
of heavy-metal carbides "fixed" in the pyrocarbon coating by diffusion or
fission recoil may vary significantly within a single irradiated fuel
compact; in this case, grinding the fuel to only -100 mesh (149 4) could
produce segregation of the dense (Th,U)C2 kernels from the relatively
light graphite fines. Also, passage of the pulverized fuel through a
sample splitter may be insufficient to produce completely homogeneous
samples with regard to the distribution of the shattered pyrocarbon coat-
ings.
5. DISCUSSION AND CONCLUSIONS
For convenience in reviewing the pertinent results of this study, all
grind-leach experiments are summarized in Table 3. The data in this table
indicate that two leaches (total contact time, 7 hr) will recover all of
the readily dissolvable (Th,U)C2; additional leaching is quite unproductive.
The recovery of heavy metals is about the same whether the leached graphite
is washed with water or with fresh leachant; however, washing with boiling
water is preferred because vacuum filtration in this instance is about
three times as fast as with concentrated nitric acid. The recovery of
heavy metals is not increased by making the leachant 0.05 M in HF; however,
leachants containing HF were used in most of the hot-cell runs on the
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Flanary, J.R. & Goode, J.H. HOT-CELL EVALUATION OF THE GRIND--LEACH PROCESS. I. IRRADIATED HTGR CANDIDATE FUELS: PYROCARBON-COATED (Th,U)C$sub 2$ PARTICLES DISPERSED IN GRAPHITE., report, January 1, 1967; Oak Ridge, Tennessee. (https://digital.library.unt.edu/ark:/67531/metadc1023399/m1/17/: accessed April 23, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.