Behavior of actinide ions during sludge washing of alkaline radioactive. Page: 4 of 12
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BEHAVIOR OF ACTINIDE IONS DURING SLUDGE WASHING
OF ALKALINE RADIOACTIVE WASTES
ANDREW H. BOND,* KENNETH L. NASH,* ARTEM V. GELIS,* MARK P. JENSEN,*
JAMES C. SULLIVAN,* LINFENG RAO**
*Chemistry Division, Argonne National Laboratory, Argonne, IL, 60439 USA
**MS 70A-1150, Lawrence Berkeley National Laboratory, Berkeley, CA 94720 USA
ABSTRACT
It is difficult to accurately predict actinide behavior during the alkaline leaching of Hanford's
radioactive sludges due to the diverse chemical and radiolytic conditions existing in these wastes.
The results of Pu dissolution during experimental washing of sludge simulants from the BiPO4,
Redox, and PUREX processes shows that 2.1% Pu is dissolved during contact with alkaline
media, but up to 65.5% Pu may be dissolved in acidic media. The dissolution of Cr, Fe, Nd, and
Mn has also been observed, and the results of solid state, radioanalytical, and spectroscopic
investigations are detailed.
INTRODUCTION
Various processes were used at Hanford for the large-scale purification of Pu for defense
uses. When production of 239Pu commenced at Hanford in 1944, Pu(IV) was coprecipitated
with BiPO4 while U(VI) remained in solution as a SQ42- complex. After solid-liquid separation,
the Pu(IV) in the BiPO4 precipitate was oxidized to soluble Pu(VI) by BiG3-, MnO4-, or Cr2072-
prior to further purifications that included LaF3 coprecipitation [1]. This solid-liquid-based
separation was replaced eight years later by the Redox process in which Pu(VI) and U(VI) were
partitioned into methyl isobutyl ketone from concentrated aqueous Al(NO3)3. Separation of
Pu(VI) from U(VI) was accomplished by addition of Fe(II), causing reduction to Pu(III) that
reports to the aqueous phase [2]. After only a few years in operation, the Redox process was
replaced by PUREX solvent extraction in which U(VI) and Pu(IV) are extracted from 3-4 M
HN03 into 30% (v/v) tri-n-butyl phosphate in an aliphatic hydrocarbon diluent. Recovery of Pu
again involves reduction to the trivalent state, principally by U(IV) or Fe(II) [2].
An understanding of the processing history and chemistry unique to each Pu purification
process is now important because wastes from these activities must be remediated. Acidic
production waste effluents were adjusted to above pH 9 with NaOH prior to tank storage. The
results of such a drastic pH change are the formation of hydrolytic polymers and large quantities
of NaNO3 that have now stratified into sludge, supernatant, and saltcake layers. Vitrification is
the preferred radioactive waste immobilization strategy, but the borosilicate glass formulations
are sensitive to the presence of the P, Al, and Cr that reside in the sludge materials. Hanford's
alkaline radioactive waste remediation strategy proposes a high temperature leach of the residual
sludge materials with 3 M NaOH [3] to remove the P, Al, and Cr. Based on the available
experimental data, it has been assumed that actinide ions will remain in the sludge during leaching
procedures. Unfortunately, this assumption does not account for the diverse redox and solution
chemistry of the actinides that is readily perturbed by hydrolysis, complexation, solubility,
disproportionation, or redox reactions involving matrix ions [4]. Because sludge washing
operations will encounter a heterogeneous mixture of solids whose thermodynamic and kinetic
behavior with respect to actinide and matrix ion dissolution is only poorly understood, the- x.. 47 ~-~-.-*-~~
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Bond, A. H.; Nash, K. L.; Gelis, A. V.; Jensen, M. P.; Sullivan, J. C. & Rao, L. Behavior of actinide ions during sludge washing of alkaline radioactive., article, November 15, 1999; Illinois. (https://digital.library.unt.edu/ark:/67531/metadc706625/m1/4/: accessed April 25, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.