PARTITIONING OF GADOLINIUM IN THE CHEMICAL PROCESSING CELL

Reboul, S. ; Best, D. ; Stone, M. ; Click, D.

Savannah River Site (S.C.)

关键词: Sludges;    Sensitivity;    Testing;    Slurries;    Neutrons;   

DOI  :  10.2172/1013048
RP-ID  :  SRNL-STI-2010-00804
RP-ID  :  DE-AC09-08SR22470
RP-ID  :  1013048

美国|英语

来源: UNT Digital Library

【 摘 要 】

A combination of short-term beaker tests and longer-duration Sludge Receipt and Adjustment Tank (SRAT) simulations were performed to investigate the relative partitioning behaviors of gadolinium and iron under conditions applicable to the Chemical Processing Cell (CPC). The testing was performed utilizing non-radioactive simple Fe-Gd slurries, non-radioactive Sludge Batch 6 simulant slurries, and a radioactive real-waste slurry representative of Sludge Batch 7 material. The testing focused on the following range of conditions: (a) Fe:Gd ratios of 25-100; (b) pH values of 2-6; (c) acidification via addition of nitric, formic, and glycolic acids; (d) temperatures of {approx}93 C and {approx}22 C; and (e) oxalate concentrations of <100 mg/kg and {approx}10,000 mg/kg. The purpose of the testing was to provide data for assessing the potential use of gadolinium as a supplemental neutron poison when dispositioning excess plutonium. Understanding of the partitioning behavior of gadolinium in the CPC was the first step in assessing gadolinium's potential applicability. Significant fractions of gadolinium partitioned to the liquid-phase at pH values of 4.0 and below, regardless of the Fe:Gd ratio. In SRAT simulations targeting nitric and formic acid additions of 150% acid stoichiometry, the pH dropped to a minimum of 3.5-4.0, and the maximum fractions of gadolinium and iron partitioning to solution were both {approx}20%. In contrast, in a SRAT simulation utilizing a nitric and formic acid addition under atypical conditions (due to an anomalously low insoluble solids content), the pH dropped to a minimum of 3.7, and the maximum fractions of gadolinium and iron partitioning to solution were {approx}60% and {approx}70%, respectively. When glycolic acid was used in combination with nitric and formic acids at 100% acid stoichiometry, the pH dropped to a minimum of 3.6-4.0, and the maximum fractions of gadolinium and iron partitioning to solution were 60-80% and 3-5%, respectively. Thus, the presence of glycolic acid increased dissolution of gadolinium, but decreased dissolution of iron. In beaker tests, the fractions of gadolinium partitioning to solution were all less than the minimum detection limits at pH 6, on the order of a few percent at pH 4, and ranging from 70-90% at pH 2. In contrast, the fractions of iron partitioning to solution were all less than the minimum detection limits at pH 6, {le} 0.04% at pH 4, and {le} 0.9% at pH 2. A possible explanation for the small magnitude of these fractions (as compared to the fractions observed in the SRAT simulations) was incomplete equilibrium, due to the short duration (30 minutes) of the beaker tests. As demonstrated by the SRAT simulations, the typical partitioning equilibration time was on the order of hours. The Fe:Gd ratio appeared to impact the extent of liquid-phase conditions under certain conditions, although the exact relationship was not clear. Temperature impacts on the liquid-phase gadolinium concentrations were modest, with liquid phase concentrations typically increasing about 25% as temperatures rose from {approx}22 C to {approx}93 C. The presence of high concentrations of oxalate did not appear to change the liquid-phase gadolinium concentrations - however, it did increase the liquid-phase iron concentrations (from being undetectable to being detectable but still minor). Additional gadolinium partitioning testing is recommended. Of greatest usefulness will be SRAT simulations focusing on a wider range of acid addition scenarios and alternate sludge compositions, particularly those specific to future sludge batches where addition of excess plutonium is being considered.