Leaching from solidified waste forms under saturated and unsaturated conditions

Oblath, Steven B.

doi:10.1021/es00067a005

Cited by 16 publications

(12 citation statements)

References 2 publications

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“…The structure of TcS x , derived from EXAFS and shown in Figure 2, yields a stoichiometry of Tc 3 S 2 (S 2 ) 4 or Tc 3 S 1 0 , which is almost identical to that determined for Tc 2 S 7 prepared under similar conditions, TcS 3.2…”

Section: Exafs Studies Of Initial Technetium Speciationsupporting

confidence: 60%

“…Sample B remained sealed throughout the experiment. 1.85M NaNO 3 , 1.07M NaOH, 0.57M NaNO 2 , 0.23M NaAl(OH) 4 , 0.16M Na 2 CO 3 , 0.14M Na 2 SO 4 , 0.02M NaCl, 0.02M Na 2 C 2 O 4 , 0.008M Na 3 PO 4 , 0.13M Na 2 S 3 2 2 1. The reductive capacity of the BFS was determined using a slightly modified version of the Angus and XAFS spectra were acquired at the Stanford Synchrotron Radiation Laboratory (SSRL) at Beamline 4-1 using a Si(220) double crystal monochromator detuned 50% to reduce the higher order harmonic content of the beam.…”

Section: Methodsmentioning

confidence: 99%

“…1,2 An important component of this effort is the use of grout based cementitious waste forms (CWFs) to immobilize the low-activity waste stream at the Savannah River Site and to stabilize the waste residues in high-level tanks at both the Savannah River and Hanford Sites. [3][4][5][6] The long-term effectiveness of these measures to prevent the migration of radionuclides is described by performance assessments that depend on the leach rates of the radionuclides. 3,5,7,8 9 9 Tc is one of the radionuclides of greatest concern for leaching from CWFs because of the high mobility and lack of sorption of pertechnetate, TcO 4 -, the most stable form of technetium under aerobic conditions.…”

Section: Manuscript Textmentioning

confidence: 99%

“…In this paper, the evolution of 9 9 Tc speciation in a series of grout samples both sealed and unsealed and with and without NO 3 -was followed for an extended period by X-ray absorption fine structure (XAFS) to determine whether NO 3 -or O 2 was responsible for oxidizing Tc(IV) species in these grout samples. 4 was determined spectrophotometrically at 289 nm (e = 2380 M l -1 cm -1 ). 1 6 UV-visible spectra were obtained using an Ocean-Optics ST2000 spectrometer.…”

Section: Manuscript Textmentioning

confidence: 99%

“…The TcS x initially present in the grout samples was steadily oxidized over 4 years. In the second series of samples, all of the TcO 4 -was not initially reduced, and the grout samples were placed in airtight containers. In these samples, the remaining TcO 4 -continued to be reduced as the samples aged, presumably due to the presence of reducing blast furnace slag.…”

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confidence: 99%

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Evolution of Technetium Speciation in Reducing Grout

Lukens

Bucher

Shuh

et al. 2005

Environ. Sci. Technol.

112

182

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Cementitious waste forms (CWFs) are an important component of the strategy to immobilize high-level nuclear waste resulting from plutonium production by the U.S. Department of Energy (DOE).Technetium ( 9 9 Tc) is an abundant fission product of particular concern in CWFs due to the high solubility and mobility of pertechnetate, TcO 4 -, the stable form of technetium in aerobic environments. CWFs can more effectively immobilize 9 9 Tc if they contain additives that reduce mobile TcO 4 -to immobile Tc(IV) species. Leaching of 9 9 Tc from reducing CWFs that contain Tc(IV) is much slower than for CWFs containing TcO 4 -. Previous X-ray absorption fine structure (XAFS) studies showed that he Tc(IV) species were oxidized to TcO 4 -in reducing grout samples prepared on a laboratory scale. Whether the oxidizer was atmospheric O 2 or NO 3 -in the waste simulant was not determined. In actual CWFs, rapid oxidation of Tc(IV) by NO 3 -would be a concern, whereas oxidation by atmospheric O 2 would be of less concern due to the slow diffusion and reaction of O 2 with the reducing CWF. To address this uncertainty, two series of reducing grouts were prepared using TcO 4 -containing waste simulants with and without NO 3 -. In the first series of samples, the TcO 4 -was completely reduced using Na 2 S, and the samples were placed in containers that permitted O 2 diffusion. In these samples, all of the technetium was initially present as a Tc(IV) sulfide compound, TcS x , which was characterized using extended X-ray absorption fine structure (EXAFS) 2 spectroscopy, and is likely Tc 2 S 7 . The TcS x initially present in the grout samples was steadily oxidized over 4 years. In the second series of samples, all of the TcO 4 -was not initially reduced, and the grout samples were placed in airtight containers. In these samples, the remaining TcO 4 -continued to be reduced as the samples aged, presumably due to the presence of reducing blast furnace slag. When samples in the second series were exposed to atmosphere, the lower-valent technetium species were rapidly oxidized to TcO 4 -.KEYWORDS technetium, cement, grout, nuclear waste BRIEFS The speciation of technetium in reducing grout samples was followed using X-ray absorption fine structure spectroscopy. The reduced technetium species were susceptible to oxidation by oxygen, but nitrate had no observable effect over the duration of the experiment. MANUSCRIPT TEXTIntroduction Immobilization of the high-level nuclear waste created during plutonium production by

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Section: Exafs Studies Of Initial Technetium Speciationsupporting

confidence: 60%

Section: Methodsmentioning

confidence: 99%

Section: Manuscript Textmentioning

confidence: 99%

Section: Manuscript Textmentioning

confidence: 99%

mentioning

confidence: 99%

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