Reductive capacity measurement of waste forms for secondary radioactive wastes

Um, Wooyong; Yang, Jung‐Seok; Serne, R. Jeffrey; Westsik, Joseph H.

doi:10.1016/j.jnucmat.2015.09.045

Cited by 22 publications

(27 citation statements)

References 17 publications

(24 reference statements)

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“…The reduced, insoluble Tc clusters present in the slagcontaining CWF possessed a first shell S coordination. Thus, reducing conditions develop in the CWF as a result of the sulfur component of the BFS, primarily present as oldhamite Ca,MgS (Um et al 2015).…”

Section: Technetium Sulfide In Waste Formsmentioning

confidence: 99%

Technetium Stabilization in Low-Solubility Sulfide Phases: A Review

Pearce

Icenhower²,

Asmussen

et al. 2018

ACS Earth Space Chem.

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Technetium (Tc) contamination remains a major environmental problem at nuclear reprocessing sites, e.g., the Hanford Site, Washington State, USA. At these site, Tc is present in liquid waste destined for immobilization in a waste form or has been released into the subsurface environment. The high environmental risk associated with Tc is due to its long half-life (213,000 years) and the mobility of the oxidized anionic species Tc(VII)O 4-. Under reducing conditions, TcO 4 is readily reduced to Tc(IV), which commonly exists as a relatively insoluble and therefore immobile, hydrous Tc oxide (TcO 2 •nH 2 O). The stability of Tc(IV) sequestered as solid phases depends on the solubility of the solid and susceptibility to re-oxidation to TcO 4-, which in turn depend on the (bio-geo)chemical conditions of the environment and/or nuclear waste streams. Unfortunately, the solubility of crystalline TcO 2 or amorphous TcO 2 •H 2 O is still above the maximum contaminant level (MCL) established by the US EPA (900 pCi/L), and the kinetics of TcO 2 oxidative dissolution can be on the order of days to years. In addition to oxygen, sulfur can form complexes that significantly affect the adsorption, solubility and re-oxidation potential of Tc, especially Tc(IV). The principal technetium sulfides areTcS 2 and Tc 2 S 7 but much less is known about the mechanisms of formation, stabilization and re-oxidation of Tc sulfides. A common assumption is that sulfides are less soluble that their oxyhydrous counterparts. Determination of the molecular structure of Tc 2 S 7 in particular has been hampered by the propensity of this phase to precipitate as an amorphous substance. Recent work indicates that the oxidation state of Tc in Tc 2 S 7 is Tc(IV), in apparent contradiction to its nominal stoichiometry. Technetium is relatively immobile in reduced sediments and soils, but in many cases the exact sink for Tc has not been identified. Experiments and modeling have demonstrated that both abiotic and biologic mechanisms can exert strong controls on Tc mobility and that Tc binding or uptake into sulfide phases can occur. These and similar investigations also show that extended exposure to oxidizing conditions results in transformation of sulfide-stabilized Tc(IV) to a Tc(IV)O 2-like phase without formation of measurable dissolved TcO 4-, suggesting a solid-state transformation in which Tc(IV)-associated sulfide is preferentially oxidized before the Tc(IV) cation. This transformation of Tc(IV) sulfides to Tc(IV) oxides may be the main process that limits remobilization of Tc as Tc(VII)O 4-. The efficacy of the final waste form to retain Tc also strongly depends on the ability of oxidizing species to enter the waste and convert Tc(IV) to Tc(VII). Many waste form designs are reducing (e.g., cementitious waste forms such as salt stone), therefore, attempt to restrict access of oxidizing species such that diffusion is the ratelimiting step in remobilization of Tc.

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Section: Technetium Sulfide In Waste Formsmentioning

confidence: 99%

Technetium Stabilization in Low-Solubility Sulfide Phases: A Review

Pearce

Icenhower²,

Asmussen

et al. 2018

ACS Earth Space Chem.

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“…If the thermal stability and chemical durability of the selected mineral are 37 sufficient, incorporated Tc may remain within the mineral structure through melting of the 38 glass, thus inhibiting Tc volatility [9]. If used in the off-gas system, the resulting Tc mineral (not 39 containing sulfur or halides) could be recycled back to pretreatment, serve as a standalone 40 waste form, or be incorporated into a low-temperature waste form that is being considered for 41 immobilizing secondary wastes from the WTP [10,11]. 42 have been shown to incorporate Tc, and are therefore potentially suitable for this use [18].…”

Section: Introduction 18mentioning

confidence: 99%

Enhanced 99Tc retention in glass waste form using Tc(IV)-incorporated Fe minerals

Luksic

Wang

et al. 2017

Journal of Nuclear Materials

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8Small-scale crucible melting tests on simulated waste glass were performed with 9 technetium-99 (Tc-99) introduced as different species in a representative low activity waste 10 simulant. The glass saw an increase in Tc-99 retention when TcO 2 ·2H 2 O and various Tc-minerals 11 containing reduced tetravalent Tc were used compared to tests in which pertechnetate with 12 heptavalent Tc was used. We postulate that the increase of Tc retention is likely caused by 13 different reaction paths for Tc incorporation into glass during early stages of melting, rather 14 than the low volatility of reduced tetravalent Tc compounds, which has been a generally 15 accepted idea. Additional studies are needed to clarify the exact mechanisms relevant to the 16 effect of reduced Tc compounds on Tc incorporation into or volatilization from the glass melt. 17

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“…SnCl2 was selected as a source of Sn(II) reductant in solution without a solid interface present. The reduction capacity of Sn-A was measured using the Ce(IV) method, details of which can be found in a recent work by Um et al [25] The performance of the materials was investigated in DIW Millipore ® water (18.2 MΩcm) and a LAW simulant (7.8 M Na Average LAW simulant, [26] referred to as LAW simulant from here on). The composition of the LAW simulant solution was based on the Hanford Tank Waste Operations Simulator (HTWOS) model [27] and is presented in Table 1.…”

Section: Methodsmentioning

confidence: 99%

The function of Sn(II)-apatite as a Tc immobilizing agent

Asmussen

Neeway

Lawter

et al. 2016

Journal of Nuclear Materials

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At the U.S. Department of Energy Hanford Site, Tc-99 is a component of lowactivity waste (LAW) fractions of the nuclear tank waste and removal of Tc from LAW streams would greatly benefit the site remediation process. In this study, we investigated the removal of Tc(VII), as pertechnetate, from deionized water (DIW) and a LAW simulant through batch sorption testing and solid phase characterization using tin (II) apatite (Sn-A) and SnCl2. Sn-A showed higher levels of Tc removal from both DIW and LAW simulant. Scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM/XEDS) and X-ray absorption spectroscopy (XAS) of reacted Sn-A in DIW showed that TcO4-is reduced to Tc(IV) on the Sn-A surface. The performance of Sn-A in the LAW simulant was lowered due to a combined effect of the high alkalinity, which lead to an increased dissolution of Sn from the Sn-A, and a preference for the reduction of Cr(VI).

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Reductive capacity measurement of waste forms for secondary radioactive wastes

Cited by 22 publications

References 17 publications

Technetium Stabilization in Low-Solubility Sulfide Phases: A Review

Technetium Stabilization in Low-Solubility Sulfide Phases: A Review

Enhanced 99Tc retention in glass waste form using Tc(IV)-incorporated Fe minerals

The function of Sn(II)-apatite as a Tc immobilizing agent

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