Getters for improved technetium containment in cementitious waste forms.

A cementitious waste form, Cast Stone, is a possible candidate technology for the immobilization of low activity nuclear waste (LAW) at the Hanford site. This work focuses on the addition of getter materials to Cast Stone that can sequester Tc from the LAW, and in turn, lower Tc release from the Cast Stone. Two getters which produce different products upon sequestering Tc from LAW were tested: Sn(II) apatite (Sn-A) that removes Tc as a Tc(IV)-oxide and potassium metal sulfide (KMS-2) that removes Tc as a Tc(IV)-sulfide species, allowing for a comparison of stability of the form of Tc upon entering the waste form. The Cast Stone with KMS-2 getter had the best performance with addition equivalent to ∼0.08wt% of the total waste form mass. The observed diffusion (Dobs) of Tc decreased from 4.6±0.2×10-12cm2/s for Cast Stone that did not contain a getter to 5.4±0.4×10-13cm2/s for KMS-2 containing Cast Stone. It was found that Tc-sulfide species are more stable against re-oxidation within getter containing Cast Stone compared with Tc-oxide and is the origin of the decrease in Tc Dobs when using the KMS-2

Single Pass Flow-Through (SPFT) Test Results of Fluidized Bed Steam Reforming (FBSR) Waste Forms used for LAW Immobilization-#12252

ABSTRACT Several supplemental technologies for treating and immobilizing Hanford low activity waste (LAW) are being evaluated. One such immobilization technology being considered is the Fluidized Bed Steam Reforming (FBSR) product, which is granular and will be monolithed into a final waste form. The granular component is composed of insoluble sodium aluminosilicate (NAS) feldspathoid minerals. Production of the FBSR mineral product has been demonstrated at the industrial, engineering, and laboratory scales. Single-Pass Flow-Through (SPFT) tests at various flow rates have been conducted with the granular products fabricated using the engineering-and laboratory-scale methods. Results show that the forward dissolution rate for the engineering-scale mineral product is 0.6 (±0.2)×10 -3 g/m 2 d while the forward dissolution rate for the laboratory-scale mineral product is 1.3 (±0.5)×10 -3 g/m 2 d

The effect of metals on zeolite crystallization kinetics with relevance to nuclear waste glass corrosion

Abstract Geologic disposal of vitrified radioactive material is planned in several countries, but there are remaining uncertainties related to the long-term stability of glass exposed to groundwater. Specifically, the crystallization of aluminosilicate zeolite minerals can accelerate the rate at which glass corrodes and radioactive material is released into the biosphere. In this study, we identify elemental species that may accelerate or suppress zeolite formation using a protocol to examine their effects on zeolite synthesis over a three-day duration. Our results are consistent with previous works demonstrating glass corrosion acceleration in the presence of calcium. Furthermore, we identify two elements—tin and lithium—as inhibitors of zeolite P2 (gismondine, or GIS type) nucleation and, thus, promising components for promoting the long-term durability of glass waste forms

Getters for improved technetium containment in cementitious waste forms.

A cementitious waste form, Cast Stone, is a possible candidate technology for the immobilization of low activity nuclear waste (LAW) at the Hanford site. This work focuses on the addition of getter materials to Cast Stone that can sequester Tc from the LAW, and in turn, lower Tc release from the Cast Stone. Two getters which produce different products upon sequestering Tc from LAW were tested: Sn(II) apatite (Sn-A) that removes Tc as a Tc(IV)-oxide and potassium metal sulfide (KMS-2) that removes Tc as a Tc(IV)-sulfide species, allowing for a comparison of stability of the form of Tc upon entering the waste form. The Cast Stone with KMS-2 getter had the best performance with addition equivalent to ∼0.08wt% of the total waste form mass. The observed diffusion (Dobs) of Tc decreased from 4.6±0.2×10-12cm2/s for Cast Stone that did not contain a getter to 5.4±0.4×10-13cm2/s for KMS-2 containing Cast Stone. It was found that Tc-sulfide species are more stable against re-oxidation within getter containing Cast Stone compared with Tc-oxide and is the origin of the decrease in Tc Dobs when using the KMS-2

Removal of TcO₄^– from Representative Nuclear Waste Streams with Layered Potassium Metal Sulfide Materials

Many efforts have focused on the sequestration and immobilization of ⁹⁹Tc because the radionuclide is highly mobile in oxidizing environments and presents serious health risks due to its radiotoxicity and long half-life (<i>t</i>_1/2 = 213 000 a). One of the more common methods for Tc removal from solution and immobilization in solids is based on reducing Tc from highly soluble Tc^(VII) to sparingly soluble Tc^(IV). Here, we report results obtained with two potassium metal sulfides (KMS-2 and KMS-2-SS) that are capable of reducing Tc^(VII) to Tc^(IV). Batch sorption experiments were performed in both oxic and anoxic conditions for 15 d in both deionized water (DIW) and a highly caustic (pH ∼ 13.6), high ionic strength (8.0 mol L^–1), low-activity waste (LAW) stream simulant solution. Tc removal for both materials in DIW is improved in anoxic conditions compared to oxic conditions as a result of a higher solution pH. In DIW and anoxic conditions, KMS-2 is capable of removing ∼45% of Tc, and KMS-2-SS is capable of removing ∼90% of Tc. Both materials perform even better in the LAW simulant and remove more than 90% of available Tc after 15 d of contact in anoxic conditions. Postreaction solids analyses indicate that Tc^(VII) is reduced to Tc^(IV) and that Tc^(IV) is bonded to S atoms in a Tc₂S₇ complex. Examination of the materials after Tc removal by X-ray diffraction shows that the initially crystalline KMS-2 materials lose much of their initial long-range order. We suggest a Tc removal mechanism wherein the TcO₄^– enters the interlayer of the KMS-2 materials where it is reduced by sulfide, which results in a distorted crystalline structure and a solid-state Tc₂S₇ complex

Micro- and Nanoscale Surface Analysis of Late Iron Age Glass from Broborg, a Vitrified Swedish Hillfort

Archaeological glasses with prolonged exposure to biogeochemical processes in the environment can be used to understand glass alteration, which is important for the safe disposal of vitrified nuclear waste. Samples of mafic and felsic glasses with different chemistries, formed from melting amphibolitic and granitoid rocks, were obtained from Broborg, a Swedish Iron Age hillfort. Glasses were excavated from the top of the hillfort wall and from the wall interior. A detailed microscopic, spectroscopic, and diffraction study of surficial textures and chemistries were conducted on these glasses. Felsic glass chemistry was uniform, with a smooth surface showing limited chemical alteration (&lt;150 nm), irrespective of the position in the wall. Mafic glass was heterogeneous, with pyroxene, spinel, feldspar, and quartz crystals in the glassy matrix. Mafic glass surfaces in contact with topsoil were rougher than those within the wall and had carbon-rich material consistent with microbial colonization. Limited evidence for chemical or physical alteration of mafic glass was found; the thin melt film that coated all exposed surfaces remained intact, despite exposure to hydraulically unsaturated conditions, topsoil, and associated microbiome for over 1,500 years. This supports the assumption that aluminosilicate nuclear waste glasses will have a high chemical durability in near-surface disposal facilities.</p

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

At the U.S. Department of Energy Hanford Site, Tc-99 is a component of low-activity 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)