Alternative Electrochemical Salt Waste Forms, Summary of FY/CY2011 Results

Abstract

SUMMARYThe Fuel Cycle Research and Development Program, sponsored by the U.S. Department of Energy Office of Nuclear Energy, is currently investigating alternative waste forms for wastes generated from the reprocessing of nuclear fuel. One such waste results from an electrochemical separations process, called the "Echem" process, in molten KCl-LiCl salt as a spent salt containing alkali, alkaline earth, lanthanide halides and a small quantities of actinide halides where the primary anion is chloride with small quantities of iodide. Pacific Northwest National Laboratory (PNNL) is investigating two candidate waste forms for the Echem spent-salt, high-halide minerals (i.e., sodalite and cancrinite) and tellurite (TeO 2 )-based glasses. Both of these candidates showed promise in FY2009 and FY2010 with a simplified nonradioactive simulant of the Echem waste. Further testing was performed on these waste forms in the fiscal (FY) and calendar (CY) year of 2011 to investigate the possibility of their use in a sustainable fuel cycle. This report summarizes the results of FY/CY2011 efforts.The chemical durability results for the tellurite glasses in FY2010 revealed a wide variety of waste form performance in these materials. The most promising glass studied was a lead-tellurite glass formulation with the Echem salt simulant. This glass showed very good chemical durability with a normalized release for sodium of 0.478 g/m 2 following a seven-day leaching test. With this in mind, the following studies were undertaken in FY/CY2011 to further investigate and optimize this type of glass as a potential waste form: 1) Expanded study on lead-tellurite glasses. The lead-tellurite glass system was further investigated in FY/CY2011 to determine the waste-loading limits and assess the tellurium-oxygen bonding structure as a function of waste loading with Raman spectroscopy and X-ray diffraction. Te-O bonding in tellurite glasses is quite complex; at least eight different structures have been presented in the literature and typically include α-TeO 4 , β-TeO 4 , TeO 3+1 , and TeO 3 structures (the α/β notation denotes the way that the TeO 4 species bond to one another, and the number after the "O" denotes the number of oxygens bonded to the central Te atom in that species). We also investigated some of the other waste-form related properties of the lead-tellurite glasses, such as the thermal diffusivity, glass transition temperature, crystallization temperature, melting temperature, and mass loss as a function of temperature. The structural investigation revealed that the medium-range order in the glass decreased and the tellurite network was depolymerized as the waste fraction was increased. It was apparent that glass-network breaking was predominantly caused by the additions of the waste cations and that the presence of chloride may have actually caused some repolymerization of the network by scavenging alkali and preventing it from acting as a network modifier. At the highest waste loadings, the glass became phaseseparated, resulting...

Solution-derived sodalite made with Si- and Ge-ethoxide precursors for immobilizing electrorefiner salt

“…As shown in Table 3 [20][21][22][23][24][25][26], the as-made S5C (Si-based) samples had high sodalite fractions at ~82-98% (of the crystalline content). In contrast to previous work where colloidal silica was used instead of TEOS [16], the use of TEOS may be the preferred silica precursor for maximizing sodalite yield in the as-made product without having to prolong the mixing duration. Figure 2 shows the XRD spectra for the as-made S5D (Ge-based) sample as well as the product after a heat-treatment at 750 °C for 8 h. The primary phase (~81.4 mass%) in the asmade sample was Na8(AlGeO4)6Cl2 (Ge-sodalite, ICSD#65664 [27], where ICSD is the Inorganic Crystal Structure Database).…”

“…The NBS-1 and NaAlP binders were provided by collaborators as powders and not made new for this study. The TePbO glass was selected because it has a high loading for the same mixed salt waste that was used in this study [14][15][16][17].…”

Glass binder development for a glass-bonded sodalite ceramic waste form

“…It is possible that this porosity could decrease with (1) larger sample volumes/masses due to more compressive force, (2) longer heat-treatment times due to increased flow of the binder glass, (3) a higher glass:SOZ ratio, (4) and/or hot pressing. In past studies for consolidating pure sodalite or glass-bonded sodalite, a range of processing conditions have been applied in addition to pressureless sintering [24] including cold-press-and-sinter [10,11,13,55], spark plasma sintering [55], and hot isostatic pressing [14,56,57]. It should be recognized that implementing a hot-pressing approach may complicate the process as the scale of the waste form increases.…”

Solution‐Derived, Chloride‐Containing Minerals as a Waste Form for Alkali Chlorides

“…A select few techniques are available for closing the porosity of the sodalite powders that are commonly applied to ceramic or metal powders and include hot pressing methods like hot isostatic pressing and spark plasma sintering, both of which are also helpful to reduce the loss of volatile constituents . We have yet to evaluate these techniques with our powders but expect that these methods would increase our pellet densities and reduce chloride volatility.…”

“…Other alternative options to sodalite are currently under consideration, including alternative naturally occurring minerals that contain the waste components . The work presented here includes a brief overview of the candidate minerals, the methods used to produce some of them, and results from the characterization of the mineral properties.…”