Alternative Waste Forms for Electro-Chemical Salt Waste

Abstract

SUMMARYThis study was undertaken to examine alternative crystalline (ceramic/mineral) and glass waste forms for immobilizing spent salt from the Advanced Fuel Cycle Initiative (AFCI) electrochemical separations process (Echem). The AFCI is a program sponsored by U.S. Department of Energy (DOE) to develop and demonstrate a process for recycling used nuclear fuel (UNF). Echem is a molten salt process for the reprocessing of UNF in an electrorefiner and generates spent KCl-LiCl salt that is contaminated with alkali, alkaline earths, halide, and lanthanide fission products (FP) that must either be cleaned of FP and other contaminants or replaced with new salt to maintain separations efficiency. Currently, these spent salts are occluded on zeolite 4A, mixed with a borosilicate binder, and heat treated to form a glass-bonded mineral waste form. The focus of this study was to investigate alternative waste forms to immobilize spent salt. A condensed version of the calculated composition of a spent salt with 20 mass% fission products is given in Table S1.[1] On a mole basis, the spent salt is dominated by alkali chlorides with minor amounts of alkaline earth and lanthanide chlorides. Borosilicate glass is the most widely studied and accepted waste form for immobilizing radioactive waste. [2][3][4][5] However, the limited solubility of chloride in borosilicate glass leads to low waste loading of spent Echem salt. To address this issue, past work with spent electrochemical salt from the electrochemical processing of sodium-bonded nuclear fuel led to the development of a two-step process that resulted in a glass-bonded ceramic waste form consisting of ~ 75 mass% minerals (primarily sodalite) bonded in a borosilicate glass network.[6] The sodalite allows higher loadings of Cl. The glass matrix encapsulates the sodalite and other minerals, supplies silica for the formation of sodalite, and solubilizes some of the FP and impurities.In the study reported here, we explored glass systems that are more compatible with Cl and have not been previously considered for use as waste forms. In addition, alternate methods were explored with the hope of finding a way to produce a sodalite that is more accepting of as many FP present in the spent salt as possible.This study investigated two different options:1. alternate glass families that incorporate increased concentrations of Cl 2. alternate methods to produce a mineral waste form.A literature survey was conducted for each option to identify high halide-containing glasses and minerals. Available fabrication and characterization methods along with available waste-form performance data were gathered from the available literature and evaluated to determine the best candidate glass systems and mineral structures for scoping studies. Scoping studies were conducted based on the conclusions from the literature surveys. [7] glass standard for high-level waste. However, they show promise and should be further developed by investigating other compositions to achieve optimized glass with improved...

Alternative Electrochemical Salt Waste Forms, Summary of FY2010 Results

“…[1] Sodalite and cancrinite were identified as the overall best candidate mineral phases for incorporation of XCl 20 , because of their high concentrations of alkali and Cl, which, when combined, comprise 97 mole% of the waste. [1] The FY2009 scoping studies showed that sodalite can be easily synthesized by the sol-gel process. However, it is produced in the form of a fine powder with particle sizes on the order of a few microns.…”

“…10 ] used in FY2009 is presented elsewhere. [1] ) XCl 10 -loadings of 5-30 mass% were prepared with TeO 2 as the main glass former in a desired ratio, and modifiers (B 2 O 3 , Li 2 O, and Na 2 O) were added separately to improve glass processability. The batches were then placed in Pt/5%Au crucibles with Pt/10%Rh lids to reduce the volatility of chlorides during melting and then melted for ~25 minutes at 600-800°C to verify an optimum melting temperature for the tellurite glasses.…”

“…The following compositions were proposed in conclusion of the FY2009 literature review and experimentation [1] : "Future work will investigate the solubility of mixed chlorides and performance of tellurite binary and ternary systems including B 2 O 3 , P 2 O 5 , Al 2 O 3 , and/or PbO with or without modifiers, e.g., alkali oxides or transition metal oxides (e.g., ZnO, WO 3 , Nb 2 O 5 ). "…”

“…NL Na denotes the sodium normalized release from the PCT. [1] Both of these candidates showed promise in FY2009, and further testing was performed on these waste forms in FY2010 to investigate the possibility of their use in a sustainable fuel cycle. This report summarizes the results of FY2010 efforts.…”

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

“…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.…”