Structural Characterization of Ternary Salt Melts for Low Activity Waste Applications
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MRS Advances © 2019 Materials Research Society DOI: 10.1557/adv.2019.136
Structural Characterization of Ternary Salt Melts for Low Activity Waste Applications Emily Nienhuis1, Muad Saleh1, John McCloy1, 2 1
Washington State University, Materials Science and Engineering Program
2
Washington State University, School of Mechanical and Materials Engineering
ABSTRACT
Reactions of alkali salts (nitrates, sulfates, carbonates, halides, borates) play a key role in the low temperature feed conversion occurring at the cold cap during processing of Hanford Low Activity Waste (LAW) glass melters. An alkali salt phase can sometimes form, and preferentially incorporate radionuclides of Cs, Cl, I, and Tc. During melting of the slurry feed, some of the feed components sequentially break down with increasing temperature to form gases (i.e., nitratesÆ NOx, carbonatesÆ CO2, and boric acid Æ H2O) or partially volatilize (halides). Sulfate, however, tends not to volatilize but has limited solubility in the final borosilicate glass waste form. To improve understanding of these low temperature processes and their composition dependencies, a scoping study was undertaken to synthesize salt systems that remain amorphous at room temperature, thus facilitating structural study. Melts of equimolar ratios of K2SO4-ZnSO4 (a known ionic glass-forming system) with added nitrates, halides, or carbonates, were melted and quenched. Some of the materials formed single phase glasses and some underwent crystallization upon quenching. Characterization of these quenched materials by thermal analysis, infrared absorption, and diffraction was performed. Addition of other anions to the sulfate base glass resulted in a distortion of the sulfate tetrahedron, as evidenced by infrared absorption. Carbonates strongly promoted crystallization, mostly of carbonate phases. Nitrates promoted crystallization of ZnO, and the nitrate volatilized with some incorporating into the glass. Halides tended to incorporate into the glass, but the small (F) and large (I) halogens promoted crystallization of sulfate-containing crystals, while moderate sized (Cl) halogens produced single-phase ionic glasses.
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INTRODUCTION: The Hanford site in Southeastern Washington State currently houses nuclear waste generated from plutonium production in the 1943-1987 timeframe, resulting in ~56 million gallons of waste that is currently stored in 177 underground storage tanks. This waste has complex varied compositions spanning much of the periodic table of elements and is heterogeneous across each of the 177 tanks. Additionally, the waste consists of water soluble salts, gels, and a broad range of crystalline materials, which separate into a sludgelike layer, a liquid supernatant layer, and sometimes a saltcake [1, 2]. There is some concern with
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