Low Temperature Sequential Melting and Anion Retention in Simplified Low Activity Waste
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MRS Advances © 2020 Materials Research Society DOI: 10.1557/adv.2020.52
Low Temperature Sequential Melting and Anion Retention in Simplified Low Activity Waste Emily T. Nienhuis1, John S. McCloy1,2 1
Materials Science and Engineering Program, Washington State University, Pullman, WA
2
School of Mechanical and Materials Engineering, Washington State University, Pullman, WA
ABSTRACT This study seeks to understand the low temperature reactions of the salt phase that occur during the vitrification of Hanford Low Activity Waste (LAW). Salts (such as nitrates, sulfates, carbonates, halides, etc.) play a key role in these low temperature reactions as they sequentially melt, decompose, and volatilize during batch-to-glass conversion. To further understand these complex processes, simplified LAW melts containing oxyanion salts (sodium salts of carbonate, sulfate, and/or nitrate) and early melting glass formers (boric acid) have been evaluated using thermal analysis, infrared absorption spectroscopy, and X-ray diffraction. Results from this study indicate that the volatilization behavior of particular salts is influenced by the presence or absence of other salts. NaNO3 volatilization is decreased by the presence of Na2SO4. The addition of either Na2SO4 or NaNO3 to the system may enhance the volatilization of Na2CO3. In all cases, Na2SO4 was retained after melting and was often found to be in two different crystalline phases upon quenching.
INTRODUCTION: Hanford Low Activity Waste (LAW) is a processed aqueous solution containing a high concentration of sodium nitrates in addition to other alkali ions and oxyanions, such as sulfate. During immobilization of Hanford LAW in borosilicate glass by the process of vitrification, a salt phase consisting of primarily sodium sulfate (Na2SO4) can form on the surface of the silicate glass melt, with small concentrations of other alkali-containing salts such as halides, chromates (CrO 42-), pertechnetates (TcO4-), etc. [1-5]. This salt layer, also known as gall or yellow-phase [6], has a lower viscosity and a lower density (in most cases) than the glass melt, resulting in the phase remaining segregated as the salts react and early batch-to-glass conversion occurs. A high salt content in the batch can result in this salt formation during the cold cap reaction 195
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sequence, where slurry feed of waste and glass-forming components react at increasing temperatures. The salt layer, when it forms, changes with temperature as it is host to a series of complex reactions while the salts melt (see Fig. 1), decompose, volatilize, and/or incorporate into the silicate glass melt. In the first reaction steps, there is a release of water and decomposition of oxyanion salts. Nitrates (NO 3-) and nitrates (NO2-) are some of the first oxyanion salts to melt and decompose to oxides
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