Cold crucible induction melter studies for making glass ceramic waste forms: A feasibility assessment

Crystallization study of rare earth and molybdenum containing nuclear waste glass ceramics

“…A glass‐ceramic waste form is being developed through the US Department of Energy Office of Nuclear Energy to immobilize nonfissionable waste streams of alkali (A, eg, 137 Cs), alkaline‐earths (AE, eg, 90 Sr), rare‐earths (RE), and transition metals (most importantly, Mo) generated during a projected scheme of aqueous reprocessing of used nuclear fuel with transuranic extraction (TRUEX plus ) . Another waste form being considered is a multi‐phase crystalline ceramic .…”

“…These benefits are realized by the partitioning of the fission product fraction insoluble in borosilicate glass into a suite of chemically durable crystalline phases through controlled cooling. In practice, waste components plus glass‐forming chemicals will be melted at high temperature (~1200‐1450°C) using a cold crucible induction melter (CCIM) and poured into a canister to air cool . During cooling, the melt phase separates into Mo‐rich and RE‐rich droplets within a borosilicate matrix, followed by crystallization.…”

“…The target crystalline phase assemblage includes powellite [(RE,A,AE)MoO 4 ], oxyapatite [(RE,A,AE) 10 (SiO 4 ) 6 O 2 ], cerianite [(Ce,Zr)O 2 ], and sometimes lanthanide borosilicate (RE 3 BSi 2 O 10 ), pollucite (CsAlSi 2 O 6 ), and/or celsian [AEAl 2 Si 2 O 8 ] . For the specific US case of glass‐ceramic waste forms, preliminary research on representative compositions has been conducted with crucible melt studies and a pilot‐scale CCIM . Actual nuclear waste glass compositions will be comprised of 30+ components, but simplified 8‐oxide systems are being currently studied in more detail, and are discussed here.…”

“…There is a trade‐off between simplifying a complex glass in order to better understand it and over‐simplifying it, such that behavior becomes substantially different than the analogous complex glass. For instance, it is known that some of the alkali behave differently in these glass‐ceramic systems, in that Cs tends to partition to a soluble molybdate or stay in the glassy phase (depending on details of the glass composition), Li goes normally to the glass, and Na can go to the glass, powellite, or oxyapatite phase . Cesium (and even more so rubidium) is expected to be a small fraction of the alkali on a molar basis .…”

“…For instance, it is known that some of the alkali behave differently in these glass‐ceramic systems, in that Cs tends to partition to a soluble molybdate or stay in the glassy phase (depending on details of the glass composition), Li goes normally to the glass, and Na can go to the glass, powellite, or oxyapatite phase . Cesium (and even more so rubidium) is expected to be a small fraction of the alkali on a molar basis . For alkaline earths, the presence of Ba and Sr cause a separate partitioning of two powellite crystalline phases with different amounts of Ca, which would not be reproduced when including only Ca; unlike the case of Cs, the molar concentrations of Ba plus Sr in the waste are expected to be about half that of added Ca .…”