TIME-TEMPERATURE-TRANSFORMATION (TTT) DIAGRAMS FOR FUTURE WASTE COMPOSITIONS
As a part of the Waste Acceptance Product Specifications (WAPS) for Vitrified High-Level Waste Forms defined by the Department of Energy - Office of Environmental Management, the waste form stability must be determined for each of the projected high-level waste (HLW) types at the Savannah River Site (SRS). Specifically, WAPS 1.4.1 requires the glass transition temperature (T{sub g}) to be defined and time-temperature-transformation (TTT) diagrams to be developed. The T{sub g} of a glass is an indicator of the approximate temperature where the supercooled liquid converts to a solid on cooling or conversely, where the solid begins to behave as a viscoelastic solid on heating. A TTT diagram identifies the crystalline phases that can form as a function of time and temperature for a given waste type or more specifically, the borosilicate glass waste form. In order to assess durability, the Product Consistency Test (PCT) was used and the durability results compared to the Environmental Assessment (EA) glass. The measurement of glass transition temperature and the development of TTT diagrams have already been performed for the seven Defense Waste Processing Facility (DWPF) projected compositions as defined in the Waste Form Compliance Plan (WCP) and in SRNL-STI-2009-00025. Additional phase transformation information exists for other projected compositions, but overall these compositions did not cover composition regions estimated for future waste processing. To develop TTT diagrams for future waste types, the Savannah River National Laboratory (SRNL) fabricated two caches of glass from reagent grade oxides to simulate glass compositions which would be likely processed with and without Al dissolution. These were used for glass transition temperature measurement and TTT diagram development. The glass transition temperatures of both glasses were measured using differential scanning calorimetry (DSC) and were recorded to be 448 C and 452 C. Using the previous TTT diagrams as guidance, subsamples of each glass were isothermally heat treated for 1 to 768 hours at temperatures between 500 C to 1000 C. Each of the heat treated samples, along with quenched and centerline canister cooled (CCC) treated samples, were analyzed using X-ray diffraction (XRD) and the PCT. Maximum crystallization was detected in samples treated at 700 C and 600 C for more than 96 hours in both glasses. Phases crystallized were similar in composition if not the same as those found in the previous TTT studies. Six different crystalline phases were detected, including nepheline, aegirine, lithium silicate, trevorite (spinel based), krinovite, and calcium magnesium iron silicon oxide. Overall, phases were spinel (iron) based, lithium metasilicate, sodium aluminosilicate or alkali transition metal silicate in composition. No new crystalline families were detected. Durability, as measured by the PCT, decreased when lithium silicate or nepheline crystals were present. The conclusions of this study were consistent with previous studies. In order to continue to meet the requirements of the WCP, a simplified strategy is suggested for the generation of future TTT diagrams. Only extremely significant changes in composition resulting from processing strategy changes would require generating more TTT diagrams.
- Research Organization:
- Savannah River Site (SRS), Aiken, SC (United States)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- DE-AC09-08SR22470
- OSTI ID:
- 986756
- Report Number(s):
- SRNL-STI-2010-00373; TRN: US1006923
- Country of Publication:
- United States
- Language:
- English
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