PSI - Issue 71

P.K. Sharma et al. / Procedia Structural Integrity 71 (2025) 126–133

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The melters have multiple heating coils in distinct zones designed to heat the mixtures of glass matrix and high level waste (HLW). These heating coils facilitate the melting of these mixtures, while pouring coils are utilized to transfer the glass matrix into the canister (IAEA Tecdoc (1992)). The vitrification process occurs in cycles of heating and cooling and pouring HLW and glass slurry mixture into the canisters. The heating stages involve evaporation, calcination, glass melt formation and soaking of the mixtures. During the soaking phase, the glass slurry and HLW can penetrate inside the grain boundaries of alloy 690 (Kaushik et al. (2013)), potentially creating micro-cracks. This phenomenon occurs due to the selective dissolution of chromium at the grain boundaries by the glass matrix, which reduces the chromium content due to formation of chromium carbides. Since chromium is essential for corrosion resistance, the formation of chromium carbides may affect the material's mechanical properties. This study investigates the interaction between the glass matrix and alloy 690 under varying exposure durations, temperatures, and stress levels. Comprehending how the glass melt interacts with alloy 690 is crucial for the long-term functionality of melter pots in vitrification facilities.

Fig. 1: Schematic of induction melter used for vitrification (Sengupta et al. (2006)).

Nomenclature t

exposure time Temperature

σ

Mean defect size Yield strength

T μ

σ y

Standard deviation of defect size Stress level used in tests Sengupta et al. (2006, 2009) explored the interaction of Alloy 690 with simulated HLW, observing Cr carbide precipitation along grain boundaries, Cr depletion in the austenitic matrix, and intergranular attack near the Alloy 690/borosilicate melt pool interface. Cr-depletion kinetics followed the form = + t 0.5 . Samantaroy et al. (2011) studied the corrosion behavior of Alloys 690 and 693 in 3M HNO₃ at 298 K and 323 K, detecting Ti -rich and Cr-rich precipitates in Alloy 690 and primarily Cr-rich precipitates in Alloy 693. Halder et al. (2016) showed that Alloy 693 developed a protective Al₂O₃ and Cr₂O₃ layer when exposed to waste glass melt, with the Al₂O₃ layer thickening over time, preventing further Cr diffusion. Day and Kim (2005) compared corrosion resistance in iron phosphate melts, finding that Inconel 693 has better properties than Inconel 690. Olson et al. (2010) ranked various alloys for corrosion resistance in FLiNaK salt with Cr content directly influencing corrosion performance. Li et al. (2021) and Xie et al. (2022) evaluated behavior of Ni-based alloys under molten fluoride and glass-forming salt environments and emphasized that microstructural stability, Cr/Al partitioning and precipitation behavior significantly affected long term corrosion resistance. Similarly, Guo et al. (2023) studied intergranular corrosion and oxidation kinetics in Ni-Cr Fe alloys pointing out that grain boundary chemistry and carbide distribution play a crucial role in corrosion damage evolution in molten salt and glassy environments. In this study, a systematic investigation was conducted to assess the impact of temperature, stress, and exposure duration of molten glass on the mechanical properties of Alloy 690. The changes in these properties were analyzed by examining the evolution of defect size, density, and distribution resulting from exposure to the molten glass environment. The findings from this research will contribute to the design and life estimation of components used in nuclear waste vitrification melters. 2. Experimental details Tensile tests were carried out at 800 °C and 900 °C after exposure to molten glass environment at different stresses and time durations. The changes in mechanical properties were evaluated before and after exposure to this aggressive σ appl