Hydride Transformations and Their Impact on Zirconium Alloy Cladding Embrittlement During Spent Nuclear Fuel Storage

Spent nuclear fuels (SNFs) are expected to be stored in dry storage systems for extended periods, making the preservation of the SNF cladding integrity critical. One of the key concerns is the embrittlement of zirconium alloy cladding at low temperatures due to hydrogen-induced ductile-brittle transition (DBT). This study examines the impact of hydrogen contents, strain rates, and temperature on the fracture toughness and DBT of Zr-2.5Nb specimens. Results show that increasing hydrogen content significantly reduces fracture toughness, with a pronounced DBT occurring between 100°C and 150°C in hydrogen-charged specimens. At room temperature, hydrides, especially circumferential hydrides, are readily fractured, leading to brittle behavior. However, at elevated temperatures (e.g. 300°C), hydrides are not fractured, resulting in ductile behavior. This study identifies that the δ-to-γ and γ-to-δ hydride transformations, driven by entropy changes and the resulting entropy-driven internal stresses, play a crucial role in determining the mechanical behavior of zirconium alloys. The entropy-driven compressive stresses from the δ-to-γ hydride transformation and hydride precipitation cause embrittlement, while the entropy-driven internal tensile stresses from the γ-to-δ hydride transformation and hydride dissolution promote ductility. The findings suggest that the long-term dry storage of the SNF cladding at temperatures below the δ-to-γ transformation temperature increases the risk of embrittlement because of the accumulated entropy-driven compressive stresses. Additionally, this study highlights the influence of strain rates on the DBT, with higher strain rates leading to higher ductile-brittle transition temperatures (DBTTs) because of increased internal tensile stresses, as observed in impact tests. Lower strain rates, such as those in ring compression tests, result in the fracturing of fewer hydrides and lower DBTTs. This study recommends prioritizing higher-strain-rate testing and monitoring hydrogen contents and hydride transformations in the SNF cladding to mitigate embrittlement risks during long-term storage. Low strain rates may be appropriate for evaluating SNF cladding behavior under creep conditions or assessing susceptibility to delayed hydride cracking; they are not suitable for accurately determining embrittlement and the DBT.