Book Description

Zirconium alloys containing about 0.5% tin, which are classified as dilute tin alloys, possess excellent uniform waterside corrosion resistance necessary for the PWR fuel applications. Mechanical and irradiation growth properties of the dilute alloys can be adjusted for specific component application by controlling the additions of other alloying elements such as iron, chromium, niobium, and oxygen. Cladding alloys with such additions have been successfully irradiated to burnups up to 69 GWd/MTU, showing significant improvements in corrosion resistance and irradiation growth characteristics compared to low-tin Zircaloy-4, one of the current standard materials. The in-PWR creep resistance of such dilute alloys is comparable to that of low-tin Zircaloy-4. Another dilute alloy with predominantly iron-containing second-phase particles that are unstable under neutron irradiation (in a cold-worked microstructure, cold work introduced prior to irradiation) appears to be most suitable for the grid strip application. Cold-worked I-spring of this alloy in a transverse stamped grid provides excellent fuel rod support by inward motion of the spring within the grid cell due to irradiation growth. The hydrogen pickup fraction of several zirconium alloys, including Zircaloy-4 and dilute alloys, exhibits a well-behaved correlation with oxide thickness under non-heat flux conditions. A similar correlation is expected under heat flux conditions. Under heat flux conditions, the hydrogen pickup fraction for Zircaloy-4 approaches a constant value of about 15% for oxide thicknesses greater than 50 ?m. For the non heat-flux conditions, the pickup fraction is less than 5% for oxide thickness values greater than 50 ?m. Possible reasons for influence of oxide thickness and heat flux on the hydrogen pickup fraction are the porosity traps in thick oxide layers and atomic vibrations of oxide lattice under heat flux conditions. The in-PWR performance characteristics of the dilute alloys such as corrosion resistance, ductility, and dimensional stability can be controlled by optimization of the composition and fabrication process. These parameters influence the composition of the second-phase particles (SPP) in the alloy microstructure, which determines the radiation stability of the SPP. Irradiation stabilityof SPP has strong impact on the in-PWR performance characteristics of zirconium alloys.