Impact of Second Phase Particles on BWR Zr-2 Corrosion and Hydriding Performance

Abstract

BWR fuel cladding corrosion and hydriding performance may limit the fuel residence time. Historically, out-of-pile high-temperature steam tests have been used to predict the BWR corrosion performance of Zircaloy materials. However, results indicate that no correlation exists in many cases between in-pile and out-of-pile Zircaloy corrosion performance. Concerning BWR hydriding performance there appears to be no reliable out-of-pile test that predicts the BWR Zircaloy hydriding performance. The Zircaloy corrosion and hydriding performance is a function of: 1. The environment such as the temperature, fast neutron flux, water chemistry. 2. The Zircaloy microstructure, such as texture, dislocation density, matrix chemical composition, and second phase particle SPP characteristics. One way to solve the problem with the lack of reliable out-of-pile tests is to correlate the BWR Zircaloy corrosion and hydriding performance to the initial microstructure. Such a correlation will of course not predict the impact of temperature, fast flux, and water chemistry on Zircaloy corrosion and hydriding performance. This paper describes the correlation between the BWR Zircaloy corrosion and hydriding performance and the Zircaloy SPP microstructure. Six Zr-2 fuel cladding rods irradiated in three different reactors to a fuel rod average burnup of >40 MWd/kgU were included in the study. The fuel claddings had varying chemical compositions and were manufactured by three different process routes, resulting in large differences in SPP characteristics. Hot cell examinations including oxide thickness and hydrogen content determinations were performed. Also SPP SEM analyses were done to assess the size distributions of the irradiated materials. In addition, SEM and TEM analyses were performed on non-irradiated achive samples from the same manufacturing lots as the irradiated materials. We further describe how the initial SPP size distribution alters with irradiation and how the BWR Zr-2 corrosion and hydriding performance correlates with the initial SPP size distribution and SPP chemistry.