The Correlation Between Microstructures and in-BWR Corrosion Behavior of Highly Irradiated Zr-based Alloys

The microstructures of four kinds of Zircaloy-2-type materials with neutron exposure up to about 15 × 10²⁵ n/m² (E>1MeV) were examined to investigate the microstructural evolution and to correlate it with the in-reactor corrosion behavior at very high fluences. The materials examined in this study included two advanced Zr-based alloys with good corrosion resistance, namely High Fe Zry and High FeNi Zry, and two other kinds of Zry-2 materials with slightly different elemental compositions and fabrication processes producing differing second phase particle sizes. All alloys showed good corrosion behavior and low hydrogen pickup when irradiated up to four cycles in dummy neutron source holders located outside of channel boxes. After six irradiation cycles, all four of the alloys had an increase in the rate of uniform corrosion and hydrogen pickup. The results of the microstructure examinations after six-cycle irradiation indicated two interesting points: (1) increased insight into the dissolution process for Zr-Fe-Cr particles, and (2) a correlation between the number density of Zr-Fe-Cr particles surviving through irradiation and the corrosion behavior at high fluences. From these results, it is suggested that increasing the initial particle size and/or controlling the alloy chemistry would be beneficial to improve the uniform corrosion performance and to lower hydrogen pickup, especially at very high fluences.