Effects of Hydrogen Pickup and Second-Phase Particle Dissolution on the In-Reactor Corrosion Performance of BWR Claddings

It is well established that the second-phase particle (SPP) size distribution and the cladding chemical composition affects the in-reactor corrosion rate of boiling water reactor (BWR) cladding, although the mechanisms are not yet fully understood. To further study these effects, the corrosion and hydriding performance was studied on four different Zircaloy-2 materials. Three of the materials were fabricated having the same chemical composition, but different SPP distributions. The fourth material served as a reference and was manufactured to have small SPPs. All four materials were irradiated in Forsmark 3 for five cycles to a burnup of >43 MWd/kgU, and poolside inspections were carried out after three, four, and five cycles of irradiation. Three cladding materials were studied by hot-cell investigations after three and four cycles of irradiation. The study shows that the dissolution of the SPPs results in a shift toward smaller particles during irradiation and that the hydrogen concentration as well as the corrosion rate decrease with increasing initial SPP size. Moreover, the hydrogen content differed significantly between the examined claddings after three cycles of operation (25 MWd/kgU), whereas the oxide thickness at this burnup level was similar for all studied materials. However, the oxidation rate for the studied clad materials diverged during the fourth irradiation cycle and, consequently, the oxide layer thickness differed between the material types after four cycles of irradiation (40 MWd/kgU). It is concluded that the initial hydrogen pickup fraction depends on the SPP size distribution of the as-fabricated material. The acceleration in corrosion rate is proposed to be a combined effect of the actual hydrogen concentration and the dissolution of the SPPs. The impact of SPP dissolution on corrosion performance and possible mechanisms behind the earlier hydrogen uptake are further discussed in the paper.