Behavior of Lithium and Boron in Irradiated and Unirradiated Oxides Formed on Zircaloy-4 Claddings

Abstract

In order to investigate the effect of lithium and boron on corrosion of Zircaloy-4 fuel cladding, secondary ion mass spectrometry (SIMS) analyses coupled with chemical analyses have been carried out on oxides of specimens corroded in water with lithium hydroxide and boric acid similar to the actual reactor conditions, and of the fuel claddings irradiated in a commercial reactor. The quantitative analyses on lithium and boron by SIMS and chemical analyses showed that the concentration of lithium in the oxides formed ex-reactor was 2 to 50 ppm when the water contained 2 to 10 ppm lithium and exhibited a strong temperature dependence. The concentration of boron in the unirradiated oxides was approximately 250 ppm when water contained 500 to 1600 ppm boron and showed some increase as the concentration of boron in the water increased. No temperature dependence was detected. The lithium concentration of irradiated oxides was systematically higher than that of the unirradiated oxides, and little difference has been seen in the 11B concentration. The SIMS analyses of the oxides of the ex-reactor corrosion specimens revealed that the depth profile of lithium concentration was flat in most of the oxide and exhibited a sharp decrease in the dense oxide layer, which corresponds to a thickness of approximately 2μm from the assumed oxide-metal interface. The boron distribution in the oxide was similar in general, but showed an obvious cyclic variation in the outer oxide region. The frequency of the variation corresponded to the rate transitions during the corrosion tests. These features in the profiles of lithium and boron have been also detected on the oxides of the irradiated claddings. The sharp decrease in the concentrations of lithium in the vicinity of the metal-oxide interface indicates that the inner layer is a barrier against the deteriorating effect of lithium. The hydrogen pickup fraction of the specimens was found to be influenced by the boron concentration in the water.