Microstructure and Corrosion Studies for Optimized PWR and BWR Zircaloy Cladding

Abstract

The increased discharge burnups envisaged today require an optimization of the Zircaloy cladding properties with regard to corrosion with different strategies for optimizing the corrosion for boiling water reactor (BWR) and pressure water reactor (PWR) applications. A broad laboratory study showed that the influence of temperature and environmental details upon autoclave test results is rather complex. Zircaloy with fine (∼ 0.05 μm), uniformly distributed intermetallic precipitates reveals nodular corrosion in steam at 420°C or below and excessive uniform corrosion in water at 350°C, but the best nodular behavior in steam at 500#x00B0;C. However, samples with larger intermetallic precipitates (>0.2 μm) exhibit a strong tendency to nodular corrosion in 500°C steam, but good behavior in steam at 420°C or below and in water at 350°C. The tendency toward nodular corrosion in 400 to 500°C steam can be suppressed by additions of oxygen; the ranking in uniform corrosion in 350°C water can be altered by additions of hydrogen. The latter was observed on samples exposed up to 1000 days in degassed water and afterwards 1000 days in water with a hydrogen overpressure. Similar correlations were found for in-pile exposure with some exceptions. Materials with very fine intermetallic precipitates, which show little tendency for nodular corrosion and behave well in-pile in oxygenated (mostly BWR) environments, exhibit accelerated uniform corrosion in a hydrogenated (PWR) environment with a corrosion rate that depends very little on temperature. However, in oxygenated coolant, materials with coarse precipitates indicate severe nodular corrosion, which is almost independent of the temperatures within the range of 290 and 350°C. As a result of these findings a dual strategy is technologically feasible by using different values of the accumulated annealing parameters (Σ Ai) for BWR and PWR application. For optimized BWR cladding tubes ΣAi ⩽ 10 -18 h is proposed; the best range for PWR cladding appears to be at 2 × 10-18 h ⩽ Σ Ai ⩽ 5 × 10-17 h.