Waterside Corrosion of Zircaloy-Clad Fuel Rods in a PWR Environment

Abstract

Waterside corrosion of fuel rods has generally not been a problem under normal pressurized-water reactor (PWR) operating conditions, although some instances of accelerated corrosion have been reported. However, an incentive exists to extend the average fuel rod discharge burnups to about 50 000 MWd/Mtu. To minimize corrosion at these extended burnups, the factors which influence Zircaloy corrosion need to be better understood. A data base of Zircaloy corrosion behavior under PWR operating conditions has been established. The data are compiled from previously published reports as well as from new Kraftwerk Union (KWU) fuel examinations. This PWR data base has been compared with the predictions of a corrosion model derived from ex-reactor data. In order to predict the in-reactor corrosion behavior from ex-reactor corrosion models, a fitting factor (F) is required. The calculated fitting factor will depend on the corrosion models used to represent the ex-reactor kinetics; for the data analyzed, it was found to vary from 0.8 to 2.8. The data show that the reactor environment increases the corrosion. For a given corrosion model, the fitting factor varies from reactor to reactor, and within a given reactor it varies from cycle to cycle. ZrO2 film thermal conductivity is another major factor that influences corrosion behavior. The thermal conductivity of the oxide layer has been determined only in out-of-pile tests on unirradiated specimens having a variety of oxide film microstructures. There is relatively large scatter in the measured data, and no such measurements exist for representative irradiated specimens. It was inferred from KWU film thickness data that the oxide film thermal conductivity may decrease once circumferential cracks develop in the layer. Measurements on irradiated specimens are needed to better evaluate the effect of the thermal conductivity of oxide films formed in-reactor on the in-reactor corrosion rate. The corrosion mechanisms and the factors influencing the waterside corrosion of PWR fuel rods are not known precisely enough to allow for a reasonable extrapolation of current operating experience to rod average burnups of 50 000 MWd/Mtu.