Revisiting weight-gain correlations for as-fabricated Zircaloy-4 cladding tubes during steam oxidation

Abstract

During a loss of coolant accident, elevated temperatures and steam environment induce fuel cladding oxidation. During cladding high-temperature oxidation, the weight increase is considered as a material property when divided by the surface exposed to the environment, this is the so-called weight-gain. Many laboratories tested Zircaloy-4 specimens and used several approaches to determine weight-gain correlations. Two frequently used methods are weight measurements and metallographic measurements of scale thickness combined with diffusion simulations. Usually, assuming that weight-gain is controlled by diffusion processes, the correlations have a parabolic dependency to steam exposure duration. In the present study, a metallographic method relying on scale thickness measurements is proposed with few assumptions on the oxygen transport process in the oxide and α(O) layers. These two layers contain the main oxygen inventory. The local weight-gain is obtained by the integration of a simplified oxygen profile across the specimen thickness. The proposed method is compared to specimen weighing approach and leads to accurate predictions. The method was applied to five experimental campaigns performed in different laboratories combining more than 375 tests. A single non-parabolic weight-gain correlation covering a large temperature range (900–1500 °C) is finally provided.