Reaction in air and in nitrogen of pre-oxidised Zircaloy-4 and M5™ claddings

International audienceHigh temperature reactivity in air and in nitrogen of pre-oxidised Zircaloy-4 and M5™ claddings has been studied by thermogravimetry. Claddings were pre-oxidised at low temperature with the aim of simulating spent fuel. Different pre-oxidation modes, inducing significant variation in the pre-oxides microstructure, were compared. The behaviour in air, investigated in the 850-1000 C temperature range, was found to be strongly dependant on the type of pre-oxide: the compact pre-oxide formed in autoclave (at temperature, pressure, and water chemistry representative of PWR conditions) significantly slows down the degradation in air compared to the bare alloys; on the contrary, a pre-oxide formed at 500 C at ambient pressure, either in oxygen or in steam, favours the initiation of post-breakaway type oxidation, which in air is associated with nitride formation. The behaviour in nitrogen has been investigated in the 800-1200 C temperature range, with Zircaloy-4 pre-oxidised at 500 C in O2. Reactivity is low up to 1000 C but becomes very significant at the highest temperatures investigated, 1100 and 1200 C. Finally, cladding segments first reacted in N2 at 1100 C, were exposed to air and show fast oxidation even at the lowest temperature investigated (600 C). © 2013 Elsevier B.V. All rights reserved