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

High 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).