Microstructure studies regarding the behavior of oxidized Zr-2.5%Nb alloy in thermal transients

The experimental simulation of LOCA scenarios in CANDU reactors assumes transients of temperature on the pressure tubes manufactured of Zr-2.5%Nb alloy. In case of any postulated accident of this kind, which is supposed to occur after some hot years in normal operating conditions, the safety analysis should take into account the oxidised state of pressure tube and the changes of its microstructure during the temperature ramps as well. The present paper focuses on experimental observations of the oxide microstructure isothermally grown at 700 deg. C and also on the microstructure of Zr-2.5%Nb alloy, followed by few transients of increasing and decreasing of temperature between 27 deg. C and 1000 deg. C. The samples were investigated by optical microscopy and scanning electron microscopy (SEM). The microstructure reveals the forming of an alpha - zirconium layer placed under oxide layer. There is a clear demarcation between oxide layer and an alpha zirconium grain layer which differs from the beta - zirconium alloy microstructure, formed at higher temperature. The grain size in alpha zirconium layer decreases towards alloy oxide interface. This microstructure can affect the thermal and mechanical properties of alloy in thermal transients. Both, oxidation processes and thermal transients are carried out in SETARAM SETSYS EVOLUTION 24 thermobalance facility in controlled environment, the weight gain being measured in situ. (authors)