Microstructural changes in oxidized Zr-Nb 2.5% alloy by thermal transients

The experimental simulation of LOCA scenarios in CANDU reactors assumes drastic transients of temperature on the pressure tubes from fuel channels. 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 ramp temperature as well. The paper investigates the micro-structural changes in oxide layers and basic materials (Zr-2.5% Nb) resulting from specific thermal transients. This study was performed on Zr-2.5 % Nb alloy samples, which were previously isothermally oxidized at temperature of 700 deg. C for different time intervals. Afterwards, the resulted samples with different thickness oxide layers were subjected to the various temperature transients at different heating/cooling rates. The oxidation process carried out in a SETARAM SETSYS EVOLUTION 24 thermobalance facility by means of steam environment. Using the thermo-gravimetric analysis (TGA) module it was possible to make in-situ measurements of weight gain. The Wetsys controlled humidity generator is used for both control and regulation of humidity level. The isothermal oxidized samples were heated and cooled in thermobalance furnace at different controlled rates. Subsequently, the oxidized Zr-2.5% Nb samples were investigated by optical microscopy and scanning electron microscopy (SEM). The paper presents some results of experimental investigations, mainly focused on the oxide layers and changes of alloy microstructures. (authors)