Manufacturing Variability, Microstructure, and Deformation of Zr-2.5Nb Pressure Tubes

The time-dependent deformation behavior of pressure tubes in CANDU (TM) (CANada Deuterium Uranium) reactors is an important property that has to be predicted for reactor life management. Measurements accumulated over many years have shown that there is considerable variability in deformation rates between different tubes. The deformation behavior not only varies from tube to tube but also varies along the length of each tube; this axial variation in itself is unique to each tube. The deformation behavior is determined by the microstructure and this is a function of the manufacturing history. An exhaustive study has been conducted to collect and compile deformation and manufacturing data for tubes in many different reactors. The manufacturing data have been analyzed with respect to the measured in-reactor performance of the pressure tubes. Data from pressure tubes fabricated over a 30-year period have been analyzed. During this time the manufacturing parameters have evolved to improve the workability and mechanical properties and also reduce tube-to-tube variability. The effect of manufacturing variables on in-reactor performance has been assessed for tubes that have been in service for many years and approaching the end of their design life. The analysis shows that the two most important factors that affect the deformation behavior of pressure tubes are the material source (ingot) and the extrusion conditions. These are related by the microstructure (texture, grain size, and dislocation density) to the deformation. The results of the statistical analyses will be presented and discussed in terms of the manufacturing conditions that bring about specific microstructures.