PERFORMANCE OF PRESSURE TUBES IN CANDU REACTORS

Abstract

The pressure tubes in CANDU reactors typically operate for times up to about 30 years prior to refurbishment. The in-reactor performance of Zr-2.5Nb pressure tubes has been evaluated by sampling and periodic inspection. This paper describes the behavior and discusses the factors controlling the behaviour of these components. The Zr–2.5Nb pressure tubes are nominally extruded at 815 °C, cold worked nominally 27%, and stress relieved at 400 °C for 24 hours, resulting in a structure consisting of elongated grains of hexagonal close-packed alpha-Zr, partially surrounded by a thin network of filaments of body-centred-cubic beta-Zr. These beta-Zr filaments are meta-stable and contain about 20% Nb after extrusion. The stress-relief treatment results in partial decomposition of the beta-Zr filaments with the formation of hexagonal close-packed alpha-phase particles that are low in Nb, surrounded by a Nb-enriched beta-Zr matrix. The material properties of pressure tubes are determined by variations in alpha-phase texture, alpha-phase grain structure, network dislocation density, beta-phase decomposition, and impurity concentration that are a function of manufacturing variables. The pressure tubes operate at temperatures between 250 °C and 310 °C with coolant pressures up to about 11 MPa in fast neutron fluxes up to 4 × 1017 n·m−2·s−1 (E > 1 MeV) and the properties are modified by these conditions. The properties of the pressure tubes in an operating reactor are therefore a function of both manufacturing and operating condition variables. The ultimate tensile strength, fracture toughness, and delayed hydride-cracking properties (velocity (V) and threshold stress intensity factor (KIH)) change with irradiation, but all reach a nearly limiting value at a fluence of less than 1025 n·m−2 (E > 1 MeV). At this point the ultimate tensile strength is raised about 200 MPa, toughness is reduced by about 50%, V increases by about a factor of 6, while KIH is only slightly reduced. The role of microstructure and trace elements in these behaviours is described. Pressure tubes exhibit elongation, diametral expansion, and sag. The deformation behaviour is a function of operating conditions and the material properties that vary from tube-to-tube and as a function of axial location. Semi-empirical predictive models have been developed to describe the deformation response of average tubes as a function of operating conditions. The effect of material variability on corrosion behaviour is less well defined compared with other properties but there are instances where tube orientation and ingot source can be identified as factors that have an effect on hydrogen pick-up.