The diametral expansion and elongation rates of Zr-2.5Nb pressure tubes in CANDU™ (CANada Deuterium Uranium) nuclear reactors are important properties that limit their useful life and the maximum power level for reactor operation.
For a given set of operating conditions there is considerable variability in the deformation rates because of the variations in as-fabricated microstructure and chemistry from tube-to-tube — specifically grain size, crystallographic texture, and oxygen content. The as-fabricated microstructure also varies within a given tube, the largest variation occurring along the length, and this is a result of cooling of the tube during the extrusion process. During service in a nuclear reactor, the microstructure evolves further, and this additional change in microstructure is primarily dependent on the rate of radiation damage (determined by the fast neutron flux), the temperature, and the time. Both the fast neutron flux and temperature vary at all points within the pressure tube.
For a given material microstructure, the deformation is a function of the operating conditions: coolant pressure (stress), temperature, and neutron flux. In principle, the deformation rate is a linear function of fast neutron flux, and this is mostly true for fast neutron fluxes of the order of 1017 n.m-2.s-1. Recent analyses of data from pressure tubes measured over long periods of operation in reactor have shown that the steady-state diametral creep rates are not linear with fast neutron flux for fluxes up to about 0.5 × 1017 n.m-2.s-1. A qualitative model has been developed to account for the observed behavior based on the modifying effects of neutron flux and temperature on the microstructure. The model describes the suppression of thermal creep and the transition from thermal to irradiation creep with increasing neutron flux.