Effects of Heat Treatment on CANDU® Pressure Tube Electrical Resistivity

Abstract

Material electrical resistivity is an important parameter for eddy current based inspections. For example, material resistivity values for the pressure tube (PT) and calandria tube (CT) are required to ensure accurate measurement of the PT-CT gap in CANDU® reactor fuel channels. Currently, gap measurement calibration is performed on a non-heat treated and non-irradiated PT, and the resistivity used during calibration is assumed to reflect that of in-reactor conditions. In contrast, changes in other parameters that affect gap measurement are compensated using in-reactor measurements. To test whether in-reactor conditions, such as exposure to elevated temperatures, could change a PT material's resistivity, this study examined the effect of heat treatment on Zr-2.5% Nb resistivity. Sectioned PT samples were held for varying periods of time at 400 °C and 450 °C, under anaerobic furnace conditions, in order to partially decompose β-Zr and produce varying fractions of ω-Zr. These temperatures were chosen to accelerate the phase transformations, which take place over a longer period of time under reactor operating conditions, where temperatures are typically between 250 °C and 310 °C. The resistivity of the heat-treated PT samples was measured using the four-point method and changes in resistivity with time at temperature were recorded. The magnitude of the resistivity was observed to decrease by up to 10% after approximately 5000 hrs at 400 °C. Reduction of resistivity with heat treatment was associated with changes in the microstructure and supported by empirical resistivity-microstructure models. Transmission electron microscopy (TEM) showed an increase in the HCP (Hexagonal Closed Packed) ω-phase volume fraction, and an associated decrease in the beta-phase network that was reducing contact between higher conductivity α-Zr grains, which resulted in an overall decrease of resistivity. These results have implications for the uncertainty of PT to CT gap measurement, where temperature variations arise along and around the channel during reactor operations.