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 Zr2.5%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.

材料电阻率是基于涡流的检查的重要参数。例如，需要使用压力管（PT）和加热管（CT）的材料电阻率值，以确保准确测量CANDU®反应堆燃料通道中的PT-CT间隙。当前，间隙测量校准是在未经热处理和未经辐照的PT上进行的，并且在校准过程中使用的电阻率被认为反映了反应器内条件的电阻率。相反，其他影响间隙测量的参数变化则使用反应器内测量进行补偿。为了测试反应器内条件（例如暴露于高温下）是否可以改变PT材料的电阻率，本研究研究了热处理对Zr2.5％Nb电阻率的影响。将切片的PT样品在厌氧炉条件下于400°C和450°C保持不同的时间，以部分分解β-Zr并产生不同比例的ω-Zr。选择这些温度以加速相变，所述相变在反应器操作条件下在更长的时间段内发生，在反应器操作条件下温度通常在250℃至310℃之间。使用四点法测量热处理的PT样品的电阻率，并记录温度下电阻率随时间的变化。在400°C约5000小时后，电阻率的幅度下降了10％。热处理后电阻率的降低与微观结构的变化有关，并得到经验电阻率-微观结构模型的支持。透射电子显微镜（TEM）显示HCP（六方密堆积）ω相体积分数增加，β相网络的相关减少，这减少了较高电导率的α-Zr晶粒之间的接触，从而导致整体电阻率降低。这些结果对PT到CT间隙测量的不确定性有影响，在反应堆运行期间，沿着通道及其周围出现温度变化。