Investigating and Characterizing Electrical Resistivity Variations Caused by Heat Treatment in Zr2.5%Nb Pressure Tubes

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Thorpe, William
Electrical Resistivity , Heat Treatment , Pressure Tubes , Zr2.5%Nb , CANDU
The electrical resistivity variations in the circumferential and radial directions in pressure tubes (PTs) at various heat treatment (HT) levels, encompassing the range of as-manufactured HT times, were characterized as these have consequences for PT-calandria tube (CT) gap measurement error in as-installed PTs. The methods employed included a combination of eddy current (EC) testing (ECT) for resistivity measurements, and scanning electron microscopy (SEM) for microstructural measurements of β_Zr ribbon thickness. A full factorial experiment was performed on the average circumferential resistivity in PT samples taken from both extrusion ends, with various HT levels, EC frequencies and probe surface placements (inner or outer PT surface). The resistivity measurements using an EC frequency of 1500 kHz and β_Zr ribbon thickness measurements in the radial direction showed a weak parabolic variation that was correlated with β_Zr ribbon thickness measurements in the radial direction. The SEM measurements showed a statistically significant difference in average β_Zr ribbon thickness of 150 nm between the outer and inner surfaces in the axial-transverse cross-section, which may explain the statistical significance of measured inner and outer surface resistivity variations in some PT samples. The multivariate analysis of variance (MANOVA) showed that the combination of HT and EC frequency was the second most significant test factor combination that accounted for about 30% of the total variance in the data, which is evidence of radial resistivity variations being created by the HT process. Measurements of resistivity in the circumferential direction showed variation of up to ± 1.25% and 1.5% of a PT’s average resistivity from the inner and outer surfaces, respectively, which may have implications for PT-CT gap measurement accuracy using analytic-based inverse algorithms that do not compensate for circumferential resistivity variations. Results obtained from mulit-frequency ECT in the circumferential direction across multiple PT samples showed that HT causes the average PT resistivity to decrease at a rate of 1.53±0.08 (μΩ⋅cm)/log⁡(hr) and 1.1±0.4 (μΩ⋅cm)/log⁡(hr) for the inner and outer PT surfaces, respectively. These results are correlated with differences in average β_Zr ribbon thickness in the axial-transverse cross-section and provide further evidence of a radial resistivity variation being created due to HT.
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