The early design of flux detectors in use in CANDU® reactors consisted of multiple-paired-coiled detectors sheathed in INCONEL® 600 and wrapped around a Zircaloy carrier rod. The signal from these detectors was carried by INCONEL 600 coaxial cables that were threaded through the carrier-rod and fed back to instrumentation outside of the reactor core. Failures occurred in early applications that were related to the environment in which the detectors operated (moist or dry air). These failures were mitigated by ensuring that the detectors operated with a He cover gas that acts both to limit corrosion and as an efficient conductor of heat (generated by nuclear heating). An improved design was introduced consisting of single-individual-replaceable detectors (SIRs) with performance improved by eliminating brazed joints that were susceptible to nitric acid attack. Post-irradiation examination of encapsulated INCONEL 600 coiled flux detectors that had failed after several years of service in a CANDU reactor showed that many of the detector wires inside the carrier rod were broken. It was concluded that failure occurred because the He cover gas was lost and replaced by air. The examination also showed that those parts of the INCONEL 600 detector wires that had operated at the highest temperature, as evidenced by the degree of localised corrosion, were the most severely embrittled. Parts of the same detector wires that were clearly operating at lower temperatures, because they were shiny, remained ductile. Calculations indicate that if the INCONEL 600 material temperature exceeds about 573K (300°C) then it is conceivable that significant swelling and associated embrittlement could be expected. This is consistent with the observations indicating that the INCONEL detector wires that had experienced operation at elevated temperatures (>300°C) were embrittled.

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