Advanced leak detection technique using a distributed temperature sensing for a double-walled pipe for a sodium-cooled fast reactor

초록

Early detection and localization of sodium leakage are important for ensuring the safety of sodium-cooled fast reactor (SFR) piping systems. Double-walled piping can mitigate the consequences of external sodium leakage; however, conventional wire-type leak detectors provide limited spatial information and may be affected by incomplete contact between the leaked fluid and sensing electrodes. This study experimentally develops and validates a quantitative leakage detection and localization methodology based on Raman-scattering distributed temperature sensing (DTS) for double-walled piping. A water-mockup test facility, WALSUM, was constructed to simulate leakage in the annular space of a double-walled pipe, and the detection performance was evaluated over leak flow rates of 0.1-7.0 mL/min and fluid temperatures of 35-90 degrees C. To improve the robustness of leakage identification against measurement noise and thermal transients, a three-point spatial temperature-ratio method was combined with a statistical detection criterion based on the standard deviation of the ratio difference. A critical threshold of 0.03 was established from a three-standard-deviation margin of the non-leakage baseline. The experimental results demonstrated that the DTS system detected the minimum tested leak rate of 0.1 mL/ min, with response times ranging from 195 to 2,579 s depending on the leak flow rate and fluid temperature. The response became faster and the leakage-induced thermal signal increased with increasing leak flow rate and fluid temperature. During a 33 h non-leakage transient test involving repeated heating and cooling cycles, no false alarms were observed. In comparative tests, the DTS system successfully detected a leakage event under conditions where the wire-type detector was affected by incomplete electrode contact. In addition, the distributed sensing capability enabled leak localization with a theoretical positional uncertainty of +/- 0.25 m based on the 0.5 m spatial resolution of the DTS. These results demonstrate the feasibility of Raman-DTS for quantitative, distributed leakage monitoring and localization in double-walled SFR piping and provide experimentally based performance metrics for further development toward practical reactor applications.

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