Leak testing of the plutonium-238 radioisotope thermoelectric generator using a quadrupole mass spectrometer

This paper presents a customized vacuum chamber leak test system based on quadrupole mass spectrometry (QMS) to evaluate the hermeticity of plutonium-238 radioisotope thermoelectric generators (RTGs) backfilled with an argon-dominated gas mixture. RTGs carried by deep-space probes are essential for survival and operation in extreme environments. They address two important challenges: providing thermal control to prevent freezing and supplying basic power during prolonged lunar nights. If an RTG leaks, the operational lifetime of the probe is inevitably shortened; therefore, precise online measurement of argon gas leakage is essential. Conventional helium mass spectrometry leak detection cannot directly verify argon tightness. To overcome this, the developed system integrates three core components: (1) an in situ QMS calibration unit to maintain signal stability under dynamic vacuum conditions; (2) a dual-standard helium leak method coupled with molecular flow theory to derive an equivalent argon leak rate; and (3) precision-calibrated argon reference leaks for quantitative real-time measurement. The total leak rate of the RTG was determined to be (6.52 ± 0.55) × 10−7 Pa m3/s (coverage factor k = 2), with a relative expanded uncertainty of 8.38% as evaluated per GUM guidelines. The reference standard and QMS nonlinearity are the most heavily weighted contributions to the expanded uncertainty. Within the operating pressure range of 10−6–10−3 Pa, the system exhibits excellent linearity, with a nonlinearity contribution to uncertainty of about 2.10%. This work establishes a reliable, traceable methodology for direct, real-time argon leak testing of sealed radioactive components where conventional methods are insufficient, emphasizing the critical role of systematic error control in low-leak measurement.