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High-temperature gas-cooled reactors (HTGRs) use a gaseous coolant for heat transfer between the nuclear core and two or more steam generators. Leakage of steam or water from the steam generators to the coolant would expose the nuclear core to water vapor. A moisture measuring system is required to determine the moisture content of the coolant gas in the range of 0.1 to 3000 volume parts per million (ppm). Another requirement is the rapid detection of large leaks resulting in 2000 ppm or more and the identification of the leaking steam generator, thus permitting isolation of the faulty coolant loop. An optical dewpoint detector has been developed that can be used either as a dewpoint monitor or as a dewpoint trip device. The response time of the device as a trip instrument is typically 1 sec in the dewpoint range of 27Â°F to 128Â°F (100 to 3000 ppm). As a dewpoint monitor, the mirror temperature can be changed at a rate of 1Â°F/sec, in the range of -87Â°F to +128Â°F (0.1 to 3000 ppm). The moisture detector head is designed to operate at the full coolant pressure of 700 psia. In the HTGR application, access to the device is difficult during reactor operation, and will be cumbersome at all times because of gamma radiation environment. Therefore, exhaustive testing of all detector head components, subassemblies, and materials selection from inorganic substances has been performed to reduce maintenance to a minimum.