5 Year In-Core Behavior of Gamma Thermometer Technology: Proof of Prototype Life ExpectancyThis paper summarizes the findings following a 5 year period of in-core operation of the Gamma Thermometer Local Power Range Monitor (LPRMs) calibration system. The 6 prototype assemblies, each containing a string of 7 thermocouples, showed a modest decrease in sensitivity of less than 0.2mV/g/W (13% loss of sensitivity) over the course of a total accumulated exposure of 33,000 MWd in an operating commercial nuclear reactor. These results reinforce the feasibility of this technology for use inactive commercial reactor cores with a minimum proven operating life of 5 years (33,000 MWd). The trend behavior of the sensitivity as the string is exposed to irradiation suggests that the operating life may be closer to 10 years before sensitivity decreases by 20% to below 1.2 mV/g/W from a 1.5 mV/g/W starting point at initial install. These results shed a favorable light on the technologies use in the upcoming GEH BWRX-300 Small Modular Reactor which incorporates GT technology as the sole calibration method for LPRMs during normal operation.