Materials inside nuclear reactors are exposed to extreme conditions, which include high temperature, high radiation doses, and corrosive conditions. Precise monitoring of a reactor environment is critical for its stability and proper functionality over the operational lifetime. To observe material performance (microstructure, chemistry, mechanical and other property changes with the changing conditions) while exposed to the reactor environment, real time monitoring of environmental conditions is required. This paper showcases the design of a novel, highly accurate, small size, reusable, real-time and reversible high temperature sensor for use within a nuclear reactor. The design is based on a hybrid plasmonic waveguide (HPW) structure comprising of chalcogenide glass (ChG) cladding on high index silicon optical waveguides. The transmitted power through the HPW structure in the transverse electric (TE) and transverse magnetic (TM) modes are simulated for both the amorphous and the crystalline states of the ChG phase change material. Our devices demonstrate a high extinction ratio of 120.4dB within a short length of 5 ?m of the waveguide, indicating the compactness of our designs. Moreover, monitoring the output power from an array of HPWs, wherein each silicon waveguide is coated with a different composition of ChG glass, provides a convenient way to monitor the temperature increase inside a nuclear reactor as a function of time.