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The objective of this program is to identify and establish the properties of reliable hightemperature thermocouple, electrical, and electronic materials to provide instrumentation and electrical components for use in fast breeder reactors. W VERSUS W -25Re THERMOCOUPLE CHARACTERISTICS AT HIGH TEMPERATUREDetermining the centerline fuel temperature of LMFBR fuel elements requires temperature measurements to approximately 2500°C. Thermocouple systems used to measure this temperature must be constructed of materials that a r e metallurgically and chemically compatible with each other and with the anticipated environment. Although these materials may be compatible, the electrical properties (resistivity and thermoelectric power) of the sheath, insulators, and environmental gas may still influence the thermoelectric signal, making validity of indicated temperatures uncertain. Loss of effective electrical insulating properties of ceramic oxides when used in conjunction with a high resistance of thermoelements has beenobserved at high temperatures by several investigators,',' and this effect was measured as a function of temperature.