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An AC-powered, radial-heat-flow apparatus has been developed for the measurement of the thermal conductivity, k, of cylindrical insulations at temperatures up to 800°C. Steady-state k measurements are made on 0.91 m-long specimens using a radiant heater as the interior heat source. The ends of a cylindrical test specimen are insulated but not actively guarded. Power to the heater is measured using a precision AC watt meter and the temperature gradient across a test specimen in the radial direction is measured by calibrated type-k thermocouples. The apparatus can be used to test cylindrical specimens with wall thicknesses up to 8 cm thick. The design of the apparatus was guided by finite-difference modeling of the two-dimension heat flow in a finite-length hollow cylinder heated on the inside. The key design parameter is the length of the specimen needed for temperature measurements at the mid-point of the specimen to be insensitive to axial heat flow.