The calibration of silicon carbide temperature monitors in a fast reactor

Silicon carbide pellets have been used as the standard temperature monitor in uninstrumented rigs in the Dounreay Fast Reactor for 10 years. The irradiation-induced expansion of the β-silicon carbide was plotted as a function of its post-irradiation anneal temperature, and the temperature at which recovery of the expansion began to take place was taken to be the irradiation temperature. Some recent research, however, suggested that silicon carbide monitor results might not exactly correspond to the actual irradiation temperatures as measured by thermocouples. In the present work, 22 silicon carbide monitors were irradiated against thermocouples in the Dounreay Fast Reactor across the temperature range 4250 to 5750C. The monitor results were directly proportional to the corresponding thermocouple readings, but with a constant silicon carbide 'hot bias' of approximately 20 deg C. Using the derived correlation equation, equivalent thermocouple temperatures can be calculated from future monitor results; the 95% confidence limits on an irradiation temperature calculated from the mean result of a group of 3 identically irradiated monitors is +- 20 to 25 deg C. (author)