The effect of inoperative thermocouples on K reactor limits

Previous operating procedures for the K reactor required management approval for continued operation if more than six monitor pin thermocouples were inoperable. Currently, there are eight inoperable thermocouples and this number is expected to increase after reactor restart. This report provides the basis and the power impact for allowing up to 20 inoperable thermocouples in the core. The SRS reactor operating limits are specified as limits on the assembly average effluent temperature, which is determined by averaging the four quadrant temperatures at the bottom of an assembly. Azimuthal asymmetries in fuel and target material distribution and in the flow channel areas contribute to an azimuthal temperature variation. Therefore, for an asymmetric assembly, each monitor pin thermocouple measures a different temperature. If a thermocouple fails in an assembly, the control computer substitutes the highest remaining temperature for the failed thermocouple and computes the average. In some cases failed thermocouples are permanently jumpered to the remaining thermocouple having the highest temperature. If the hottest thermocouple fails in an assembly, the calculated average temperature determined from thermocouple substitution is lower than the actual assembly average temperature. For this reason, allowance must be made for the error in the average temperature measurement due to a failed thermocouple