Verification of fuel thermocouple readings during IFA-431 irradiation

Part of the IFA-431 irradiation test involved power decreases from approximately 10 kW/ft to 8 kW/ft (peak power) in 20-30 seconds. During these transients, output was recorded from all 12 fuel thermocouples and from a cobalt neutron detector centered in the assembly. A special data-logging system was used to do this, and a full set of data was recorded once every 3 seconds. Several of the power decreases were found to be sufficiently linear to permit straightforward analysis of the time-dependent fuel centerline thermocouple response. This response can be used to predict the true value of the initial fuel centerline temperature, under the assumption that (1) the power decrease is truly linear and (2) the transfer function between rod power and centerline temperature is independent of the power change. The bias introduced by assumptions (1) and (2) is not insignificant, but is smaller than the scatter in the present data, and is predictable for each case. When the estimated bias is accounted for, we find the fuel thermocouple readings to be in agreement with their predicted values within 750C