Study on transient temperature measurement at fuel clad surface in NSRR experiments

Vincent, B.*; Sugiyama, Tomoyuki ; Fuketa, Toyoshi

In NSRR experiments, fuel clad temperature is measured by thermocouples welded on the clad surface. This report presents evaluation of temperature measurement error due to thermal conduction to the thermocouple. Analyses using the CASTEM code showed that a transient capacitive phase occurs while clad temperature remains below 400 C. The temperature error reaches the maximum in this phase; -100 C (underestimation) without zirconia layer at the clad outer surface and +150 C (overestimation) with 100m zirconia layer. A "fin effect" phase starts when the clad temperature exceeds 400 C and the film boiling regime is clearly established, during which the error stabilizes between -20 C without zirconia layer and +50 C with 100m zirconia layer. The influence of the thermocouple is limited within a distance of about 0.5 mm. A transfer function was determined from the calculation results to estimate the accurate clad temperature from the thermocouple records.