An evaluation of thermowell's integrity in RCP test facility

An RCP test facility has been constructed at KAERI. During the commissioning startup operation, one of thermowells was cracked due to the high-speed coolant velocity. To complete the startup operation, a modified design of the thermowells was proposed, and all the original thermowells were replaced by the modified ones. From an evaluation of the original and modified designs of the thermowells according to the recent PTC code, the original design did not satisfy the new or old requirements, and the modified design simply satisfied the old requirement. The cracked surface of the original one was owing to the fatigue stresses in transverse oscillations. Two of three modified thermowells were found to be bent after about a 4.1 × 108 fatigue cycle operation. Their tapered-shanks came in contact with the nozzle walls and each showed a rim of deformation on the root. The modified ones suffered the fatigue stresses in in-line oscillations and became bent in in-line direction. A new design of thermowells with a straight shape and shorter unsupported length satisfying the current PTC code requirement was developed and installed in the RCP test facility. For the new design, additional modal analysis based on the finite element method was performed to check the dynamic behaviors using the ANSYS code and it was found that the PTC code predicted more conservative frequencies for the evaluation of dynamic behaviors in thermowell design.