Failure Analysis of 6.8 Evaporator Thermowell

The Savannah River Technology Center Materials technology Section was requested to determine the cause of failure for a resistance temperature device and thermowell assembly that was used in the 6.8 Low Activity Waste evaporator. A methodical and well planned approach was used for the failure analysis task with special precautions because of the high radiation and contamination levels. Two sections of the failed assembly were selected for thorough analysis, the bottom of the inner tube with the end-cap and a piece of inner tube at the vapor/waste interface. The failure analysis consisted of macroscopic examination and metallographic analysis. Intergranular attack (IGA) was found to be the primary corrosion mechanism that led to eventual failure. IGA of the end cap occurred because of a presumed preferential microstructure and accelerated the corrosion rate over that of inner-tube side wall. Once the end cap was breached, the waste quickly attacked the RTD components, leading to the low resistance to ground readings that indicated the initial RTD malfunction. A metallographic analysis of an unexposed end cap is recommended to confirm the suspect microstructure