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Material Reliability Study on Buried High Level Radioactive Waste Storage Tank Using Remote Ultrasonic Testing

Radioactive waste is stored in high level radioactive waste storage tanks at the Savannah River Site (SRS). Some of these tanks have been in service for over 50 years. Periodic visual and ultrasonic (UT) nondestructive examinations (NDE) have been performed to monitor the effects of service. Inspections revealed that several of the older, non-stress-relieved tanks developed stress corrosion cracking (SCC). An in-service inspection (ISI) program has been implemented to provide for detection and characterization of cracking, thinning, or pitting of the waste tanks. The program specifi es examination of regions of the tank that would be most susceptible to corrosion attack, to detect and characterize any fl aws and demonstrate acceptance to protect against potential leakage and instability. Several cracked areas on one of the older tanks were re-examined after fi ve years to evaluate materials performance and crack growth. A critical review of the work performed to understand stress corrosion crack behavior for the SRS waste tanks was completed. Each crack was then evaluated for service exposure history, consistency of the crack behavior with the current understanding of stress corrosion cracking, and present and future impact to the structural integrity of the tank. Crack instability calculations were performed on each crack. In all cases, the crack behavior was determined to be consistent with the previous understanding of stress corrosion cracking in the SRS waste tank environment. This paper provides a description of the evaluations performed, focusing on the nondestructive examination techniques utilized to perform the remote ultrasonic (UT) examinations of a high level radioactive waste (HLW) storage tanks at SRS. All inspections are performed relying completely on remotely operated equipment and cameras as personnel can not be in the area due to access constraints and radiation levels. The inspections are performed from the annular space of the buried, ~3.9 million liter waste storage tanks. A steerable, magnetic wheel wall crawler is inserted into the annular space through small risers (120 – 200 mm diameter) in the tank top. The crawler carries the equipment to simultaneously collect data with up to 4 UT transducers and 2 cameras.

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