Liquid radioactive waste is stored in large, underground carbon steel tanks at the United States Department of Energy’s Savannah River Site in Aiken, South Carolina. As a component of the site’s Structural Integrity Program, in-service inspection of these tanks is conducted to ensure tank integrity and to verify the effectiveness of waste chemistry requirements for mitigation of corrosion degradation. The inspections are performed using a projection image scanning (P-scan) automated ultrasonic testing device, which is remotely operated on a magnetic wall crawler. The inspections focus on gathering data related to the primary corrosion mechanisms of concern: general corrosion, pitting corrosion, and stress corrosion cracking. The inspections provide a real-time assessment of actual conditions to confirm that the corrosion control program is effectively mitigating corrosion.

A horizontal band of incipient pits was identified on one of these tanks over 25 years ago. As a precaution, the site has monitored this band of pits to determine if they were actively growing (i.e., pit depth increasing). In 2012, the location of nearly 100 of these pits were referenced. In 2022, the inspection plan for this tank required that these same pits be measured. A statistical analysis was performed to determine if a difference between the pit distributions measured in 2012 and 2022 exists. Statistical tolerance limits for pit depth were also calculated assuming the Largest Extreme Value (LEV), negative Weibull and log-Normal probability distributions for comparisons. The analysis confirmed that there was no significant difference between the two distributions and hence pit growth is not occurring. The analysis provided the basis for discontinuing future inspection of the horizontal band on this tank.

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