Acceptance criteria for ultrasonic flaw indications in the inner liner of double-shell waste storage tanks

Radioactive defense waste, resulting from the chemical processing of spent nuclear fuel, has been stored in double-shell tanks (DSTS) at the Hanford Site since 1970. As part of the program to assure that the DSTs maintain their structural integrity, an inspection plan is being developed and implemented. This report provides recommendations and technical bases for acceptance criteria for flaw indications detected during ultrasonic inspection of inner liners of the DSTS. The types of indications addressed are crack-like flaws, wall thinning, and pitting. In establishing acceptable flaw sizes, the evaluations have taken into consideration the potential for crack growth by the mechanism of stress corrosion cracking. Consideration was given to technical approaches used in ASME Codes, for reactor tanks at the Department of Energy Savannah River facilities, and in recommendations by the Tank Structural Integrity Panel. The goal was to ensure that indications discovered during inspections are not large enough to ever cause a leak or rupture of the tank inner liner. The acceptance criteria are intended to be simple to apply using a set of tables giving acceptable flaw sizes. These tables are sufficiently conservative to be applicable to all double-shell tanks. In those cases that a flaw exceeds the size permitted by the tables, it is proposed that additional criteria permit more detailed and less conservative evaluations to address specific conditions of stress levels, operating temperature, flaw location, and material properties