Environmentally Assisted Cracking Susceptibility Assessment of AP1000^® RCP Flywheel Retainer Ring A 289 18Cr-18Mn Steel Material Through Slow Strain Rate Testing Under Simulated Primary Water Environment

The AP 1000 reactor coolant pump design included the selection and scrutiny of a Flywheel Retainer Ring Material to ensure its performance during its service life. This paper summarizes the results of an autoclave slow strain rate test (SSRT) program undertaken to establish the Environmentally Assisted Cracking (EAC) susceptibility of AP 1000 RCP Flywheel Retainer Ring (A 289 18Cr-18Mn Steel) material under simulated RCS environment. The tests program represented “beyond design basis” condition of leakage in the Alloy 625 hermetically sealed flywheel can. There is very little service experience or test data available in the literature on the performance of A289 material under exposure to primary water environment. The current program is undertaken to support the ACRS request. The testing was done under two phases. The first phase consisted of preliminary screening testing at 1×10−6/sec. The second phase represented slower strain rate at 2×10−7/sec. Both phases accompanied control sample tests under nitrogen. The primary coolant environment was maintained close to 1000 ppm boron as boric acid and 2 ppm lithium as lithium hydroxide which corresponded to a 300?C pH value of 7.0. The concentration of hydrogen was maintained at 25 cc/kg and the test temperature was maintained at 260 to 280?F. A number of SSRT parameters were employed in assessing the material susceptibility to EAC. The fracture morphologies were correlated by Scanning Electron Microscopy. Overall, the 18Cr-18Mn material SSR tests demonstrated sufficiently adequate resistance to SCC in primary water. Details of test parameters and test results will be presented.