An assessment of the CRDM alloy 600 reactor vessel head penetration PWSCC remedial techniques

This paper summarizes the evaluations and results of an autoclave accelerated SCC (stress corrosion cracking) test program designed to assess the effectiveness of selected surface remedial techniques to mitigate Alloy 600 PWSCC (primary water stress corrosion cracking) in the reactor vessel head penetration base and weld materials in PWRs. The testing employed ring samples representing commercial heats of penetration tube material in service. The test rings were stressed by cyclic ovalization to represent the penetration weld residual stresses and were treated with specified repair and mitigative remedial techniques prior to exposure in the autoclaves. A reference sample fabricated from the Ringhals 2 susceptible penetration tube material with known cracking history was included in the testing for the purpose of 'real time bench marking' the test results. The results of the accelerated SCC testing suggested that depending on the treatment employed, the assessed remedial treatments could be effective in extending the penetration life by a period ranging from a few cycles to beyond the licensed life of the plant. The anticipated benefit is largely determined by the details of the application of the process and the vessel head (service) temperature. There appears to be room for improvements to be made in all processes, but sufficient information is available for utilities to make informed judgments. (authors)