“Mitigation of Stress Corrosion in Reactor Nozzles and Core Structures” J.S. Porowski, B Kasraie, E.L. Danfelt

Keywords: Inconel transition pieces; differential thermal expansion; vessel penetrations; Inconel 600 safe-ends; Control Rod Drive Mechanisms; MSIP

Inconel transition pieces are often used in nuclear vessels to reduce differential thermal expansion stresses in low alloy vessel penetrations connected to austenitic pipes. Inconel 600 have been used at most of the nozzles in BWR reactors for such applications. When cracking was observed in a reactor with Inconel 600 safe-ends after only six years of operation, many of these safe-ends were replaced during the early stages of life in new reactors. Cracking was also observed in Inconel stubs of Control Rod Drive Mechanisms (CRDM) nozzles attached to reactor head penetrations by Jwelds. Furthermore, at certain locations, such as pressurizer nozzle weldments, axial cracks have been detected in the nozzle tube at a location adjacent to the J weldment. In addition, sever damage due to stress corrosion cracking was found in BWR core shrouds……Mechanical Stress Improvement Process has been used to improve weldments in Light Water Reactor piping systems, including RPV nozzles and safe ends since 1986. The present paper describes the results of current work on improving stress patterns in different types of reactor nozzles in both BWR and PWR vessels.

The Mechanical Stress Improvement Process (MSIP) has been used to improve weldments in Light Water Reactor piping systems, including RPV nozzles and safe-ends since 1986. Recent work on CRDM nozzles has shown that the tensile stresses can also be effectively redistributed using simple mechanical means for this relatively complex connection geometry. The present paper describes the results of current work om improving stress patterns in different types of reactor nozzles in both BWR and PWR vessels.

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