Cracking and occasional leaks have been reported in some Boiling Water Reactor (BWR) control rod drive (CRD) stub tubes. Roll expansion of the housing against the Reactor Pressure Vessel (RPV) bottom head penetration has been used successfully to provide a leak barrier. The recently approved ASME Code Case N-730 “Roll expansion of Class 1 Control Rod Drive (CRD) Bottom Head Penetrations in BWRs, Section XI, Division 1” provides the specific criteria for the application of roll expansion. The minimum roll band length in the Code Case was based on the requirement that the roll joint capability exceed the scram forces on the CRD. The roll joint capability was based on a simplified analytical model with assumed friction factors. The predictive model was then compared with the results of extensive testing on mockups. This paper describes the results of the testing that has been performed to determine the load capability of roll repairs for different roll band lengths, material combinations (stainless steel and Alloy 600), percent wall thinning, thermal cycling and surface condition. The mock-ups were rolled using procedures and rolling equipment similar to those used in actual plant application. The mock-ups were tested in a testing machine by applying a ‘push force’ on the housing. In addition to measuring the force using a load cell, strain gages were also used to measure the strains on the housings. LVDTs were used to monitor the displacement during the test. The results showed that the resistance of the rolled joint (i.e. the load capability) is proportional to the roll length. The load capability was not a strong function of wall thinning or thermal cycling. It was strongly affected by the surface condition (e.g. oxidation) and the housing material yield strength. The predictive model was consistent with the test results and confirmed that the roll expansion joint has substantial load capability. Thus, the roll joint is not only a leak barrier, but also a structural load-carrying joint that is sufficient to resist the upward scram loads on CRDs.

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