LABORATORY ANALYSIS OF CRACKED CRDM HOUSINGS FROM PALISADES

摘要

This paper describes the results of laboratory analyses performed on three cracked control rod drive mechanism (CRDM) housings removed from locations #8, #23, and #25 at Entergy's Palisades Nuclear Station. In-situ eddy current testing from the inner diameter (ID) revealed that all three housings exhibited indications on the inner diameter (ID) surface at the elevation corresponding to the machined weld buildup region designed for aligning the internal CRD mechanism. Housings #23 and #25 each contained one indication, whereas housing #8 contained 26 indications. Laboratory investigations included visual inspections, penetrant testing (PT), dimensional measurements, residual stress measurements using X-Ray diffraction (XRD), optical metallography, Vickers microhardness, scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS). All of the cracks examined initiated at the ID surface in the weld deposit. Weld pores were typically present at the initiation site at or near the ID surface, although no pore or weld defect was observed for two of the cracks. The absence of a weld pore at the initiation of these two cracks indicated that the weld pores are not a necessary condition for cracking. Visual examination of the laboratory-opened cracks showed distinct crack growth events, or beach marks. The deepest cracks contained six or seven beach marks, whereas shallower cracks contained three to five beach marks. Cracking was generally oriented in the axial direction, although angled cracks with circumferential features were also present. This finding suggested the stresses responsible for the cracking were not oriented strictly in the hoop direction, but included an axial component as well. The ID surface residual stress measurement results for the housing (measured in the hoop direction only) indicated the highest residual surface tensile stresses were generally located at the lower edge of the machined weld deposit where many of the cracks initiated. Stresses as high as +745 MPa (+108 ksi) were measured. Fractographic and cross sectional examinations revealed that cracks exhibited a fan-shaped cleavage morphology with minor crack branching. Crack propagation of this type is consistent with transgranular stress corrosion cracking (TGSCC). Material, stress, and environmental conditions that contributed to the TGSCC of these housings are discussed, and remedial actions taken by the site to mitigate or eliminate future failures of CRDM housings are reviewed.