Laboratory Analysis of Lower End Cap Weld

Abstract

This paper covers examinations designed to characterize the integrity of a lower end cap to fuel cladding weld produced using a high-pressure resistance welding process from a U.S. Boiling Water Reactor. The work scope included visual inspections, replication, isotopic characterization, X-ray diffraction (XRD), machining, scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS), and material microstructure characterizations. The purpose of this testing was to assess the integrity of the weld and determine if it might be a contributing factor in the fuel cladding failure.

Initial reports indicated that the end cap separated from handling of the fuel rod during poolside exams. Subsequent laboratory examinations of the fracture indicated the separation occurred completely within the cladding. The predominant cladding failure mode was brittle fracture with a trace amount of ductility. Several ID-initiated radial cracks were observed throughout the circumference of the cladding.

Results indicated that the hydriding and cracking were ID-initiated, resulting in brittle failure of the cladding. Hydride concentrations were significantly higher along the cladding ID, which indicated that the source of the

hydrogen, and oxygen, was likely a point remote from the lower end cap weld region examined, consistent with secondary hydriding damage. The weld bondline was difficult to discern and did not exhibit any evidence of disbond or lack of fusion. Based on this, and other points to be discussed, there was no evidence that the weld contributed to the cladding fracture and eventual separation of the lower end cap from the fuel rod.