Examination and Evaluation of in-Reactor Fracture of Shroud Tubes in Mixed-Oxide Fuel Experiment X159

During disassembly and subsequent visual examinations of X159 (a Mark A-19A type of subassembly containing mixed-oxide fuel), 11 of 19 shroud tubes were found fully or partially severed. Several of the capsules within their shroud tubes were distinctly kinked at axial locations near those at which the shroud tubes were severed. The examination also disclosed that the shroud-tube fractures occurred during reactor operations.