Case Studies in Root Cause Analysis: Identifying Opportunities for Improved Maintenance of CANDU Fuelling Machines

Abstract

The charge tube assembly of the CANDU® fuelling machine contains numerous moving/rotating components (ball screws, linear and rotating bearings, mechanical gears, mechanical seals, etc). Premature wear-out and/or failure of key charge tube components can result in the need for unscheduled or excessively frequent overhauls and repairs, along with its associated negative effects. This paper presents two examples of root cause investigations involving the charge tube and ram assembly. The first case study details the metallurgical failure examination of a failed ram assembly that fractured at the threaded connection between a ram drive screw and ram extension tube. The failure was discovered several hours after the ram was driven manually to a stall condition in response to ram encoder errors. Detailed metallurgical examinations of the failed ram identified the primary fracture mode as stress corrosion cracking, which occurred during the application of a sustained tensile and torsional load while the ram was in the full retract position. Difficulties encountered during installation of the dowel pin resulted in the formation of microcracks, which subsequently grew by stress corrosion cracking. This was the root cause of the failure. In the second case study, the fuelling machine (FM) head H5B at Bruce B was taken out of service for a partial overhaul. At that time, it was discovered that the charge tube rear ball nut had sustained excessive ball wear damage which appeared to be confined to only one groove. The damaged charge tube ball nuts, balls, deflectors, and ball return tubes were examined. The rear charge tube ball nuts displayed evidence of alternating or intermittent wear on the outboard surfaces of the front and rear ball tracks. In contrast, the front ball nut exhibited more uniform and less severe wear of the ball tracks. A wear mechanism is presented that is consistent with the observed damage pattern. The proposed wear mechanism is based on the assumption that the balls are not free to move along the ball track as the ball screw is rotated. The wear experienced by the ball nuts results in a loss of pre tension, affecting the fine positioning of the charge tube. In each case, the root cause analysis determined that there were opportunities for improvements in maintenance practices that will improve the overall reliability and performance of the fuelling machines at Bruce Power.