This paper summarizes the fuel bundle damage characterization done by Nuclear Safety Solutions Limited (NSS) and the out-reactor flow visualization tests done at Atomic Energy of Canada Limited (AECL) to reproduce the damage observed on irradiated fuel bundles. The bearing pad damage mechanism was identified and the tests showed that a minor change to the fuelling sequence would eliminate the mechanical interaction. The change was implemented in January 2005. Since then, the bearing pad damage appears to have been greatly reduced based on the small number of discharged bundles inspected to date.
This paper presents the results of a comparison of time-averaged channel powers with the channel powers obtained from using time-averaged burnups. The analysis was carried out on a Bruce A and Bruce B reactor using the core tracking SORO program, which calculates the flux, burnups, bundle powers, and channel powers. The time-averaged channel powers from the two methodologies were found to be in good agreement.
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.
Bruce Units 1 to 4 were shut down during the 1990s, largely as a consequence of tube degradation resulting from inappropriate steam generator secondary side water chemistries. Following a condition assessment, Bruce Power restarted Units 3 and 4 and is currently refurbishing Units 1 and 2. In order to assess the condition of the Unit 1 and Unit 2 preheaters and determine their suitability for extended operation, inspection, maintenance and assessment activities have been conducted. Eddycurrent and visual inspection have revealed vessels in generally good condition. Secondary side internals appear largely undegraded. Some tube to support fretting has been observed, and a number of tubes have been removed from service because of debris fretting concerns. To prepare for return to service, the primary side divider plates have been replaced and the tubes have been ID cleaned to restore the preheater to its original condition. This paper summarizes the inspection planning, findings, assessment for extended operation and maintenance activities undertaken.
The Canadian CANDU Industry is implementing a composite analytical solution to demonstrate, with high confidence, adequate safety margins for Large Break Loss of Coolant Accidents (LBLOCA) in existing CANDU reactors. The approach involves consolidating a number of individual approaches in a manner that alleviates reliance on any single analytical method or activity. Using a multi-layered approach, the objective of this composite solution is to use a variety of reinforcing analytical approaches such that they complement one another to collectively form a robust solution. The composite approach involves: i) systematic reclassification of LBLOCA to beyond design basis events based on the frequency of the limiting initiating events; ii) more realistic modeling of break opening characteristics; iii) application of Best Estimate and Uncertainty (BEAU) analysis methodology to provide a more realistic representation of the margins; iv) continued application of Limit of Operating Envelope (LOE) methodology to demonstrate the adequacy of margins at the extremes of the operating envelope; v) characterizing the coolant void reactivity, with associated uncertainties; and vi) defining suitable acceptance criteria, accounting for the available experimental database and uncertainties. The approach is expected to confirm the adequacy of existing design provisions and, as such, better characterize the overall safety significance of LBLOCA in CANDU reactors. This paper describes the composite analytical approach and its development, implementation and current status.
