EVALUATION AND IMPLEMENTATION OF METHODOLOGY FOR DISPOSITIONING FLAWS IN Zr-2.5 Nb PRESSURE TUBES OF CANDU~? POWER REACTORS

摘要

Adequate assessment of detected flaws is a critical step in demonstrating the fitness for service of the affected components and thereby assuring safe operation of nuclear power plants. Methodologies for the assessment and disposition, for some key structures, systems and components, could significantly impact the safety and economics of operating power reactors. This paper reviews a process, employed by staff of Canadian Nuclear Safety Commission, for the evaluation and implementation of a new methodology for assessing flaws in operating CANDU pressure tubes against initiation of delayed hydride cracking. It, thus, illustrates from perspectives of regulatory authority staff, what areas are of the most concern in evaluating a new flaw assessment methodology, and what measures might be applied in order to minimize any possible risks or uncertainties associated with its applications. While the implementation of the new methodology has resulted in the avoidance of many unnecessary replacements of pressure tubes, recommendations from regulatory authority staff also lead to further understanding of the degradation mechanisms and to the foreseeing of challenges for long-term operation. The process and focal points of regulatory evaluation might need to be taken into consideration for future new assessment methodology developments for the licensees and their R&D community.