Applications of Probabilistic Fracture Mechanics for Pressure Tubes

Abstract of the technical paper/presentation presented at:
Structural Mechanics in Reactor Technology Conference (SMiRT 25)
August 4–9, 2019

Prepared by:
B. Wasiluk, K. Tsembelis and J. Jin
Canadian Nuclear Safety Commission


Over time, pressure tubes in a Canada Deuterium Uranium (CANDU™) reactor experience in-service degradation including the development of three types of volumetric flaws: a) debris fretting flaws, which are created when foreign materials are trapped between a fuel bundle and the pressure tube, b) bearing pad fretting flaws, which are developed at contact points between fuel bundles and pressure tubes, and c) crevice corrosion flaws, which are related to localized corrosion being affected by lithium. Fitness for service of pressure tubes is demonstrated on an ongoing basis using several methodologies fundamentally based on fracture mechanics. The Canadian Nuclear Safety Commission (CNSC) closely monitors industry fitness-for-service programs, and performs detailed reviews of industry fitness-for-service evaluations for inspected pressure tubes and reactor core assessments. Additional evaluations of the full reactor core composed of both inspected and uninspected pressure tubes in accordance with CSA N285.8, Technical requirements for in-service evaluation of zirconium alloy pressure tubes in CANDU reactors Clause 7, entitled “Assessment of reactor core”, requires evaluations for a) protection against fracture for all service level transients, b) evaluation of pertinent degradation mechanisms related to flaws, and c) leak before break (LBB), described below. This clause permits the application of either deterministic or probabilistic methodologies. The evaluation for fracture protection is currently performed using a deterministic methodology outlined in CSA N285.8, while a new, industry-developed probabilistic fracture protection methodology and acceptance criteria are currently under review by the CNSC. The assessment of pertinent degradation mechanisms related to flaws uses the probabilistic core assessment (PCA) methodology. This reactor core-based methodology evaluates crack initiation and growth from postulated volumetric flaws followed by an LBB evaluation. The estimated pressure tube rupture frequency per year must be lower than the allowable values provided in table C.1 of CSA N285.8. The probabilistic LBB evaluation methodologies are based on an evaluation of either a single bounding pressure tube or the entire reactor core. The results must be lower than the maximum allowable conditional probabilities of pressure tube rupture given a through-wall crack as per CSA N285.8. The areas requiring further development are discussed.

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