Applications of Probabilistic Fracture Mechanics for Pressure Tubes With Flaws

Abstract of the presentation presented at:
2nd International Seminar on Probabilistic Methodologies for Nuclear Applications
October 25-26, 2017

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

Abstract

Pressure tubes operated in CANDU reactors experience several degradation mechanisms, including those related to volumetric flaws. There are three main types of such flaws: (a) debris fretting flaws mostly created at the beginning of reactor operation, (b) bearing-pad fretting flaws due to fretting between fuel bundles and pressure tubes, and (c) crevice corrosion flaws related to localized corrosion with the assistance of lithium.

The inspection findings for volumetric flaws are dispositioned to demonstrate protection against crack initiation using methodologies based on deterministic fracture mechanics. In accordance with CSA Standard N285.8, the disposition of inspection findings shall be accompanied by evaluations for the entire reactor core, including protection against fracture for all service level transients, evaluation of all pertinent degradation mechanisms related to flaws and leak before break (LBB). These assessments can be performed either deterministically or probabilistically. Currently, while protection against fracture is demonstrated deterministically, the other two assessments are performed probabilistically using Monte Carlo methods. The first assessment of pertinent degradation mechanisms related to flaws was carried out by the Canadian nuclear industry as early as 2006 using the probabilistic core assessment (PCA) methodology. This reactor core-based methodology combines an assessment of related flaw types with a LBB evaluation. Volumetric flaws are postulated for the entire reactor core and allowed to potentially initiate cracks. Three crack initiation mechanisms are currently known: (a) delayed hydride cracking (DHC), (b) hydride region overload (HROL) and (c) fatigue. Initiated cracks are simulated to grow by DHC and can ultimately penetrate through the entire wall of a pressure tube. In turn, through-wall cracks are assessed for LBB. The result of the assessment is the pressure-tube rupture frequency per year, which must be lower than the allowable frequency in table C.1 of CSA Standard N285.8. The probabilistic LBB evaluation methodologies recently introduced by the Canadian nuclear industry are based on simulations of either a single bounding pressure tube or the entire reactor core. The acceptance criterion is the maximum allowable conditional probability of pressure-tube rupture given through-wall cracking as per CSA Standard N285.8-15. The paper discusses lessons learned from applications of probabilistic fracture mechanics for pressure tubes with flaws, the areas remaining areas for further development and possible international collaboration.

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