Study of Grain-Level Stresses in CANDU Pressure Tubes

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Zirconium alloy pressure tubes comprise the in-core portion of the primary pressure boundary of CANadianUranium Deuterium (CANDU) reactors. These tubes are susceptible to degradation associated with theabsorption of hydrogen from coolant. The absorption of even small amounts of hydrogen can causesusceptibility to fracture or reduction in fracture toughness due to the precipitation of brittle hydride phases. Aspressure tubes age, engineers are required to continually develop more advanced evaluation procedures. Theseprocedures are based upon improved understanding of the nature of these degradation mechanisms and arevalidated against experimental data.Microscopically, pressure tube material consists of crystals where atoms are stacked with specific orientations.Each crystal is also known as grain. It has been shown that pressure tubes with different manufacturinghistories have different susceptibilities to hydride embrittlement. Although the macroscopic loads acting on thepressure tubes are well understood, there is a lack of data on the grain-level stresses, particularly close tohydrides. As hydrides and fracture are active on this level, it is likely that understanding this length scale couldhelp explain variability in hydride embrittlement.This project focuses on numerical modelling of grain-level stresses in pressure tubes. The modelling will bedone by a Crystal plasticity finite element code that can simulate the effects of material microstructures. Theexistence of possible relationship between microstructure and hydride embrittlement will be explored. Also, thefeasibility of a modern diffraction technique for measuring such stresses will be explored. This project will beconducted in collaboration with Kinectrics Inc. The outputs will support industrial understanding of the natureof hydride phases, and can be used by industrial partners to develop more advanced engineering codes.