In CANDU® reactors, zirconium alloys are used for three tubing components. Zircaloy-4 is used for the fuel sheaths. Zr-2.5Nb alloy is used for the pressure tubes, and Zircaloy-2 is used for the calandria tubes. These three tubes are manufactured using different processes, and they have different crystallographic textures and microstructures due to the different processes used to manufacture the tubes. These tubes normally operate at low temperatures (< 350°C). In a hypothetical loss-of-coolant accident (LOCA), these tubes may be subjected to high temperatures up to 1000°C or higher. During the temperature transient, a phase transformation occurs that changes the microstructure which affects the material‘s high-temperature deformation behaviour.
This thesis investigated the texture evolution, phase transformation and mechanical anisotropy in as-manufactured and two modifications ('A' and 'B') of CANDU Zr-2.5Nb pressure tube materials using neutron diffraction during heating up to 1050°C and on cooling. These investigations provided a further understanding of the anisotropic deformation behaviour observed in high temperature-ramped tensile deformation tests. The stable evolution of the hexagonal close-packed (hcp) α-Zr texture and changes in volume fraction of body-centered cubic (bcc) β-Zr, resulting from temperature changes during heating (or cooling) has provided key information to help explain the mechanical anisotropy response of as-manufactured pressure tube material. CANDU-type Zircaloy-4 fuel sheaths with as-manufactured and two simulated braze zone microstructures (one for the spacer location and the other simulating the heat-affect- zone) were also investigated. An in-situ approach has been developed using SELFPOLY (a polycrystalline model) to determine the Kearns (texture) numbers and Hill's anisotropy factors with texture (Orientation Distribution Function) data obtained on fuel sheaths with previously known anisotropy factors measured using strain tests. The same approach was applied to the two modified Zr-2.5Nb pressure tube materials for comparison with the mechanical anisotropy of differently textured fuel sheaths. Further research work to validate the anisotropy factors are suggested using ballooning tests on fuel sheaths and high temperature-ramped tensile tests on the two modified pressure tube materials.
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