Analysis of steady-state thermal creep of Zr-2.5Nb pressure tube material
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ZIRCONIUM alloys are used extensively in CANDU* *CANDU is a trademark of Atomic Energy of Canada Ltd., Mississauga, Ontario, Canada.
power reactors and their behavior under different states of stress can be highly anisotropic. In these reactors, the alloy Zr-2.5Nb is used in the production of pressure tubes and its creep properties are a strong function of temperature, applied stress, and texture.[1–4] The anisotropy of deformation of Zr2.5Nb pressure tubes during service is related to the anisotropic physical properties of the hexagonal crystal structure of zirconium. During fabrication, the anisotropy of the single crystal contributes to the development of an anisotropic microstructure, including crystallographic texture, grain morphology, and dislocation structure.[5–10] The pressure tubes in CANDU reactors are manufactured from Zr-2.5Nb by hot extrusion and cold work. They are about 6-m long, with an inside diameter of 100 mm and a N. CHRISTODOULOU, Senior Scientist, is with the Deformation Technology Branch, Atomic Energy of Canada Ltd., Chalk River, ON, Canada K0J 1P0. Contact e-mail: [email protected] C.K. CHOW, Senior Scientist, is with the Fuel Channel Engineering Branch, Atomic Energy of Canada Ltd., Mississauga, ON, Canada L5K 1B2. P.A. TURNER, Assistant Professor, is with the Instituto de Fı´sica Rosario, University of Rosario, 2000 Rosario, Argentina. C.N. TOME´, Senior Scientist, is with the Materials Science and Technology, Los Alamos National Laboratories, Los Alamos, NM 87545. R.J. KLASSEN, Assistant Professor, is with the Department of Mechanical and Materials Engineering, University of Western Ontario, London, ON, Canada N6A 5B9. Manuscript submitted August 10, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS A
thickness of 4 mm. During service, they elongate several millimeters a year and they also increase in diameter. In reactor components, the presence of a flaw can be a consequence of a manufacturing defect or it can be created by an operating event. This event may comprise contact with another component or debris in the primary coolant leading to scratching or indentation of the material. The presence of a notch or volumetric flaw, which is defined as a flaw that occupies a volume and is generally blunt, will result in an increase in stress at the root of the flaw that could be many times that of the far-field stress. In the vicinity of a notch, the stress distribution and its evolution with time depend strongly on the plastic and creep response of the material, both of which are highly anisotropic in zirconium alloys.[11,12] In a recent publication, Leitch et al.[13] used neutron diffraction to measure the stress/strain field in the vicinity of a notch in a Zr-2.5Nb compact toughness specimen under an externally applied load. These authors showed that the stress around the notch relaxed with time due to thermal creep. The aim of the present work is to present the model that was used by Leitch et al. to describe the steadystate creep rate of this material as a function of the stres
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