STP1423

    Variability of In-Reactor Diametral Deformation for Zr-2.5Nb Pressure Tubing

    Published: Jan 2002


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    Abstract

    The diametral expansion of pressure tubes in CANDU™ reactors due to irradiation creep and growth is an important property that may limit the useful life of the tubes. Measurements accumulated over many years have shown that there is considerable variability in diametral strain rates between tubes. There is also considerable variability in the creep and growth response as a function of axial location, which is due to axial variations in operating temperature and flux, and to a gradual change in grain structure and crystallographic texture from one end of the tube to the other. The net effect is that pressure tubes tend to deform at a faster rate when the back end of the tube (i.e., the end leaving the extrusion press last) is installed at the fuel-channel outlet. The primary cause of the difference in microstructure along a given tube is the temperature change during the extrusion process. This end-to-end variation itself varies from tube to tube, due to variations in extrusion conditions from one extrusion run to the next, and also due to variations in ingot chemistry and billet processing.

    A semiempirical predictive model has been developed previously to represent the irradiation creep and growth behavior of a generic pressure tube, with a standardized microstructure, as a function of temperature and neutron flux. The diametral strain data from one hundred and twenty-five Zr-2.5Nb pressure tubes have been compared with the model. Deviations from predicted behavior have been correlated with the available microstructure, chemistry, and manufacturing data. Apart from obvious microstructural dependencies of diametral strain, such as the relationship with texture, grain structure is also a significant parameter that varies considerably from tube to tube and correlates strongly with diametral strain. The textures and grain structures, themselves, are related to manufacturing conditions (billet processing, extrusion pressures, temperatures, and soak times) and also, to some extent, on the impurity content of the material (due to the modifying effects on the Zr-Nb phase diagram).

    Keywords:

    Zr-2.5Nb, pressure tubes, zirconium alloys, nuclear reactor materials, neutron irradiation, deformation, creep, growth, flux, temperature, X-ray diffraction, microstructure, texture, grain size, oxygen, chemical analysis, statistical analysis, predictive models, rate theory


    Author Information:

    Griffiths, M
    Senior Researcher, AECL, Chalk River Laboratories, Chalk River, Ontario

    Davies, WG
    Senior Researcher, AECL, Chalk River Laboratories, Chalk River, Ontario

    Causey, AR
    Senior Researcher, AECL, Chalk River Laboratories, Chalk River, Ontario

    Moan, GD
    Manager, Reactor Engineering Services, AECL, Sheridan Park, Mississauga, Ontario

    Holt, RA
    Division Director, AECL, Chalk River Laboratories, Chalk River, Ontario

    Aldridge, SA
    President, Nu-Tech Precision Metals Inc., Arnprior, Ontario


    Paper ID: STP11417S

    Committee/Subcommittee: B10.02

    DOI: 10.1520/STP11417S


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