Changes in shape of internally pressurized tubes caused by operating temperatures and pressures are enhanced by fast neutron irradiation. Lengths and diameters of Zr-2.5Nb pressure tubes in CANada Deuterium Uranium-Pressurized Heavy Water (CANDU-PHW) power reactors and test reactors have been monitored periodically over the past 20 years. Axial and transverse strain rates have been evaluated in terms of operating variables and the crystallographic texture and anisotropic microstructure of the extruded and cold-drawn tubes. The anisotropic deformation occurring during steady-state irradiation creep and growth is described by a self-consistent model that takes into account the presence of intergranular stresses without building up any discontinuities of strain and stress at the grain boundaries. In this model, it is assumed that climb-assisted glide of dislocations on prismatic, basal, and pyramidal planes is the dominant creep mode and that growth occurs by net fluxes of interstitials and vacancies to a non-random distribution of dislocations and grain boundaries. The predictions from a deformation equation based on data from the Pickering and Point Lepreau Nuclear Generating Stations and the WR1, Osiris, DIDO, and NRU test reactors are in good agreement with measurements of pressure tubes in Bruce units. The equation has been employed as a material subroutine in the 3-D finite element code H3DMAP for predicting the detailed shape change of pressure tubes. The prediction from H3DMAP is a more complete description of shape change than that obtained from the closed-form expression.
The effect of texture and dislocation structure on irradiation creep of Zircaloy-2 (irradiated at about 340 K) and Zr-2.5 wt% Nb alloys (irradiated at about 558 K) is studied by means of a self-consistent model. The model relates the creep behaviour of polycrystals to that of single crystals by taking into account the crystallographic texture, dislocation density, grain shape and the intergranular stresses generated due to the crystallographic anisotropy. Three independent creep compliances of the polycrystal obtained from creep tests on a reference material are used to derive the single crystal creep compliances. These are used to calculate the polycrystalline compliances for the remaining materials. At low irradiation temperatures the predicted polycrystalline compliances agree well with the measured values. The observed behaviour can be described by a climb-assisted glide mechanism in which the creep strain is accommodated mainly by prismatic slip with smaller contributions from basal and pyramidal slip systems. At higher irradiation temperatures, the self-consistent approach can also describe well the creep behaviour of Zr-2.5 wt% Nb samples.
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).
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