Modeling In-Reactor Deformation of Zr-2.5Nb Pressure Tubes in CANDU Power Reactors

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.