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.

A method has been developed to incorporate the local three-dimensional shell behavior of two concentric tubes in the two-dimensional beam modeling of the problem. The two dimensional modeling of fuel channels in CANDU pressurized heavy water nuclear reactors is used in lieu of a more accurate three dimensional finite element approach in order to reduce the on-line simulation time which greatly affects the SLAR (Spacer Location And Repositioning) maintenance operation cost during outage. However, effort must be made to include the three-dimensional shell behavior of these channels into the two-dimensional modeling. In recent studies a nonlinear force-dependent model for contact stiffness between the calandria tube and pressure tube has been developed. However, local deformation of calandria the tube at spacer locations due to in-reactor creep leads to settling of the spacer into the calandria tube that consequently reduces the gap between the two tubes. In this work, the effect of local deformation (elastic and creep) of calandria tubes on modeling of contact at spacer locations is assessed using a three dimensional finite element code. The result is incorporated into a two-dimensional beam model of the problem as a reduction in size of the spacers that separate the two tubes. It is shown that the proposed method increases the accuracy of prediction of contact time and the spacer. In general, the method described in this paper suggests a way to incorporate local shell deformation into beam models of slender shell structure.