A numerical algorithm for analyzing piping response to thermal transients and seismic excitations

This paper describes a three-dimensional finite-element method for the structural response analyses of reactor piping systems subjected to thermal transients and seismic excitations. In the thermal-transient analysis, the algorithm appropriately accounts for stresses arising from the temperature dependence of the elastic material properties, the thermal expansion of the materials, and the change in the temperature-dependent yield surface. It includes a thermoviscoplastic constitutive equation that incorporates capabilities of treating thermal softening, failure, strain rate, creep, and stress ratcheting. In the piping seismic analysis, a time-history method is utilized to account for possible nonlinear phenomena as well as to reduce the solution conservatism obtained from response-spectrum analyses. Variation in dominant stress frequencies and translational frequencies are easily handled. The algorithm described herein has been integrated with the implicit time-integration scheme for treating hoop, flexural, axial, and torsion deformations induced by these accident loads. It is valid for large displacement and rotations, but small strains. Both geometrical and material nonlinearities are considered. To illustrate the methodology, several sample problems are given. Results show that this approach is extremely efficient and is suitable for long-duration analyses of reactor piping systems. (orig.)