Nonequilibrium Transient Analysis of Nuclear Steam Generators.

Abstract

This thesis describes the development and verification of a nonequilibrium drift-flux steam generator model to simulate and analyze various transients encountered in nuclear plants utilizing light water reactors. The nonequilibrium drift-flux model is based on a one-dimensional, separated-flow, single-channel formulation. The five fluid conservation equations used in the formulation include the continuity, momentum, and energy equations for the mixture and the liquid and vapor equations. The liquid and vapor equations are derived by combining the continuity equation with the energy equation for each phase. The conservation equations are derived in a form that allows for easy comparison among various two-phase flow models. The finite-difference form of the nonequilibrium drift-flux equations is obtained through the staggered mesh structure of the implicit continuour-fluid Eulerian (ICE) scheme. The actual numerical solution utilizes a three-step iterative method, which is designated as the EICEN scheme, or the nonequilibrium version of the extended ICE method. The heat flux terms for the liquid and vapor equations are obtained through a proposed energy partition model which accounts for the nonequilibrium effects of both vapor and liquid, and satisfies the constraints in the steady-state limit. Specific models for the integral economizer once-through steam generator (IEOTSG) and the U-tube steam generator (UTSG) are developed utilizing the EICEN nonequilibrium two-phase flow models. A lumped-parameter steam dome model has been developed to represent natural circulation paths in the UTSG designs. For the three-equation slip-flow option, a quasi-moveable boundary formulation has been also developed to allow a smooth representation of the boiling boundary movement. The steady-state and transient steam generator models developed in the present study are incorporated into the TRANSG-N computer code. Several test calculations were performed to assess the accuracy and applicability of the TRANSG-N models. Comparison between the calculated results and the experimental data are generally favorable. Comparison is also made of the various two-phase flow models including the three-equation slip-flow and the four-equation drift-flux formulations.