(An) advanced numerical scheme for transient thermal hydraulic analysis in a nuclear power plant

Abstract

A new computational method, named HEMFIS (Homogeneous Equilibrium Model Fully Implicit Scheme), was developed to simulate thermal hydraulic operational transients in a real-time accident simulator for PWR. The fluid model is based on one-dimensional, single-fluid conservations for mass, energy, and momentum, including spatial acceleration, compressibility, and thermal expansion effects. In the finite difference form, we use the nonconservation form for the energy and momentum equations. The fully implicit scheme is used to eliminate a timestep limitation for simulation of slow transients: both of convective and sonic terms are treated implicitly. The nonlinear finite difference equations are linearized by the Newton-Raphson method for quick convergence. First, the energy equation is solved to obtain enthalpy. The spatial acceleration term in the momentum equation is manipulated with the mass equation so that the mass flow rate can be expressed only in terms of pressure. Putting the mass flow rate expressed in terms of pressure in the mass equation, we finally obtain the linear equation with the pressure variables. The main advantage of the present numerical scheme over the typical safety analysis codes such as WFLASH and RELAP-4 is its ability of fast running, maintaining its accuracy comparable to them. The numerical scheme was tested with analytical methods and LOFT blowdown experiments. Excellent accuracy and no stability limitation with fast running were proved.