New computational method with a fully-implicit scheme for a real-time accident simulator

Abstract

A new computational method was implemented in the FISA code (Fully-Implicit Safety Analysis code) to simulate PWR operational transients in a real-time accident simulator. The fluid model is based on the one-dimensional, homogeneous-equilibrium model including spatial acceleration, compressibility, and thermal expansion effects. In the finite difference form we use the non-conservative form for energy and momentum equations. The fully-implicit scheme is used to eliminate a time-step limitation for simulating slow transients. The Newton-Raphson method is used as an iteration scheme for quick convergence. First, the energy equation is solved to obtain enthalpy. Spatial acceleration terms in the momentum equation are manipulated with the mass equation so that mass flow rates may be expressed in terms of pressure only. Putting the mass flow rate expressed in terms of pressure into the mass equation, we finally obtain linear equations with pressure variables only as unknowns. The main advantage of the present numerical scheme over typical saftey 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 semiscale tests. Excellent accuracy and no stability limitation with fast running were proved.