A Hybrid Computer Simulation of the U-Tube Dynamics in Two-Phase Control Absorbers for Nuclear Reactors

Abstract

This report presents the development of a mathematical model describing the dynamic behaviour of a two-phase (gas and liquid) control absorber. This new type of absorber, invented in Italy, is being developed at the Reactor Control Branch of the Chalk River Nuclear Laboratories, Atomic Energy of Canada Ltd., for possible use in future CANDU nuclear reactors. The purpose of the mathematical model is: (1) to provide a tool for evaluating experimental data from a two-phase absorber test rig. (2) for use in larger simulations to investigate the controllability of future CANDU power plants. The mathematical model of the two-phase absorber presented here includes the U-tube dynamics, return line dynamics, gas line dynamics and liquid line dynamics. However, only the U-tube model has been implemented on the Hybrid Simulator. It is based on the conservation laws (mass, energy and momentum) using cross sectional averaged variables. A continuous space discrete time scheme is used to solve the void propagation equation. The U-tube model has been tested with perturbations in both time and frequency domains. However, the remainder of the simulation needs to be implemented on a hybrid computer to provide the parameters for direct comparisons with experimental data from the two-phase absorber test rig.