The Dynamic Simulation of a Two-Node Non-Equilibrium 600MW CANDU Pressurizer System

Abstract

The dynamic simulation of a pressurizer is presented. The pressurizer is modelled as a two-node (liquid and vapour) non-equilibrium system. The dynamics of the final control elements (steam relief valve and heaters) are incorporated into the model. The system is defined by the energy and material balances written for both the liquid and vapour nodes. The liquid node may be saturated or subcooled and the vapour node may be saturated or superheated. Depending on the state of each node, flashing (of liquid) and the condensation (of vapour) are accounted for. The thermodynamic state of heavy water is determined from the Helmholtz free energy. Numerous thermodynamic partial and total derivatives of heavy water are needed in the energy and material balances. These terms are derived using thermodynamic derivations and Maxwell relationships and they are expressed as simple algebraic functions of the Helmholtz free energy and its first and second derivatives. The usual pressure-volume search for the solution into the next time step has been transformed into pressure-enthalpy search by expressing the rate form of the equation of state. The simulation code is run on microVax-II (or on any other microcomputer) and Mac II equipped with 68881 coprocessor. The simulation code has been written such that it can readily be integrated into the DSNP (Dynamic Simulator of Nuclear Plants) language with dynamic displays of the liquid level, pressure and temperature and with setpoint control interrupt by the operator (Mac 11).