Dynamic modelling of the natural convection water cooling principle

Abstract

The aim of this project is to investigate the operating characteristics of the Reactor Cavity Cooling System (RCCS) of the Pebble Bed Modular Reactor (PBMR) demonstration power plant during the active and passive operation conditions. Fortunately, the RCCS experience no circulation anomalies during active operation conditions, due to the pumps circulating the water. However, in the event of the passive operating conditions, the pumps are not functioning, and the system is dependant on natural convection to circulate the water within the system. Some circulation anomalies develop during the passive operation conditions and the system tends to overheat. During the passive operation conditions of the RCCS, the heat generated by the Reactor Pressure Vessel (RPV) is transferred to the standpipes of the system and natural convection is set into action. As the temperature in the system rises, the natural convection speeds up. Unfortunately, when the temperature surpasses a certain temperature the system tend to stop circulating properly and the water in the system start to boil, thus resulting in the system overheating. The study aims to gain a better understanding of the dynamics of the RCCS by using dynamic modelling methods. A previous study on the RCCS has been done and a Flownex® simulation model of the system does exist. The simulation provides a representation of the hydraulic behavioural characteristics of the system during passive operating conditions. By developing a dynamic electrical circuit model of the existing RCCS model, it will be possible to study the natural convection phenomenon in another energy domain. The hydraulic simulation performed by means of Flownex will serve as reference for the results obtained from modelling the system by means of the developed equivalent electrical model. By manipulating the analogue circuit components analytically and by utilising fundamental circuit laws which includes Kirchoff's Law and Ohm's Law, state-space equations will be derived. The research can be used to refine the design of the RCCS, thus ensuring circulation without anomalies, during passive operation conditions. This will enhance the safety factor of the PBMR nuclear plant dramatically. The state-space equations can be utilised to determine the poles of the system. The pole positions convey valuable information regarding the stability of the system elements. There is still a lot of suspicion concerning the safety aspects of nuclear energy. The PBMR technology addresses and eliminates most of these suspicions and with the energy crisis the world is currently experiencing, nuclear energy is steadily becoming a more viable and much needed resource option.