Automated control of a pebble bed core thermal flow test unit / by Jan H.J. Prinsloo

Abstract

The HTTF (Heat Transfer Test Facility) is a unique project verifying the only pebble bed correlations currently used by PBMR (Pty) LTD. They are developing a new concept nuclear power station and are at present in the preparation phase of the conshuction of the worlds first PBMR (Pebble Bed Modular Reactor). The PBMR required the HTTF to be built at the North-West University in Potchefstroom. The HTTF consists of two separate test facilities: the H7TU (High Temperature Test Unit) and the HPTU (High Pressure Test Unit). The focus of this project will be on the HPTU. The HPrU is a unique test plant making a high range of test and operating conditions possible. The plant's test vessel can be loaded with eleven types of separate test sections, enabling it to do these tests. Pressure ranges and mass flow conditions vary in every test that is conducted. A design like this requires a complex control system able to control the plant during these variable test conditions. The HFTU has a very high safety requirement as it will be operated at extremely high pressures and, primarily because it will enable PBMR (Ltd) Pty to develop an inherently safe nuclear power plant. An automated control system needs to be developed to ensure the safety of this plant. The purpose of this study is to develop and deliver this safe, automated and user friendly control system that will be able to control the HPTU throughout its operating ranges. Research had to be done on its design to determine the plant's operating criteria. Furthermore, an investigation of the HTPU's characteristics and behaviour is necessary to fully understand the operation arrangement of the plant in order for it to be controllable. For the development of a complex, but absolutely safe protection systems, the operating margins have to be gathered. The plant will be operated for many hours at a time with limited number of operating personnel, which underline the necessity of research in the development of a modem plant user interface, as it will be the only communication path between the highly complex HPTU and the newly trained operators. It is not always possible to tune and simulate controllers for large plants because of their complexity. Additional tuning methods are required to do PID (Proportional Integral Differential) variable tuning. Most of these tests are conducted in the actual plant. A background study therefore had to be conducted on the development and tuning of industrial PID controllers. A control system previously developed for PBMR project that was completed at the end of 2002. This plant is called the Pebble Bed Micro Model (PBMM) and was, up until now, one of PBMR's proudest achievements. This control system was investigated to determine the control and protection system criteria. It was used as a resource of information for an equally complex and similar in size plant's control systems. The HPTU's automated control system consists of an OCS (Operational Control System) and an EPS (Equipment Protection System). The OCS will contain all the software necessary to control and protect the HPTU throughout all the operating conditions. It physically controls the plant by manipulating the actuators of the plant to perform the required functions. The EPS is a backup protection system for the OCS to ensure that critical plant operating parameters are not exceeded. This system is developed to protect and control the plant throughout all the possible operating scenarios. Prior to the possibility to develop a protection system like this, it was essential to fully understand and analyse the HPTU's design. To determine the required operating conditions, the modes and states were investigated. High risk machines and equipment were then identified to determine whether extra backup protection hardware would he necessary for the specific equipment. A simulator was developed for the HPTU to simulate and predict the operating behaviour of the plant and to design and test all the relevant PI controllers. The control system was designed and developed during the construction of the plant. Tuning of the controllers was done during the commissioning of the HPTU and a study of the results determined the performance of the controllers. The user interface is the interface between the operator actions and the plant. Modem engineering development like the HPTU required a modem user interface. Research was conducted to determine the effect that the conventional user interfaces had on operators in order to determine a optimum way to design and implement the system. Modem user interface was investigated to develop a control system that would allow good cooperation between operators and control systems. The hardware and control room setup was also designed to represent a quality control interface.