Start-up and shutdown control of a three-shaft Brayton cycle based power conversion unit / by Stefan A. Laubscher
Laubscher, Stefan Andries
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The Pebble Bed Modular Reactor (PBMR) is a graphite-moderated, helium-cooled reactor that uses the Brayton direct gas cycle to convert the heat, which is generated in the core by nuclear fission. The heat is transferred to the coolant gas (helium), and converted into electrical energy by means of a gas turbo-generator. The Pebble Bed Micro Model (PBMM) is a model that was developed to demonstrate the operation of the closed, recuperative three-shaft Brayton cycle in order to gain a better understanding of its dynamic behaviour and to demonstrate the control strategies that will eventually be used in the PBMR. It is also used to demonstrate the ability of Flownex to simulate the integrated performance of the cycle. Flownex is a general-purpose thermal-fluid network analysis code. It solves the flow, pressure and temperature distribution in large unstructured thermal-fluid networks and provides the engineer or designer with essential information about the interaction between network components. Flownex can handle a wide variety of network components such as pipes, pumps, orifices, heat exchangers, compressors, turbines, controllers and valves. A complete Flownex model of the PBMM exists and will be used throughout the entire project. The start-up and shutdown sequences of the Pebble Bed Modular Reactor (PBMR) is characterised by numerous checks and control actions that occur in a very specific order. This order of events and continuous checking of important system parameters are intuitively based on rules that inherently describe the safe and preferred operating region of the plant. Presently most of the sequences and actions are performed by an expert who knows the system very well. Some of the sequences and actions are controlled by simple controllers and are only activated by the operator. A need therefore exists to capture the different control events of the start-up and shutdown sequences in a control model or method. A control model or method was developed to control the start-up and shutdown sequence of the PBMM model in Flownex. The control method was implemented in SIMULINK® as there is an existing interface between Flownex and SIMULINK®. The control method directly controls the procedures of start-up and shutdown through the Flownex/ SIMULINK® interface. The most important system parameters are monitored and adjustments are automatically made by the control method in response to system changes. A graphical user interface allows a user to manually activate start-up and shutdown. The aim of the project is to develop a controller that can control the PBMM through start-up as well as shutdown. Special emphasis is placed on critical transitions and pressure considerations due to the control of the start-up blower system rotational speed. The project establishes expertise in the control of the start-up and shutdown sequence of the PBMM in Flownex. Flownex was successfully used as a modelling platform to simulate the normal and dynamic operation of the PBMM. Identification of start-up and shutdown was a result of investigating the start-up and shutdown checklist documents as well as a detailed consideration of data captured by the PBMM. A control method was implemented in SIMULINK® and controls start-up and shutdown through the Flownex/SIMULINK® interface. A graphical user interface allows in time system changes by a user. Data captured by the simulation model was evaluated against data captured from the real system.
- ETD@PUK