Simulation of operational and accidental behaviour of modular high temperature reactors with Brayton cycle power conversion unit

Abstract

The present work analyses and investigates the behaviour of a High Temperature Reactor (HTR) with a Pebble Bed core connected to a Brayton cycle Power Conversion Unit (PCU) during operational and accident conditions. The modelling of a complete circuit including both the PCU and the Pebble Bed Reactor has been performed with the commercial thermal-fluid analysis simulation code Flownex. Flownex has been developed for High Temperature Pebble Bed Reactor applications, and has been extensively validated against other codes. As the reactor core model incorporated in Flownex is a simplified model based on 0D point kinetics, the extended 1D WKIND core model was implemented in the analysis calculations using a special coupling methodology. This study introduces a new sub-routine which enables the coupling of the WKIND reactor core model to the Flownex PCU model via an external interface. The interface facilitates the data exchange between the two codes, allowing for necessary manipulations and synchronisation of the coupled codes. By doing so, the 1D diffusion equation solution implemented in WKIND core model replaces the point kinetics model implemented in Flownex. This replacement allows for a detailed accurate solution even for very fast transients, through the treatment of the space-dependent heat conduction from the graphite matrix to helium. Flownex component models have been validated against the experimental results of the 50 MWel direct helium turbine facility Energieversorgung Oberhausen (EVO II). This provided the opportunity to validate Flownex calculations against experimental data derived from a large-scale helium Brayton cycle installation. Small differences observed in the results could be explained. Based upon steady state and transient analysis it is concluded that Flownex models simulate accurately the behaviour of the components integrated in the EVO II plant. Such models could be applied to analyse the transient behaviour of the total system of the reactor and the PCU. In the present thesis, both the reactor core and the PCU have been modelled with a very high level of details. Due to the direct coupling, the reactor core and the PCU have a large and fast influence on each other. Hence, it is important to investigate the interactions between the two for the safety analysis of the compete plant. Furthermore, a comparison between two system layouts of the PCU was investigated in this study, namely a single and a three shaft configurations. With the complete system model created it is possible to precisely simulate a great variety of operations which were demonstrated in several selected cases. These include the withdrawal of control rods, turbo-machinery trip, load following and a helium leak. Transient simulations results incorporated also both shaft configurations. The results show that the point kinetics core model is sufficiently accurate, with an exception to strong reactivity transients. In such cases, the analyses using the Flownex point kinetics model over-predicts the core thermal power. Further investigation is needed for improving the coupling methodology and the data exchange between the codes. It was proven that from the thermo-dynamical behaviour point of view, a quick response to a range of power demands, using a simple design of the control system, advocates the single shaft system configuration. However, further investigation should be done to rectify this, especially during long-term part load performance of the system.