Modelling, simulation and analysis of a supercritical carbon dioxide decay heat removal system

Abstract

In case of a station blackout and loss of the ultimate heat sink, as occurred in the Fukushima accident, the supercritical carbon dioxide (sCO2) decay heat removal system can be used to transfer the decay heat to the ambient air. This innovative, compact and self-propelling system consists of a compact heat exchanger, a gas cooler and the turbomachine, a compressor and a turbine mounted on a single shaft. The discussion of the state of the art revealed that further research is required on all topics related to this system, e.g. modelling, validation, design, control and simulation. In this thesis, the models of the thermal-hydraulic system code ATHLET were improved, extended and validated for the simulation of this system. This includes the fast and accurate calculation of the sCO2 properties, heat transfer and pressure drop correlations for sCO2, water and air and heat exchanger and real-gas turbomachinery models. Subsequently, a cycle design strategy was proposed which maximizes the excess power at the highest ambient temperature to ensure a self-propelling operation at any boundary condition. These conditions include conservatively high as well as conservatively low decay heat curves and an ambient temperature range from -45 °C to +45°C. Furthermore, the modular design, with several sCO2 cycles with a design heat removal capacity of 10 MW per cycle, is applicable to different nuclear power plant types and sizes. Several operation and control strategies, which were developed in this thesis, facilitate reliable operation, even far from the design point. Operation at any ambient temperature is enabled by keeping the compressor inlet temperature constant at its design point. The turbine inlet temperature control combined with the successive shutdown of single cycles allows smooth operation along the decay heat curve. Further strategies reduce the thermal stresses, enable a relatively fast start-up or increase the heat removal capacity of the system further. For detailed analysis, the system was coupled to a generic Konvoi pressurized water reactor with a thermal power of 3840 MW, considering a combined station blackout and loss of the ultimate heat sink scenario. The results demonstrate that a system with four CO2 cycles provides sufficient heat removal for more than 72 h. Several failure analyses, e.g. failure of single cycles, control or fans, indicated that the system can even tolerate most of these unlikely events. Beyond nuclear safety, the code development, modelling, validation and simulation efforts contribute to research on sCO2 cycles in general, e.g. with regard to future power generation.