Novel Heat Transfer Models to improve the Performance Prediction of Passive Residual Heat Transfer Systems

Passive safety systems are an economically interesting alternative to conventional active systems, which are also more robust against many external influences, as they do not rely on an external, active drive. Accordingly, they continue to function even if large parts of the plant infrastructure are damaged or unavailable, as it was the case in Fukushima Daiichi accident, for example. However, passive heat removal systems in particular pose major challenges for designers and the thermalhydraulic calculation tools they use. One reason for this is the coupling or feedback between the heat flow that is introduced into the coolant and the mass flow of the coolant through the system, which results from the heat input. In addition, state-of-the-art heat transfer models obviously cannot capture the heat transfer for passive systems precisely enough. As a result, attempts to recalculate experimentally determined heat transfer rates of passive heat removal systems with fluid dynamic codes have sometimes resulted in considerable deviations between experimental and numerical data. This paper presents two new heat transfer models developed specifically for passive systems. It describes how they can help to better calculate and predict the performance of related systems. The models were developed primarily based on experimental data recorded at the COSMEA and NOKO test stands and published by the Helmholz Center Dresden Rossendorf. In addition, the model development was supported by CFD calculations executed to better understand the underlying mechanisms.