Towards CFD modelling of critical heat flux in fuel rod bundles

The paper describes actual CFD approaches to subcooled boiling and investigates their capability to contribute to fuel assembly design. In a prototype version of the CFD code CFX a wall boiling model is implemented based on a wall heat flux partition algorithm. It can be shown, that the wall boiling model is able, to calculate the cross sectional averaged vapour volume fraction with good agreement to published measurements. The most sensitive parameters of the model are identified. Needs for more detailed experiments are established which are necessary to support further model development. Nevertheless in the paper the model is applied for the investigation of the phenomena inside a hot channel in a fuel assembly. Here the essential parameter is the critical heat flux. Although subcooled boiling represents only a preliminary state toward critical heat flux essential parameters like the swirl, the cross flow between adjacent channels and concentration regions of bubbles can be determined. By calculating the temperature at the rod surface the critical regions can be identified which might later on lead to departure from nucleate boiling and possible damage of the fuel pin. The application of up-to-date CFD with a subcooled boiling model for the simulation of a hot channel enables the comparison and the evaluation of different geometrical designs of the spacer grids of a fuel rod bundle.