Validation of a transient thermal-fluid systems CFD model for a packed bed high temperature gas-cooled nuclear reactor

This paper provides an overview of the theoretical basis for a new thermal-fluid systems CFD simulation model for high temperature gas-cooled reactors, contained in the Flownex software code. Flownex provides for detailed steady-state and transient thermal-fluid simulations of the complete power plant, fully integrated with core neutronics and controller algorithms. The reactor model is founded on a fundamental approach for the conservation of mass, momentum and energy for the compressible fluid flowing through a fixed bed, as well as the heat transfer in the pebbles and core structures. The time-wise integration of the resulting differential equations is based on an implicit pressure correction algorithm. This allows for the use of rather large time steps making it very suitable for simulating the slow transients that can be expected to follow incidents like reactor shutdowns. The paper also compares the Flownex results for four transient tests with the measured results from the SANA test facility as well as to the results of simulations with the Thermix/DIREKT code that were done at the Research Centre, Jülich. The Flownex results compare well with the Thermix/DIREKT results for all the cases presented here. Good comparison was also obtained between the simulated and measured results, except at two points within the pebble bed near the inner wall. The fact that quick computer simulation times were obtained indicates that the new model indeed achieves a fine balance between accuracy and simplicity. However, the discrepancies obtained at the two points near the inner wall, together with the fact that additional uncertainty was introduced in the original SANA test set-up by not being able to control the temperature of the outer wall, highlight the need for additional systematic tests to be performed in order to better validate the new model.