CFD simulations of separate effects in an HTGR lower plenum under air ingress condition

Abstract: The local heat transfer in an HTGR lower plenum is investigated at the Czech Technical University (CTU) in Prague with collaboration with the U.S. Nuclear Regulatory Commission (NRC). NRC has initiated efforts to build a technical infrastructure necessary to support licensing activities for HTGR''s. These efforts include the development and evaluation of computational fluid dynamics (CFD) tools to analyze the system during various conditions. In this case air ingress condition is investigated. The local heat transfer under air ingress condition is strongly affected by the physical phenomena such as molecular diffusion and natural circulation. These separate effects are not yet well understood. Therefore, in the first phase of research it is necessary to perform separate-effects simulations. The CFD solver is exercised and the results are compared with experimental data. The separate-effect benchmarking of the CFD code includes simulations of two separate phases of the JAERI experiments: isothermal molecular diffusion of a binary gas mixture and non-isothermal diffusion and natural circulation of a binary gas mixture. Computational model of experimental apparatus consists of a vertical inverted U-tube connected at the bottom to a cylindrical tank. Commercial CFD solver Fluent 6.3 was used for the simulations. The influence of the size of computational mesh and length of time step of unsteady solver was studied. For the simulation of isothermal molecular diffusion the coarser mesh with about 9000 hexahedral cells was suitable. The length of time step about 0.05s seems to be optimal. The non-isothermal diffusion and natural circulation simulations were made and compared with experimental data. The onset time of the natural circulation was simulated in good agreement with experiment with small deviation of 2.3%. [Copyright &y& Elsevier]