Modelling a pebble bed high temperature gas-cooled reactor using a system-CFD approach

Abstract

The objective of this study was the development of a systems-CFD model of the PBMR reactor that used the minimum number of grid points to achieve grid independence. The number of grid points is reduced by increasing the accuracy of the discretisation scheme in the reactor model. Any reduction in the number of grid points leads to an increase in calculation speed, which is critical for systems simulation codes that are used for optimisation or transient simulations. While some previous reactor models had been developed for systems simulation codes, their discretisation schemes have not been optimised to use the minimum number of grid points and some heat transfer phenomena were neglected without knowing the effect. Therefore, there was a need to optimise discretisation schemes as well as investigate the effect of including certain heat transfer mechanisms. Modelling methods for several phenomena were developed and implemented in a reactor model in the Flownex systems simulation code, which is used to simulate the PBMR. Subjects of investigation included pebble bed convection discretisation, fuel sphere discretisation, the effect of the radiation heat transfer modelling approach as well as conjugate conduction and radiation across the helium riser channels in the reactor side reflector. After testing the phenomenological models in isolation, the comprehensive reactor model was tested by simulating the SANA experiment and HTR-10 reactor experiments published by the IAEA. Several sensitivity studies were performed to assess the effect of physical as well as numerical parameters. Two reactor discretisation schemes were also evaluated, namely the control-volume based scheme and the element-based scheme. The control-volume based scheme was found to provide a simpler and more intuitive framework for implementing mathematical models, but not to increase accuracy directly. The most significant finding was that the newly developed second-order accurate convection heat transfer scheme gives the greatest improvement in calculation speed by requiring the least number of pebble bed increments. The other important finding was that the methods currently used in many reactor simulation codes for fuel sphere discretisation and radiation heat transfer approximation are appropriate and give adequate accuracy.