Thermal Analysis of A Pebble Bed High Temperature Gas Cooled Reactor

Abstract

The pebble bed type of high temperature gas cooled nuclear reactor is a promising option for next generation reactor technology and has the potential to provide high efficiency and cost effective electricity generation. The reactor unit heat transfer poses a challenge due to the complexity associated with it. In the pebble bed, heat is generated in kernels inside the fuel sphere. The generated heat is conducted to the sphere surface where it is transferred to the coolant (helium gas) by means of convection. In the bed itself when a temperature gradient exists across the bed, heat is transferred between pebbles by means of conduction and interstitial radiation in the radial and axial directions. This necessitates a heat transfer model that deals with radiation as well as thermal convection and conduction. Modeling such a complex thermal hydraulic system requires the use of variety of techniques and simulation tools ranging from a one-dimensional model to large scale three dimensional model using computer codes based on analytical or computational fluid dynamics. In this research, a simplified analytical model of the pebble bed type high temperature nuclear reactor heat transfer was developed to analyze the convection, conduction and radiation heat transfer phenomena, as well as the loss of pressure through friction in the pebble bed. Correlations derived from experimental data or directly from experimental testing were used to approximate the thermo-physical properties of the pebble bed. The developed model was implemented on a personal computer using a simulation code PEBTAN (PEbble Bed Thermal Analysis code) which was written in FORTRAN 95 programming language. Simulation and numerical experiments were conducted, and the results showed very good agreement with published data. The model can adequately account for the heat transfer phenomenon, and the loss of pressure through friction in the pebble bed type high temperature nuclear reactor. The model can serve as the bases for a more detailed three dimensional computational fluid dynamic analysis.