Improvement of core effective thermal conductivity model of GAMMA+ code based on CFD analysis

• We assessed the core effective thermal conductivity (ETC) model of GAMMA+ code.
• The analytical model of GAMMA+ code was compared with the result of CFD analysis.
• Effects of material property of composite and geometric configuration were studied.
• The GAMMA+ model agreed with the CFD result when the fuel gap is ignored.
• The GAMMA+ model was improved by the ETC model of fuel compact including fuel gap.

The GAMMA+ code has been developed for the thermo-fluid and safety analyses of a high temperature gas-cooled reactor (HTGR). In order to calculate the core effective thermal conductivity, this code adopts a heterogeneous model derived from the Maxwell’s theory that accounts for three distinct materials in a fuel block of the reactor core. In this model, the fuel gap is neglected since the gap thickness is quite small. In addition, the configuration of the fuel block is assumed to be homogeneous, and the volume fraction and material properties of each component are taken into account. In the accident condition, the conduction and radiation are major heat transfer mechanism. Therefore, the core effective thermal conductivity model should be validated in order to estimate the heat transfer in the core appropriately. In this regard, the objective of this study is to validate the core effective thermal conductivity model of the GAMMA+ code by a computational fluid dynamics (CFD) analysis using a commercial CFD code, CFX-13. The effects of the temperature condition, material property and geometric modeling on the core effective thermal conductivity were investigated. When the fuel gap is not modeled in the CFD analysis, the result of the GAMMA+ code shows a good agreement with the CFD result. However, when the fuel gap is modeled, the GAMMA+ model overestimates the core effective thermal conductivity considerably for all cases. This is because of the increased thermal resistance by the fuel gap which is not taken into account in the GAMMA+ model. By adopting the effective thermal conductivity of the fuel compact including the fuel gap, the accuracy of the GAMMA+ model was improved.