Analytical and experimental study on the effective thermal conductivity of VHTR fuel block geometry with multiple cylindrical holes

In Korea, Very High Temperature gas-cooled Reactor (VHTR), PMR200, is being developed in Nuclear Hydrogen Development and Demonstration (NHDD) project. Hexagonal graphite fuel block of PMR200 contains lots of cylindrical coolant holes and fuel compacts. Due to the lots of holes and fuels, transverse heat transfer phenomenon in fuel block becomes very complicated. Especially in the accident situations when forced convection is lost, the heat flows in radial direction through the large number of coolant holes. It makes more complex heat transfer phenomena as the radiation heat transfer and natural convection is added to the heat transfer modes. Despite the complexity in heat transfer modes, the accurate analysis on the heat transfer in fuel block is necessary since it is directly relevant to the integrity of nuclear fuel embedded in fuel block. However, the effective thermal conductivity (ETC) in which the effects of all of heat transfer modes are lumped is necessary for calculating the heat transfer of a fuel block. Because the detailed calculation of numerous fuel blocks and coolant holes needs excessive computing resources. There can be some uncertainties in the ETC correlations, and therefore the accuracy of those should be assessed properly. In this study, GAMMA + model is introduced, which is a set of correlations to calculate the ETC of the fuel block in the GAMMA+ code, which is developed to analyze VHTR thermo-fluid transients at KAERI. And the accuracy of the correlation is assessed by the commercial CFD code, CFX-13. GAMMA+ model consists of three parts. At first, Radiation heat transfer model gives equivalent radiation conductivity, which is added to gas conductivity to produce net conductivity of coolant hole. And this coolant hole conductivity and graphite conductivity is homogenized to the ETC by the analytical model. At last, the effect of bypass gap is reflected to ETC of graphite block by bypass gap model. The results of CFD analysis were consistent with GAMMA+ model. Although a little disagreement was shown for the case of high temperature, the result of GAMMA+ model was relatively accurate for the low-temperature cases. In the calculation of the VHTR fuel block geometry, the existence of fuel gap makes large uncertainties in evaluating the ETC. Therefore, more researches on this topic seem to be necessary. The corresponding experiment is also being prepared for the experimental assessment of GAMMA + model.