Spherical fuel elements are distributed randomly in the pebble bed reactor core and helium flow through the pebble bed to remove nuclear reaction heat. Pebble bed reactor core is usually treated as a uniform porous media flow in thermal hydraulic research. However the porosity distribution is nonuniform and the porosity near the wall increase sharply. A new random model is developed in this paper to investigate thermal hydraulic characteristics of pebble bed reactor core. Porosity assumption is based on porosity measurement of other research. Porosity simulation is divided into three parts according to the distance from wall. In the center of core, porosity is assumed to obey normal distribution, where average porosity is from the experimental relation based on statistical results. The mean and standard deviation of porosity distribution near the wall will increase because of the wall effect, where the distance from the wall is about three times of fuel ball’s diameter. The third part is zone from three times to five times of ball’s diameter departed from the wall. The wall effect of this zone is between center and the wall zone. Based on above assumption, a random porosity simulation is completed to apply in this research. COMSOL Multiphysics 3.5a software is used in this research. COMSOL Multiphysics are a calculation platform using proven Finite Elements Methods (FEM). In this research, Brinkman equation for porous media flow is applied in the simulation. Non-thermal Balance model is used in local heat transfer research between gas and pebble bed. A geometry model is built to simulate HTR-10. Temperature profile of variant porosity is gained from stationary analysis and comparison with uniform porosity is also discussed in the paper. For transient analysis, four cases simulation is carried out in the research. Case 1 and 2 simulate heat transfer phenomena with forced cooling system and with passive cooling system after reactor shut down. Way-Wigner-curve is used in Case 1 and Case 2 to simulate decay heat in the calculation. Case 3 and Case 4 simulate ATWS phenomena with natural convection and without natural convection system when blower is trip off in normal operation. Simulation results also are compared with some ATWS experiments and some discussion is done in the paper. From the results, it can be seen that random porosity will affect temperature distribution near the wall and make outlet temperature non-uniform greatly. The maximum temperature of variant porosity is much greater than the maximum temperature of uniform porosity at the same condition. Transient analyses of variant model show that passive cooling system can remove residual heat even in accident conditions when the blower trip off whether reactor shut down or not and the analyses results correspond substantially with experimental results. In general, variant porosity should be considered in the thermal hydraulic research of pebble bed reactor core. Variant porosity model can provide good prediction of heat transfer phenomena than uniform porosity model. Especially it can explain some transient analysis results.

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