Research Of Packing Characteristics And Effective Thermal Conductivity For Pebble-Bed Fluoride Salt Cooled High Temperature Reactor

The 10 MW Pebble-Bed Fluoride salt cooled High temperature Reactor(PB-FHR)is an important reactor that integrates the coolant and fuel technology from the liquid fuel molten salt reactor and high temperature gas cooled reactor to improve safety margins and commercial availability.Reactor safety design requires that the core decay heat can be discharged by passive means such as heat conduction,natural convection and thermal radiation under any accident conditions.This passive heat transfer mode is closely related to the effective thermal conductivity of the pebble bed,which directly affects the core temperature distribution.Previous studies on the effective thermal conductivity of pebble-bed are mostly focused on high temperature gas cooled reactor.Considering the different characteristics of pebble-bed stacking in PB-FHR,especially the great difference between the thermophysical properties of core coolant FLiBe and helium gas,such as the stronger natural convective heat transfer of FLiBe than helium gas and the different influence on radiation that lead to the relevant theoretical models and experimental results can hardly applied in PB-FHR.From the reactor safety design,it is necessary to study the natural convective heat transfer and obtain the effective thermal conductivity of the actual 10 MW PB-FHR pebble bed core.This is of great significance to understand the heat transfer process correctly and to carry out the safety analysis during the loose of forced coolant accident.The main research contents include the following three parts.Firstly,the particles movement under buoyancy in 10 MW PB-FHR is simulated by DEM to investigate the effects of particle diameter,the friction coefficient between particles and the particle number on the steady state packing pebble bed structures.The simulation results show that packed bed with smaller diameter can weaken the oscillation of the axial and radial porosity,which will help to smooth the power distribution in the reactor core.By reducing the friction between particles,thevibration process can be well reflected and the relationship between the friction coefficient and packing porosity as well as the coordination numbers agree with the negative exponential distribution.The bed average porosity and coordination number tends to be constant with the value of 0.43 and 5.6 respectively,when the filled particles number exceeds about 10000,after which the influence of particle number minimized,then the packing bed with 10000 filled particles can be used as a representative of the full packed bed to study the thermal-hydraulic numerical analysis of the natural circulation in 10 MW PB-FHR,which will help saving the computational resources.Secondly,based on the discrete element method(DEM)and computational fluid dynamics(CFD)method,a general numerical methodology for analyzing the natural convective heat transfer in the pebble bed core is established,and the validity of numerical methodology is demonstrated by comparing with the experimental results operating in the gas cooled reactor(HTR).Meantime,the lacking experimental data of the limit temperature ranges are also predicted by the numerical method to improve the data integrity and the corresponding effective thermal conductivity of HTR is between 0 ~ 32 W/(m·?).The effects of helium and nitrogen cooling media on the steady-state temperature distribution of the pebble bed are compared.The results show that the fluids with strong natural convective capacity have greater influence on the temperature distribution and tend to flatten the radial temperature distribution of the pebble bed core.Because the numerical methodology takes into account the heat conduction,radiation and natural convection effects and there is no simplification in the heat transfer process,it can be used in the simulation of fluoride salt cooled pebble bed reactor.Finally,based on the results and method of previous studies,the heat transfer process in the 10 MW PB-FHR core is simulated.The results show that the natural convective mechanism of FLiBe plays an important role in the total heat transfer,and the change of its thermal radiation absorption coefficient has little effect in determining the effective thermal conductivity.Because of the strong convective characteristics of FLiBe,the bed temperature along the axial direction of the bedpresents a distinct temperature gradient,and a large temperature gradient is found in the boundary layer near the hot and cold wall,while the temperature distribution in the bulk region keeps almost unchanged where the natural convection is relatively weak.The molten salt flows are mostly concentrated between the hot and cold walls to form a single circulation motion.The effective thermal conductivity in PB-FHR under the limited temperature ranges is about 100 ~ 280 W/(m·?).