Experimental And Numerical Study On Structural,Thermal,and Mechanical Characterizations Of Tritium Breeder Pebble Beds

Solid tritium breeders have been widely used in the solid blanket design of the fusion reactor due to their advantages of low material corrosion and no MHD effect.The Helium-Cooled Ceramic Breeder(HCCB)blanket of the China Fusion Engineering Testing Reactor(CFETR)uses the monosized Li4SiO4 pebble bed as the tritium breeder and the Be pebble bed as the neutron multiplier.The Water-Cooled Ceramic Breeder(WCCB)blanket utilizes the mixed bed of Li2TiO3 and Be12Ti pebbles to breed tritium and multiply neutrons.The thermo-mechanical properties of the pebble bed are directly related to the heat transfer and mechanical properties of the blanket,and also affect the reliability and safety margin of the blanket operation.Therefore,the purpose of this paper is to carry out experimental and simulation research on the packing structure properties,heat transfer characteristics,and thermo-mechanical properties of the pebble bed,so as to provide data support for the development of blanket technology.First of all,this paper carried out the experimental characterization of the pebble bed packing structure based on X-ray tomography.The overall and local packing characteristics of the binary mixed pebble bed are studied,and the packing structure properties of the pebble bed are obtained.The unique porosity distribution and typical oscillation characteristics of the binary mixed pebble bed near the wall are found.There are n=D/d peaks of the radial porosity distribution within the range of one large particle diameter D from the wall for the binary pebble beds with different particle size ratios δ.A semi-analytical correlation is given to predict the average coordination number of the binary pebble bed with different δ and XL.The cumulative distribution functions for the contact angle for pebble beds are similar to the curve 1-cos(θ),which does not change with the δ and XL.Secondly,an experimental platform including a test section and a helium loop is designed to measure the effective thermal conductivity(ETC).The test section is based on the hot wire method.For the Li2TiO3 pebble bed and the Li4SiO4 pebble bed,experiments are performed in the presence of stagnant helium at atmospheric pressure.The uncertainty of the measurement result of the ETC is about 5.5%.When the temperature increases from 20 to 400℃,the ETC of the Li2TiO3 pebble bed and the Li4SiO4 pebble bed in helium increases from 0.83 W/(m·K)to 0.96 W/(m·K)and from 0.54 W/(m·K)to 0.61 W/(m·K),respectively.Based on the DEM-CFD oneway coupling method,the influence of solid contact conduction and solid-fluid-solid conduction on the ETC of pebble beds are studied.The ETC of the pebble bed with size distribution,the monosized pebble bed with different particle sizes and the binary pebble bed are simulated.When ks/kf more than 20,the contact conduction causes a nonnegligible effect on the overall heat transfer of pebble beds.For lithium titanate pebble beds with similar packing factors,in the case that the radiation heat transfer can be ignored,the effect of the particle size distribution on the ETC of the monosized pebble bed can be ignored.During the simulation,the monosized pebble bed can be used instead of the pebble bed with size distribution to simplify the modeling work.In addition,a device for conducting thermo-mechanical experiments on pebble beds is designed and built.The ETC experiments of Li2TiO3 pebble beds and Li2TiO3&304SS pebble beds at different temperatures and different stresses are carried out,as well as uniaxial compression tests at different temperatures.An improved empirical correlation for the ETC of the pebble bed,which is related to temperature and packing factor,is given.By Reimann fitting,the experimental input parameters of elastoplasticity in thermo-mechanical constitutive models of the Li2TiO3 pebble bed and the Li2TiO3&304SS pebble bed are obtained.Finally,based on the FEM method,the thermo-mechanical constitutive models of different pebble beds are constructed and compared with the experimental results.The thermo-mechanical performance analysis of the sandwich model of the HCPB blanket and the typical structural model of the WCCB blanket is carried out.The temperature field distribution,volumetric inelastic strain distribution and hydrostatic pressure distribution of the pebble bed are obtained.The maximum hydrostatic pressures of the Be pebble bed and the Li4SiO4 pebble bed in the sandwich model are 2.0 MPa and 2.6 MPa,respectively.