Thermo-mechanical Study Of Pebble Beds For Water Cooled Ceramic Breeder Blanket

As one of the breeding blanket candidates for the China Fusion Engineering Test Reactor(CFETR),a design of the water cooled ceramic breeder blanket(WCCB)is performed in China.Both the beryllium pebble bed and the Li2Ti03&Be,2Ti mixed pebble bed are configured as multi-layered zones in the WCCB to multiply the neutron and breed the tritium.Thermo-mechanical performance of pebble beds significantly impact the reliability of thermo-hydraulic and mechanical design of WCCB.Therefore,the object of this work is to study the thermo-mechanical characteristics of the concerned pebble beds and thereby provide the basic engineering data for the development of WCCB.Firstly,the discrete element method(DEM)method is applied to produce prototypical pebble beds with mono-sized particles for fusion blankets,as well as those with multi-sized Li2TiO3&Be12Ti particles for WCCB.After that,packing structure of the concerned pebble bed is investigated and finally found the possible packing assemblies of Li2TiO3&Be12Ti mixed pebble bed for WCCB pebble beds.Secondly,an improved theoretical M-SZB model evolved from the SZB model was proposed to evaluate the effective thermal conductivity(keff)of binary pebble beds composed of different particle materials.After compared with the existing experimental data published in literatures,the results calculated by this model were found to agree well with the existing experimental data.Based on this theoretical model,the keff of the Li2TiO3&Be12Ti mixed pebble bed for the WCCB is found between 2.0 and 4.0 W/(m K).It means the mixed pebble bed conducts much more heat(about 2?4 times)than typical unitary lithium ceramic pebble beds.Besides,a coupled DEM-CFD method was employed to simulate heat transfer and estimate the pebble bed effective thermal conductivity.The current numerical model presented a good performance to calculate keff of unitary pebble beds(Be,Li4SiO4 and Li2TiO3 pebble bed).According to the current DEM-CFD modeling of a typical Li2TiO3/Be12Ti mixed pebble bed,keff is roughly ranged between 2.0 and 4.0 W/(m·K),which is in accordance with the estimation of M-SZB model.Finally,a two-way coupled numerical model was constructed by using DEM to analyze thermo-mechanical behavior of prototypical fusion blanket pebble beds.The suitability and validity of the current numerical model were evaluated by comparing with the previous experimental or numerical data.Finally,the DEM model was used to analyze the bed thermo-mechanical behavior under different load modes(load/unload/periodical load).