Flow Characteristics Study Of Purge Gas In The Pebble Beds Of Water Cooled Ceramic Blanket

Fusion blanket is directly related to the self-sustaining of trituim and energy conversion,a key component for fusion reactor.As one of three candidate blankets for Chinese Fusion Engineering Test Reactor(CFETR),the water-cooled ceramic blanket(WCCB)is intended to adopt separate layout with berylliun pebble bed and multi-sized mixed pebble bed with different diameter ratios of Li2TiO3 and Be12Ti pebbles,so as to meet the neutron multiplication and tritium self-sustaining requirements.And low pressure helium(0.1%content of H2)is used to sweep the bred tritium out.Flow characteristics of helium in the pebble beds determine the extraction efficiency of the tritium,and the pressure drop of the pebble beds also affects the design of the sweeping system for fusion blanket.Study on the flow characteristics would contribute to better understanding of tritium transport process in the pebble beds.In order to obtain the detailed flow parameters for the pebble beds in the CFETR-WCCB design,both numerical simulation and experiment studies have been conducted and presented in this paper.In the numerical simulations,flow characteristics of a mono-sized pebble bed was first carried out to validate the feasibility of DEM-CFD coupling method,and influence of different contact point processing methods on pressure drop calculation was compared.After feasibility verification,multi-sized mixed breeding pebble beds satisfying the requirement of neutronics were established based on the DEM-CFD method,and mainly the dual-sized pebble beds with diameter ratios 6:1 and 7:1 were analyzed.Detailed flow characteristics such as porosity distribution,velocity distribution,especially the pressure drops in the mixed pebble beds with different diameter pebble ratios and packing factors were obtained.In the experimental study,a tritium sweeping simulation device was designed,and the preliminary experiments for pressure drop measurement were performed.The device consists of a helium cycle heating system and a pebble bed experimental section.The helium cycle heating system is mainly used to provide helium with certain flow rate,pressure and temperature,consisting of vacuum pumping system,gas circulation system,cooling and heating system,measurement and control system,and valve and piping system.The pebble bed experimental section is mainly used for measurement of the flow resistance of the helium flowing through the pebble beds.This device is capable of measuring the pressure drops in the pebble beds with different pebble diameter ratios and packing factors at different temperature/inlet pressure/velocity of purge gas,and will be quite useful for achieving database for future design of CFETR tritium cycle extraction system.The pressure drops of mono-sized pebble beds(1mm,2mm and 5mm)and a dual-sized mixed pebble bed(3mm and 5mm)were measured for device commissioning and validating functions of the control and measurement system for the next hot state operation.The flow characteristics of helium in the pebble beds obtained in these studies,including velocity distribution,pressure drop and so on,would provide important references for structure design of WCCB,study on tritium sweeping efficiency and design of tritium extraction system.