Fusion DEMO BoP Efficiency Analysis And Thermal Performance Evaluation Of CO₂ Cooled Pebble Bed Blanket

Based on the development of ITER,a new kind of demonstration fusion reactor has been proposed in Europe.EU DEMO is the next generation fusion reactor and it is the last experimental reactor for fusion commercial power plant.And blanket is the most important component,the goal of which includes enough tritium breeding,energy conversion and radiation shielding.Reasonable blanket design and material selection are very important for the safety performance and thermoelectric conversion efficiency of fusion reactor.Nowadays there are 4 candidate blankets for EU DEMO:helium cooled pebble bed blanket(HCPB),helium cooled lithium lead blanket(HCLL),water cooled lithium lead blanket(WCLL),dual coolant lithium lead blanket(DCLL).Apart from water,these coolants for these blankets all adopted the helium gas.Due to the low density of helium,it needs a higher circulator power which will cause the lower net electic power of EU DEMO.CO2 has been applied into nuclear fission since 1950's for the MAGNOX Reactor and Advance Gas Reactor.The density of CO2 is 11 times higher than helium that the CO2 circulator power will much lower than helium.And the storage of CO2 is much eaysier than helium too.The price of CO2 is cheap which can largely decrease the cost of EU DEMO.This paper first investigated the comparative analysis of the power generation efficiency of fusion DEMO reactor with helium and CO2 used as coolants respectively in the blanket.The design scheme of integrated Balance of Plant system(BoP)for fusion demonstration reactor is given firstly,including PHTS(Primary Heat Transfer System),IHTS(Intermediate Heat Transfer System)and PCS(Power Conversion System).Then,the EU DEMO efficiency analysis with helium and CO2 as coolants in the PHTS was studied.And it showed that CO2 had a higher net electric efficiency than helium under the same design conditions.And at the end,the influences of coolant outlet temperature in the PHTS and water vapor pressure in the PCS on the system efficiency were analyzed.The higer CCPB outlet temperature could improve the EU DEMO net efficiency.However,to improve the inlet and outlet temperature at the same time could not improve the net efficiency.At the condition of 600 ? for the outlet temperature,it was good to improve the steam pressure of PCS for the net efficiency.When the pressure drop was 1.5 times than the original,CO-2 net efficiency was still higher than that of helium.Based on the power generation efficiency analysis,this thesis proposed C02 as the blanket coolant instead of helium,a new CO2 cooled pebble bed blanket(CCPB).The inlet and outlet temperature of CCPB are 300 ? and 500?,the coolant pressure is 8 MPa.The structural material of CCPB is RAFM steel(EUROFER97),the tritium breeding material is Li4SiO4,the neutron multiplier material is Be12Ti,and the plasma facing component material is tungsten.The first wall(FW)is a blanket component directly facing the plasma and needs to bear loads such as high heat flux from the plasma,high-energy particle flow,etc..In this paper,the thermal performance of CCPB FW was analyzed and evaluated at first.In the EU DEMO,the No.6 inboard blanket suffering the highest particle heat flux,the No.15 outboard blanket suffering the highest radiation heat flux,and equatorial typical blanket No.12 were chosen to be evaluated for the thermal hydraulic performance.Considering the roughness effect of pratical coolant channel,thermal hydraulic analysis results with different wall roughness in different positions were given.Also,heat flux sensitivity analysis was carried out for the first wall of blanket No.6 and No.15.When the total heat flux in the first wall of the No.6 blanket was increased to 1.6 MW/m2,the pressure drop is 3 bar,lower than the pressure drop limit of 3.7 bar.While the total heat flux in the first wall of the No.15 blanket was increased to 1.2 MW/m2 with the mass flow rate 0.48 kg/s needed,the pressure drop is 3.4 bar,still lower than the pressure drop limit of 3.7 bar.The thermal hydraulic of equatorial typical blanket breeding zone was studied in the fllowing part.After preliminary analysis,the structure of breeding region was optimized,mainly considering the Pin numbers and dimensions,and adding the graphite cylindrical rods.After iterative optimization,12 pin designs were adopted and the pin size increased to 86 mm for the breeding region.And the temperatures of structural material and breeding material are all lower than temperature limitation.Both the highest temperature 512.5 ? of FW appearing at the bend of the first wall and the highest temperature 513.1 ? of Pin-cladding are all lower than the temperature limit 550 ?.And the highest temperature 895.9 ? and 917.6 ? for Be12Ti and Li4SiO4 are lower than the relative material limit 900 ? and 920 ?respectively.At last,the thermal analysis of the back plate of the blanket was carried out,and the temperature profile was given for the furth stress analysis in the future.The research in this paper can also provide important reference for the analysis of thermoelectric conversion efficiency and optimization of blanket design of Chinese fusion reactor.