The Effect Of Solid Blanket Service Environment On Mechanical Performance And Release Behavior Of Hydrogen Isotopes For Tritium Breeders

D-T fusion has been thought as the most likely way to solve the energy crisis of human due to its abundant fuel source,safe,and almost zero waste.In order to realize D-T fusion,China,the European Union,India,Japan,Korea.Russia and the United States reach an agreement to for the most ambitious energy projects,to build the world’s largest magnetic fusion device-International Thermonuclear Experimental Reactor(ITER).Our country has decided to build Chinese Fusion Engineering Test Reactor(CFETR)to bridge the gap between ITER and DEMO,using the physics and engineering technology studied from ITER.Tritium breeding blanket as one of the most important components for fusion reactor will ensure the tritium fuel self-sufficiency by tritium breeding materials.HCCB is the designed TBM of our country to be tested in ITER,which adopt Li4SiO4 as tritium breeding material.CFETR will adopt Li4SiO4 and Li2TiO3 as tritium breeding material for its designed blanket concept Helium Cooled Ceramic Breeder Blanket(HCCB)and Water cooled Ceramic Breeder Blanket(WCCB)respectively.During real blanket operation,the ceramic breeder may be fractured due to thermal stresses,which was induced by thermal expansion,thermal gradients,and thermal shocks.One aim of this work is to investigate the factors that influence the mechanical property of Li4SiO4,Li2TiO3 and evaluate the mechanical property thermal stability of this two traditional ceramic breeder and advanced core-shell Li2TiO3-Li4SiO4 during long-term annealing at high temperature.Tritium breeder also will be bombarded by energetic particles in blanket,so we also investigated the microstructure and mechanical property of Li2TiO3 irradiated by helium ion.In order to realize self-sustaining D-T fusion reactions,the bred tritium must be collected as much as possible to achieve sufficiently high TBR.In this work,we also investigate the hydrogen isotope release behavior of these three kinds breeders by neutron irradiation and deuterium gas exposure.Hydrogen release behavior change before and after long-term annealing at high temperature was also studied.Hydrogen release behavior also has strong relationships with surface chemical environment of breeders.Investigations of change of surface chemical environment induced by ion irradiation were conducted.For investigations of mechanical property,(1)Grain growth at temperature higher than 750℃ is mainly responsible for the decrease of mean crush load of Li4SiO4 pebbles.The kinetic investigation of grain growth for Li4SiO4 pebbles indicates that the migration and coalescence of pores is the mechanism of grain growth whose activation energy Q equals 140 kJ/mol far less than reported 400 kJ/mol of Li2TiO3 pebbles.It means that Li4SiO4 will be easier to lose mechanical stability than Li2TiO3 due to grain growth under the same temperature condition.(2)After long-term annealing at 900℃in 0.1%H2/He flow gas for 1000h,the mean crush loads of Li4SiO4 pebbles are reduced by about 41%,the mean crush loads of Li2TiO3 pebbles are increased by about 9%,the mean crush loads of core-shell Li2TiO3-Li4SiO4 during are decreased by about 23.3%。For Li2SiO4 pebbles and core-shell Li2TiO3-Li4SiO4 pebbles,the mean crush load drastically decrease process happened by a 100h annealing.Weibull distribution analyses of failure behavior of these three ceramic breeder pebbles after long-term annealing show the following ranking for thermal stresses needed to achieve the same failure probability:LiSiO4 pebbles<Li2TiO3 pebbles<core-shell Li2TiO3-Li2SiO4 pebbles.The microstructures of Li4SiO4 pebbles changed enormously,its grain growth,open porosity increase,increase in larger pores and lithium loss all occurred during one 100h annealing.The microstructure of Li2TiO3 pebbles keep stable during long-term annealing,its open porosity,pore size,and lithium content all show no significant change.The microstructure of core-shell Li2TiO3-Li4SiO4 pebbles show limited change.The grains of Li4SiO4 phase show restrained growth due to the impeding of Li2TiO3 phase.Open porosity increases and some small pores change to larger pores;the behavior of lithium loss is determined by lithium evaporating from two phases,but more relevant to Li4SiO4 phase.(3)Nanoindentation investigations of Li2TiO3 irradiated with 2 MeV helium ions indicate that hardness is influenced by irradiation damage.At low dose(about 0.018 dpa),the decrease of hardness is caused by dislocation motion and decrease of bond density per unit volume.At high dose(about 0.86 dpa),the formed defect clusters impede the mobility of dislocations,which is the main reason responsible for the increase of hardness.Based on hardness theories and Raman spectra,the decrease of bond length is considered as another reason for the increase of hardness.For release behavior of hydrogen isotopes from breeders:(1)Tritium release behaviors of Li2SiO4,Li2TiO3 and core-shell Li2TiO3-Li4SiO4 pebbles have been compared under same neutron irradiation condition.Out-of-pile tritium release behaviors with different heating rates have been conducted to obtain the apparent desorption activation energy of bred tritium.The apparent desorption activation energy of main tritium release peak of Li4SiO4 and Li2TiO3 is obtained to be 0.29eV and 0.49eV.There are two explicit tritium release peaks for core-shell Li2TiO3-Li4SiO4 pebbles,and the apparent desorption activation energy is evaluated to be 0.92eV and 1.78eV,respectively.Isothermal desorption behaviors for these three kinds of breeders have been conducted.Diffusion controlled kinetic process of tritium release is used to fit the curve of tritium release amount versus time.With the isothermal desorption time increasing,diffusion-controlled process gradually becomes the rate-determining step during tritium release from Li4SiO4 pebbles.With the isothermal desorption temperature increase,the matching between fitted line and experimental curve becomes better.Lots of tritium retained when temperature below 673K for core-shell Li2TiO3Li4SiO4 pebbles.When the temperature increased to 673K,almost all tritium can be released and diffusion process is the rate-determining step.(2)Deuterium mainly released at low temperature for Li2SiO4 and core-shell Li2TiO3-Li2SiO4 pebbles,and all shows relationship with their H2O release behavior.The low-temperature release process indicates that deuterium mainly adsorbed on the surface will be released by isotope exchange reaction with H2O.Deuterium mainly released at high temperature for Li2TiO3 pebbles,which reflected the detrapping process from E’-centers and decomposition of hydroxyl group(O-D).(3)Hydrogen isotope release behavior before and after long-term annealing has been investigated.A large number of open pores and lithium vacancies were formed in Li4SiO4 pebbles due to 1000h annealing leading to the release peak shift to lower temperature.The transformation of closed pores to open pores avoids the present of high temperature release peak due to deuterium trapped by closed pores.The influence of ion irradiation on the chemical environment for Li2TiO3 tablets:(1)After deuterium ion irradiation,the hydroxyl groups increased obviously compared with virgin.TDS was used to study the deuterium release behavior after deuterium ion irradiation.Deuterium release curve also shows two high temperature release peaks,which is composed of deuterium detrapping from oxygen vacancy and breaking of OD bonds.(2)Oxygen vacancy defects induced by helium ion irradiation promote the adsorption of water for Li2TiO3 tablets and forming multilayer H2O.