Study On The Defects Evolution Of Li₂TiO₃ Ceramic Breeder And Its Effect On Release Behavior Of Hydrogen Isotopes

Hydrogen isotopes,i.e.,deuterium(D)and tritium(T)are fuels for nuclear fusion reactors.Due to lack of tritium in the nature,a blanket is designed to produce tritium in fusion reactor.Because of good chemical stability and tritium release behavior,Li2TiO3 is considered as one of promising candidate ceramic breeder materials.There are some researches on irradiation effect and tritium release mechanism of Li2TiO3,but the understanding is still not enough.At present,it is difficult to obtain neutron sources in the laboratory condition,with similar neutron energy spectrum and flux to those generated from D-T reactions(peak at 14 MeV).According to the R&D requirements of ITER TBM module and CFETR solid breeder blanket,a systematic study was carried out on irradiation damage,defect evolution,release behavior of hydrogen isotopes and their interaction mechanism.The research work and achievements is shown as follows.(1)Tritium release experiments were performed to study the release behavior of Li2TiO3 pebbles which were irradiated in a research reactor.The activation energy of tritium in the Li2TiO3 pebbles is evaluated to be 0.42eV.The temperature range of tritium release is 200～500℃.Meanwhile,deuterium was introduced into Li2TiO3 by ion implantation and the deuterium release behavior was then investigated by means of thermal desorption spectroscopy.The activation energy of deuterium in Li2TiO3 is evaluated to be 0.53eV.The temperature range of deuterium release is 250-550℃.This shows that Li2TiO3 ceramic exhibit good release performance of hydrogen isotopes.(2)The correlation between defect evolution and release behavior of hydrogen isotope was studied by electron paramagnetic resonance technique.It could be found that E-center is the main defect type after deuterium irradiation.The evolution of irradiation defects consists of fast and slow annealing process.Based on the release behavior of hydrogen isotopes,the correlation between irradiation defects and release behavior of hydrogen isotopes was summarized.(3)The effects of deuterium ion irradiation on the mechanical properties of Li2TiO3 were analyzed by micro-hardness measurement.The results show that the Vickers hardness of Li2TiO3 decreased with increase in irradiation dose.The Meyer coefficient was calculated as 1.65,which is consistent with Meyer’s law.The Vickers hardness of Li2TiO3 was also measured after the defects annihilated,and the results show that deuterium ion irradiation has an irreversible effect on Vickers hardness.(4)2MeV helium ion irradiation was applied to simulate 2.1MeV helium ions generated in the real tritium breeding reaction.XRD and Raman results show that the microstructures of Li2TiO3 are damaged by helium ion irradiation,and the degree of damage increases with the radiation dose.E-center is mainly responsible for microstructure damage caused by helium ion irradiation.10MeV oxygen ion irradiation was applied to simulate primary knock-on atom(PKA)generated by neutrons in Li2TiO3.XRD and Raman results show that the oxygen ion irradiation also damage the microstructure of Li2TiO3,and the variation tendency is similar to that of helium.But the damage is caused by several types of defects,including E-center、NBOHC、POR and Ti3+.The change of microstructure disorder after helium and oxygen ion irradiation was summarized.For the similar dpa,the destruction of Li2TiO3 in oxygen ion irradiation is more serious than helium ion irradiation.It indicates that the type and energy of ions determine the damage degree of the materials for different kinds of ion irradiation.(5)The evolution behavior of irradiation defects was studied by isothermal and isochronous heating treatments.During the isochronous heating treatment,the number of E-center defects decreases as the temperature increases,while Ti3+ defects gradually appear.Thus,the evolution of defects after helium ion irradiation can be divided into two process:annihilation of E-center defects and generation of Ti3+ defects.During the isothermal heating treatment,the number of E-center defects will increase firstly and then decrease.The evolution mechanism of E-center defects for isothermal and isochronous heating treatments is different,but heating temperature determines whether E-center defects could be ultimately annihilated.It indicates that the behavior of helium ions in Li2TiO3 is closely related to the evolution of defects.(6)According to the study on the discoloration of Li2TiO3,there are three types of discoloration during the experiments.Firstly,due to the generation of E-center during irradiation experiments,the color of Li2TiO3 changed.Secondly,the color of Li2TiO3 changed after heating in vacuum atmosphere.The formation of E-center is mainly caused by the absorbed moisture on the surface of Li2TiO3.The E-center defects produced by vacuum heating treatment have an effect on the microstructure of Li2TiO3 and the release behavior of hydrogen isotopes.Thirdly,according to the study on the discoloration of Li2TiO3 in reducing atmosphere of deuterium,the main type of defect is E-center.The microstructure and release behavior of Li2TiO3 were investigated after heated in the deuterium atmosphere.The results are similar to that of vacuum heating treatment.At the same time,the formation of E-center defects will result in Ti4+ becoming Ti3+.In conclusion,the discoloration of Li2TiO3,which is mainly caused in the reducing atmosphere,is a combination effect of E-center and Ti valence changes.(7)Based on the above results,the influence of adsorbed moisture content on defect formation was further studied.It can provide reference for the preservation,transportation and assembly of Li2TiO3 in the future.The E-center defects produced by vacuum heating have an effect on the microstructure and release behavior of Li2TiO3.