Deuterium Behavior And Correlation With Irradiation Defects Of Lithium Ceramic Microspheres Release

Tritium breeder is considered as the key function material of the tritium breeding blanket in a fusion reactor. The tritium release behavior and the irradiation properties of tritium breeder in neutron fields are the important targets for the deuterium-tritium fuel cycle technology and the engineering design of the blankets. The preparation work of the solid tritium breeders, such as LiAlO2, Li2ZrO3, Li2TiO3and Li4SiO4, has been carried out for more than20years in China, but the properties of these materials are studied less, especially the tritium performance during irradiation. Therefore, the lack of the understanding about the tritium issues and irradiation effects can not meet the engineering requirements. According to the current research needs without fusion neutron source, out-of-pile annealing experiments are performed to study the tritium release behavior of the lithium ceramics which are irradiated in fission reactors in this work. The influencing factors, including the heating rate, the isothermal temperature and the purge gas, are investigated respectively to discuss the rate-controlling step during the tritium release process, y-ray, electron beam and neutron irradiation are used to produce defects in lithium ceramic pebbles, the changes of the microcosmic structure before and after irradiation are compared, and the characteristics of irradiation defects are detected by means of electron spin resonance (ESR). The annihilation kinetics of the irradiation defects in neutron-irradiated Li4SiO4is investigated using isochronal and isothermal annealing methods to accelerate the recover rate of the defects. The correlation between annihilation of the irradiation defects and tritium release is analyzed, and then the model of the mutual action between defects and tritium is established. The surface irradiation defects and the hydrogen isotopes desorption behavior of Li4SiO4are investigated by D2+implantation.Li4SiO4ceramic pebbles (-80%T.D), which are made of LiOH with freezing-shaping technology, show good tritium release performance in the low temperature region of500-800K. The apparent desorption activation energy of the bred tritium on the pebble surface is evaluated to be40.0±4.2kJ/mol based on the TDS experiments at different heating rates. The apparent desorption activation energy of tritium on the pebble surface was consistent with the diffusion activation energy of tritium in the crystal grains, indicating that tritium release was mainly controlled by diffusion process. The diffusion coefficients of tritium in the crystal grains at temperatures ranging from450K to600K are obtained by isothermal annealing tests, and the Arrhenius relation is determined to be D=1x10-7.0exp(-40.3×103/RT) cm2s-1. The tritium diffusivity of the Li4SiO4ceramic pebbles in this work is close to that of Li4SiO4crystal powder in the literature, implying that the diffusion of tritium through grain boundaries has little effect on the tritium release rate of Li4SiO4ceramic pebbles. The effect of irradiation fluence on tritium diffusivity in the samples is observed as reduction of the tritium diffusion coefficient with increasing the neutron fluence in the studied range. The bred tritium requires a high temperature region of750-1000K to be liberated from LiAlO2ceramic pebbles corresponding to the apparent desorption activation energy, which is evaluated to be128.7±28.6kJ/mol.The release forms of the bred tritium in lithium ceramics influenced by purge gas composition are investigated. He, He+0.1%H2and He+0.1%H2O are used as purge gas respectively in the TDS experiments. The experimental results show that the released pieces of tritium are co-controlled by the desorption reaction and the hydrogen isotope exchange reaction on the pebble surface, indicating that hydrogen addition to the purge gas plays small role on the released forms of the tritium for Li4SiO4because of its quick diffusivity and low desorption activity energy, but can significantly change the released form of the tritium for LiAlO2because of its slow diffusivity and high desorption activity energy. Catalytic metals loaded on the lithium ceramic pebbles can enhance the hydrogen isotope exchange reaction between tritium on the solid surface and hydrogen in the purge gas, and accelerates the recovery rate of the molecular tritium (HT) in the low temperature range.With respect to the irradiation effects of Li4SiO4ceramic pebbles, y-ray, electron beam and thermal neutron are used in a low dose range. The experimental conditions can not produce serious damage on the surface or in the body of the specimens, but induce some kinds of defect centers, which change the specimens’color. Electron spin resonance technique (ESR) was employed to analyze the paramagnetic defects in the specimens before and after irradiation. It is found that F+center is the principal paramagnetic defect existing in Li2O after irradiation, and there are E’-center, O--center and O2--center in ternary lithium ceramics after irradiation. The ESR spectra of Li4SiO4irradiated by un-neutron irradiation methods show similar characteristics with the spectrum of the specimen irradiated by thermal neutron when the irradiation dose is more than500kGy. This result provides a reference to study the irradiation effects of material exposed to neutron field.The annihilation kinetics of the irradiation defects in neutron-irradiated Li4SiO4is investigated with annealing methods to accelerate the reaction rates related to the defects. Trapped electrons and various kinds of defects recover via diffusion at a heating rate of5K/min from R.T. to798K, then the defects gradually annihilate. some holes are released from the recombination reactions of the E’s and PORs to enhance the production of the amorphous silicon centers in the present Li4SiO4when the annealing temperature is higher than798K. Tritium compounds (Si-T or Li-T) formed by colloids trapping tritium may be the considerable reason of tritium inventory in high temperature region. Basing on the first-order reactions, the annealing experiments of ESR show that there are fast and slow annihilation processes of the irradiation defects for neutron-irradiated Li4SiO4. The activation energies for the two processes are0.18and0.57eV respectively. The fast annihilation process may be attributed to diffusion of trapped electrons into defects, and the slow annihilation process may be attributed to the annihilation of E’-center via recovering oxygen.Isochronal annealing experiments of ESR for neutron-irradiated Li4SiO4are performed to contrast the annihilation process of irradiation defects and the tritium release process. It is observed that the tritium release rate speeds up when the annihilation process of the irradiation defects on the verge of ending, thus the correlation between the slow annihilation process and the tritium release is speculated. According the difference of activity energy of the slow process and the diffusivity activity energy of oxygen, two theoretical models are suggested to interpret the mutual action of oxygen vacancy and tritium. One model supposes that the recovering oxygen ions triggers the tritium release, but a relaxation energy of-0.60eV will be produced after the recover of oxygen ions. The other model considers that he slow annihilation process is related to the diffusion of hydroxyl radical and the recovery of oxygen owing to the same energy barrier of0.56-0.58eV for both the slow annihilation process and the hydroxyl diffusion.The effect of surface defects on the tritium release is simulated by D2+implantation. The chemical states and the deuterium desorption behavior of Li4SiO4implanted with3KeV D2+are investigated by in-situ X-ray photoelectron spectroscopy, the FT-IR spectroscopy and the thermal desorption spectroscopy. The experimental results show that deuterium implantation can change the chemical states on the surface of Li4SiO4, and produce many irradiation defects and binding groups, such as defects trapping D to form Li-D, therefore the thermal desorption behavior of molecular deuterium is related to the surface states. The un-O-D states of deuterium implanted into the sample convert to O-D states during the heating process, which has some relationship with the thermal desorption behavior of deuterium water (HDO). The above results are helpful to understand the tritium release behavior of lithium breeder affected by irradiation defects.The above results are suitable to the experiments with low dose irradiation, but can not extend to reflect the tritium release and defects behavior in the fusion blankets.