| The selection of tritium breeder material is crucial in the blanket design of a fusion reactor. Lithium based ceramics have been considered as promising tritium breeder materials due to their inherent thermal stability and chemical inertness. Currently, the most attractive ceramic breeder materials are Li4SiO4 and Li2TiO3. In the design of China ITER-TBM, Li4SiO4 is the preferred tritium breeder material with Li2TiO3 as a backup. Tritium release performance is an important aspect for evaluating a ceramic breeder material. From the viewpoint of designing a blanket and tritium recovery system, it is of great significance to have a deep understanding of the mechanisms of tritium migration and release in the ceramic breeder materials.The behavior of tritium release from ceramic breeder materials is not only very complicated but also affected by many factors. Since 1980s, a lot of tritium release studies have been carried out throughout the world by the methods of both in-pi le and out-of-pile experiments, modeling and theoretical calculations. However, a thorough understanding of the tritium release mechanisms has not been achieved yet. To have a deeper understanding of the mechanisms of tritium release from Li4SiO4, the factors such as high magnetic field (MF), surface impurities and materials properties that affect tritium release behavior from Li4SiO4 were investigated systematically by out-of-pile experiments in this thesis. Based on the experimental studies, the kinetics of tritium release from Li4SiO4 was also analyzed.Based on a 7 T superconducting magnet, out-of-pile tritium release experiments on Li4SiO4 within high MF were designed and conducted. The experimental results showed that the effect of high MF on tritium release behavior from Li4SiO4 was very small. No obvious MF effect was observed even under the conditions that the MF intensity reached 7 T and the grain size reached 100～300 μm (larger than 4 times the Larmor radius). In comparison with the high MF, the factors such as purge gas composition, water vapor adsorption and materials properties had more significant influences on the behavior of tritium release from Li4SiO4. Therefore, the high MF is not a key factor affecting tritium release behavior from Li4SiO4.In order to have a deep understanding of the effect of surface impurities on tritium release behavior from Li4Si04, the impurity components and their thermal desorption behavior on Li4SiO4 were first investigated. The XRD and XPS results showed that the main impurities on Li4SiO4 were LiOH (or LiOH · H2O), Li2CO3 and SiO2, which were the chemisorption products of water vapor and carbon dioxide (CO2) on Li4SiO4. The TPD, TG-DSC and TG-IR results showed that the main desorption products from Li4SiC>4 were water and CO2, among which the ratio of water accounted 93～94 mol%. The water desorption included three processes:The first process (RT-150℃) which had two characteristic peaks at around 100 and 135℃, respectively, was attributed to physisorbed water and chemisorbed water through hydrogen bonds; The second process (150～300℃) which also had two characteristic peaks at around 207 and 260℃, respectively, was attributed to hydroxyl groups such as Li-OH and Si-OH; The third process (300-450 ℃) which had only one characteristic peak at around 415℃ was attributed to the decomposition of LiOH. The desorption of CO2, which had a characteristic peak at around 700 ℃, was attributed to the decomposition of Li2CO3. Based on the above results, the effect of surface impurities on tritium release behavior from Li4SiO4 was mainly focused on the water adsorption impurities (H2O,-OH, LiOH). The experimental results showed that the existing of water adsorption impurities could enhance the tritium release from Li4SiO4. The main tritium release peak shifted from 500 to 250 ℃ after hygroscopic treatment of the irradiated Li4SiO4 samples. It was found that the water desorption peaks and the tritium release peaks overlapped very well, indicating a strong correlation exists between the two processes. The effect of water adsorption/desorption on tritium release behavior from Li4SiO4 can be explained by the exchange reaction between hydrogen isotopes, namely that the tritium atoms can detrap from the surface trapping sites through isotope exchange reactions and desorb as tritiated water in the water layer.The effect of materials properties on tritium release behavior from Li4SiO4 was investigated by systematically contrasting and analyzing the wet sample and melt sample. First, the microstructures of the two samples were contrasted and obvious differences in the surface morphology, inner morphology, grain size, pore structure, density and porosity were found. According to the tritium release results, the microstructrure of the wet sample was more favourable for tritium release behavior. Further analysis results indicated that the difference in tritium release behavior between the two samples was mainly related to the grain sizes and surface conditions, whereas little to do with the pore structures. Using ESR method, the irradiation defects and their annihilation behavior of both samples was studied comparatively. Under the same irradiation conditions, the ESR signals of the two samples were consistent in g-values but different in relative intensities. On the whole, the concentration of irradiation defects in the wet sample was obviously higher than that in the melt samle, indicating that the formation of irradiation defects arc related to materials properties. In terms of the annihilation behavior of irradiation defects, the final annihilation temperature was at about 600 ℃ for both samples, however, the starting annihilation temperature was obviously lower for the wet sample. Concerning the ESR spectra, the signal g=2.0495 almost disappeared at 225 ℃ in the wet sample, while it disappeared at higher than 325℃ in the melt sample. Besides, a new signal (g=2.0838) appeared at around 425℃ in the melt sample but not in the wet sample. The kinetic analysis results showed that the activation energies of the annihilation of irradiation defects in the wet sample were much lower than that in the melt sample for both the fast and slow annealing processes. The annihilation behavior of irradiation defects were affected by not only the materials properties, but also the neutron energy and neutron flux. It was also found that a large amount of tritium began to release just when the annihilation of irradiation defects nearly came to the end (500～600℃), which indicates that the tritium release behavior is strongly correlated with the annihilation behavior of irradiation defects. Since the tritium release behavior from Li4SiO4 was controlled by surface desorption process, the interactions between irradiation defects and tritium should mainly take place at the grain surface. Accordingly, a tritium release model was proposed including the following steps:(a) diffusion of tritium from the bulk of grain to the grain surface; (b) trapping of tritium by surface defects; (c) detrapping of tritium triggered by annihilation of surface defects; and (d) desorption of detrapped tritium on the grain surface.Based on the above experimental studies, the kinetics of tritium release from Li4SiO4 was analyzed by using diffusion and desorption models. The analysis results further indicated that tritium release from Li4SiO4 was mainly controlled by surface desorption process. Under the purge gas of He+0.1%H2, the kinetics of tritium release from Li4SiO4 was well described by first order desorption model, and the desorption mechanism was the isotope exchange reaction between the surface tritium and H2 in the purge gas.According to the experimental studies and kinetic analyses, the key factors affecting tritium release behavior from Li4SiO4 should include temperature, purge gas compositions, grain sizes, surface chemical states and irradiation defects. This thesis not only further deepened the understanding of the mechanisms of tritium release from Li4SiO4, but also pointed out the directions for further studies. |
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