Lithium Ceramic Proliferative Agent Li ₄ SiO ₄ Surface Study Tritium Release Behavior

Deuterium/tritium fusion energy is a key way for solving energy issue in the future. Nuclear fusion including many science, engineering and technology issues, tritium self-sustainment is a key issue among them for a deuterium/tritium fusion reactor. Since there is only trace of tritium exists in the nature, except for deuterium/tritium gas that is supplied to the circular vacuum vessel at its initial operation phase, tritium fuel required by fusion reactor must be self supplied by irradiating lithium containing material with neutron during the reactor’s stable operation stage. Understanding tritium release behavior and mechanism of lithium containing breeder is the research foundation for designing tritium extraction loop and realizing tritium self-sustainment, it is also a research difficulty. Thus, this paper focused on studying tritium release behavior on Li4SiO4which is the main solid lithium ceramic breeders in China. Great differences in tritium release chemical forms and temperatures of Li4SiO4in references indicate that diffusion is not the unique factor affecting tritium release behavior. On the contrary, these results imply that surface reactions have great effects on tritium release behavior of Li4SiO4. Consequently, this paper focused on studying effects of surface reactions on tritium release behavior on LiSiO4.Effects of surface reactions on tritium release behavior on Li4SiO4are studied through out-of-pile tritium release experiments in this paper, including adsorption/desorption reactions as well as isotope exchange reaction. Competition mechanism and reaction priority is analyzed according to experimental data. And the reasons for getting different tritium release results in references are analyzed and explained basing on the research results of this paper. The research results will be useful for designing tritium extraction loop. Since gas flow rate and temperature ramping rate can affect tritium release temperature, experimental data in this paper are obtained under conditions of50mL/min and5℃/min, except for special indication.The main conclusions are as follows:(1) For the orthosilicate, except for "free" tritium gas released at room temperature, tritium exists mainly in the form of-OT, which can be released either in the form of tritiated water through recombination/desorption reaction of-OT/-OH or in the form of tritium gas through recombination/desorption reaction of two hydrogen isotope ions by breaking O-H bonds, which would not happen on the sample with high water concentration. The former released at lower temperature than the latter. Discovery of direct release of tritium gas from Li4Sio4complements knowledge of tritium release mechanisms, which were rarely mentioned in the published references. It was found that releasing of tritium gas only occurred on "dried" samples. After adsorbing water on the surface of Li4SiO4, the concentration of H2O/-OH on the surface of Li4Sio4increases, which makes the-OH/-OT recombination/desorption reaction or H2O isotope exchange reaction precede with priority and tritium release in the form of tritiated water, thus fractions of gaseous tritium decrease.(2) Li4Sio4is a material with large water uptake capability. Multi adsorption sites, including physic-and chemi-adsorbed sites exist on the surface of Li4SiO4, which can be changed according to pretreatment conditions and storage conditions. Physic-adsorbed water would affect tritium release temperature through H2O isotope exchange reaction. Chemi-adsorbed water would affect tritium release behavior through-OH/-OT recombination/desorption reaction. Multi-adsorption site of H2O/-OH on the grain surface of Li4SiO4result in multi desorption temperature peaks of tritiated water, which would change according to adsorbed water on the surface. Research of reaction dynamics on the grain surface indicate that desorption reaction of tritiated water on the surface is the second order reaction, each desorption site corresponds to different activation energy which were calculated in this paper according to experimental results.(3) For H2doping sweep gas, both of H2isotope exchange reaction and water formation reaction with H2gas occur, which are called H2effects in this paper. The lower threshold value for H2isotope exchange reaction between H2molecule and surface tritium appears above400℃, which is lower than that of HT/T2recombination/desorption reaction (563℃and722℃), which indicate that adding H2in the sweep gas would decrease gaseous tritium release temperature. Temperature peak of water formation reaction by adding H2gas in the sweep gas appears at668℃.(4) When both of H2O and H2effects exist, H2O effects occur at temperature lower than H2isotope exchange reaction, which makes tritiated water released with priority to tritium gas. Thus, effects of H2isotope exchange reaction on tritium release behavior would be reduced after adsorbing water on Li4SiO4samples. Understanding the mechanism affecting H2effects is important for designing tritium extraction loop and such research has rarely studied in the previous work.This paper studied effects of surface adsorbed water on tritium release behaviors. Competition mechanisms between H2O effects and H2effects are also studied and analyzed. Results in this work indicate that tritium release is strongly affected by surface effects, at least for wet orthosilicate. Tritium gas release data at high temperatures (563℃and722℃) were got firstly in this paper, which were not seen in the literatures. Basing on these researches, tritium release characteristics under different surface conditions of the sample are generalized and reasons for getting different tritium release results in the literatures are analyzed. In addition, reaction kinetics characteristics on the surface of Li4SiO4were calculated in this paper. Researches in this paper deepened understanding of tritium release mechanism and provided valuable experimental data for designing Chinese TBM blanket and tritium extraction loop. Obtained data in this paper also provide valuable input characteristics (activity energies) for modeling tritium transport process in the lithium ceramic breeders and thus serve better to the deuterium and tritium fuel cycle system design.