| Nuclear power sources has many advantages such as high power, long life, small volume, light weight, good safety, high reliability and does not rely on solar energy, and it is an important tool for deep space exploration. The shield of nuclear power sources can reduce the damage of neutron and gamma ray. On the design of shield for nuclear power sources, LiH was usually selected as the neutron shielding material. LiH has a high hydrogen density, high melting point and low density, high absorption cross section and low molecular mass, lithium hydride is an ideal shielding material for its low weight and high efficiency. Lithium hydride will produce a large number of vacancy and interstitial hydrogen ions after long time irradiation. The advantage of LiH as a neutron shield material is dependent upon the total amount of hydrogen present and the uniformity of the hydrogen distribution. Hydrogen migrates against the thermal gradient leaving the hot regions depleted in hydrogen. Based on hydrogen migration study, we can provide theoretical and experimental basis for the design of lithium hydride shield.CASTEP which based on density functional theory was used to do some first-principles calculations on Hydrogen migration. Static calculation was used to study the migration behavior of interstitial hydrogen ion along,and directions. At the same time, the behavior of migration lead by the nearest neighbor hydrogen ion was studied. We get two conclusions:hydrogen ion migration along gives the lowest activation energy, about 0.1882 eV, hydrogen ions which migrate to the nearest clearance and the second neighbor clearance are more likely leaded by its nearest hydrogen ion. In addition, Hydrogen has a large thermal neutron scattering cross section, hydrogen ion can be easily knocked out of by the neutron in the neutron irradiation conditions, so we conducted the research to the Frenkel defect. We find that the hydrogen ions most likely migrate to the second and the fourth neighbor clearances. These two positions are the best ones for the formation of Frenkel defects. Then the migration barrier along direction for hydrogen ion in different lattice constants corresponding to the temperature from 300K to 800K was calculated, and molecular dynamics method was used to calculate the diffusion coefficient for hydrogen ion from the 300K to 800K. We conclude that the migration barrier of hydrogen ion reduces gradually with the increase of temperature. The diffusion coefficient for hydrogen ion in 300K is 1.4491×10-7cm2/s. The migration barrier which calculated by molecular dynamics is close to the migration barrier of direction with static calculation.Ion implantation was used for deuterium ion doping, the depth distribution of deuterium variation with time will provide the information of the deuterium diffusion in the lithium hydride. Secondary ion mass spectrometry and nuclear reaction analysis was adopted to analysis the depth of deuterium in lithium hydride. Secondary ion mass spectrometry couldn’t got the depth distribution of deuterium accurately, but it shows that lithium hydride containing Cl", F", O2- and other impurities. The results were helpful to nuclear reaction analysis. Nuclear reaction analysis depend on the reaction of 3He(D,p)4He for the quantitative analysis of deuterium. The incident energy of 3He were 800 KeV,1000 KeV,1400 KeV,1800 KeV and 2400 KeV, and different incident energy show the information of deuterium concentration of different depth. The experimental results show that few deuterium remained on the surface, most of them were overflow from the surface. Then we can get internal deuterium ion depth distribution. Fick’s second law was used to estimate the diffusion coefficient of deuterium in lithium hydride, the result is 1.32×10-7 cm2/s, similar to the molecular dynamics simulation results. |
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