A. Yoshikawa

Hydrogen retention of JT-60 open divertor tiles exposed to HH discharges

Hydrogen retention in graphite tiles exposed to hydrogen discharges at the JT-60 open divertor has been investigated by means of thermal desorption spectroscopy (TDS). Most of the plasma facing area was covered with re-deposited layers of maximum thickness of about 70??m appearing at the inner divertor region. Major parts of retained hydrogen were thermally desorbed as hydrogen molecules with a peak temperature of around 970?K. Almost all the hydrogen atoms were retained homogeneously in the re-deposited layers with an averaged hydrogen concentration of ~0.03 in H/C, which is much smaller than the saturated hydrogen concentration (H/C = 0.4?1.0). Since the saturated hydrogen concentration in carbon materials decreases with increasing temperature, the re-deposited carbon layers are very likely subjected to higher temperatures during the discharges, which are supported by the higher release temperature of hydrogen in TDS. This result suggests that hydrogen retention can be significantly reduced with higher wall temperatures.

STUDY ON CORRELATION BETWEEN TRITIUM RELEASE BEHAVIOR AND ANNIHILATION BEHAVIOR OF IRRADIATION DEFECTS IN NEUTRON-IRRADIATED Li4SiO4

Abstract Tritium release from thermal neutron-irradiated Li4SiO4 is initiated with the annihilation of E’-centers by recovering O- with diffusion of O-. Electron Spin Resonance (ESR) shows that differences in the formation of irradiation damage between 14 MeV and thermal neutrons in Li4SiO4 result in different tritium release behaviors. The kinetics for the annihilation of irradiation defects has been determined. The contribution of elastic collisions by 14 MeV neutrons was much higher than that of thermal neutrons. Isothermal annealing experiments show that annihilation of irradiation defects consisted of two processes: namely, the fast and slow annihilation processes. Their activation energies were determined to be 0.13 and 0.39 eV, respectively. Comparing the experimental results for the thermal and 14 MeV neutronirradiated Li4SiO4 shows that the activation energies of the slow annihilation process were significantly different. These results relate to the density of irradiation defects, which in turn depend on the contribution of the recoil particles produced by nuclear reactions to form irradiation damaged sites.

Role of Lithium on Chemical States and Retention Behavior of Tritium in Li2TiO3

Abstract Deuterium ion implantation experiments into Li2TiO3 and TiO2 were performed with various ion fluences to elucidate the role of lithium on deuterium retention behavior in Li2TiO3. The experimental results showed that there were four deuterium trapping states in TiO2; two of them were interacted near the surface and the others were deuterium trapped by E’-center and bound to oxygen with forming TiO-D bond in bulk. For Li2TiO3, there were five trapping states; four of them were the same as those in TiO2 and the other was that bound to oxygen with forming LiO-D bond. The implanted deuterium was preferentially trapped by E’-center with forming hydroxide. LiOD phase was formed as increasing ion fluence. The retention of deuterium trapped by E’-center for Li2TiO3 was less than that for TiO2, indicating that the migration of lithium via irradiation defects during implantation refrains the deuterium retention in Li2TiO3.

Correlation between annihilation of radiation defects and tritium release in neutron-irradiated LiAlO2

The annihilation behaviors of radiation defects in neutron-irradiated LiAlO2 were investigated by means of Electron Spin Resonance (ESR). It was found that the annihilation of radiation defects consisted of two processes, the fast and the slow processes. The activation energies of them were determined to be 0.14 ± 0.01 eV and 0.58 ± 0.01 eV, respectively. The F+-center was found to act as a trapping site of tritium by comparing its annihilation behavior with that of tritium release. Taking the results obtained in the present and the previous works in consideration, it can be said that the annihilation process of oxygen vacancies is of very important because tritium release from the bulk of a breeder starts just after the slow annealing process becomes dominant. Therefore, to understand the slow annihilation process of radiation defects is an important key to clarify the mechanism of tritium release from ceramic breeder materials.

Study on Chemical Behavior of Energetic Deuterium-implanted into Li2TiO3

Abstract Deuterium ion implantation and subsequent X-ray Photoelectron Spectroscopy (XPS) and Thermal Desorption Spectroscopy (TDS) experiments were performed with varying implantation temperatures to reveal chemical behavior of tritium produced in Li2TiO3. These experimental results showed that there were four deuterium trapping states; two of which were interacted with and without oxygen near the surface, and the other two were interacted with E’-center and with oxygen with the formation of O-D bond in the bulk. These trapping states of deuterium in the bulk were almost the same as those of tritium generated in thermal neutron-irradiated Li2TiO3. The total amount of deuterium retention in the bulk was almost constant until O-D bonds formed in the bulk were decomposed, indicating that tritium trapping could proceed under hot atom chemical reactions. It was concluded that E’-center could trap the implanted deuterium more frequently than oxygen with the formation of O-D bonds in the bulk. Annihilations of them due to oxygen recovery could increase the retention of D with the formation of O-D bonds, resulting in the almost constant deuterium retention ratio up to its decomposition temperature of 573 K.

Helium implantation effects on retention behavior of hydrogen isotopes in oxygen-contained boron films

Abstract He+ implantation effects on the Retention behavior of hydrogen isotopes implanted into 35% oxygen-contained boron film was studied by means of SEM, AFM, XPS and TDS. It was found that the D retention for only D2+ implanted film was the highest and it decreased for pre-He+ implanted film and post-He+ implanted film. From the SEM and AFM images, the surface morphology of the oxygen-contained boron film was partly cracked, indicating that B2O3 was formed in the film. From the TDS and XPS results, the defective structure and the formation of B-D-B bond, B-D bond and B-O-D bond were observed by He+ and D2+ implantation, respectively. It was suggested that oxygen was trapped as B-O bond. The reaction with implanted D2+ was preceded in different mechanism.