Daiju Yamaki

Tritium release behavior from neutron-irradiated Li2TiO3 single crystal

Li 2 TiO 3 single crystals with various grain size (1-2 mm) were used as specimens. After the irradiation up to 4 × 10 18 n/cm 2 with thermal neutrons in JRR-2, tritium release from the Li 2 TiO 3 specimens in isothermal heating tests was continuously measured with a proportional counter. The tritium release in the range from 625 to 1373 K seems to be controlled by bulk diffusion. The tritium effective diffusion coefficient (D T ) in Li 2 TiO 3 was evaluated to be D T [cm 2 /s]=0.100 exp(-104[kJ/mol]/RT), 625 K < T < 1373 K. In this temperature region, the tritium effective diffusion coefficients in Li 2 TiO 3 are close to those of Li 2 O irradiated with thermal neutrons of 4 × 10 16 and 2 × 10 19 n/cm 2 . It indicates that the tritium release performance of Li 2 TiO 3 is essentially as good as that of Li 2 O.

Development of a triple beam irradiation facility

Abstract A triple beam ion irradiation facility has been developed to study the synergistic effects of displacement damage, helium and hydrogen atoms on microstructural changes of materials under irradiation environments simulating a fusion reactor. The system consists of a vacuum chamber and three beamlines, which are connected with each electrostatic accelerator. Samples can be irradiated in the wide temperature range from liquid nitrogen to 1273 K in the chamber by replacing two kinds of sample stages alternatively. An austenitic stainless steel was simultaneously irradiated with triple beam of nickel, helium and hydrogen ions at 573–673 K using this facility and TEM observations were carried out from a cross sectional view normal to the incident surface. It was shown that the number density of dislocation loops decreased in the region where hydrogen and helium were deposited in comparison with ones in the region where only displacement damage was induced to a similar damage level.

Microstructure in pure copper irradiated by simultaneous multi-ion beam of hydrogen, helium and self ions

Abstract Pure copper was irradiated at 300–500°C by 5 MeV Cu ions (single beam) and Cu ions plus gas atoms (H and He) (dual beam irradiation) simultaneously. The high energy ion irradiation was carried out with the accelerator TIARA at the Takasaki-establishment of JAERI. The ions stop within a few microns from surface level and damage was formed up to this depth. The damage structure was observed as a function of the depth utilizing a focused ion beam (FIB) device. Below 300°C irradiation with a single beam produced a high density of stacking fault tetrahedra (SFT) but void formation was not observed. Large voids were observed with single beam irradiation at 500°C. In specimen irradiated with a dual beam of helium and Ni ions, the number density of voids was increased significantly. In copper irradiated with hydrogen and Ni ions, the number density of voids was not so large. Experimental results show that helium atoms promote void formation. Hydrogen atoms have less effect on void formation than helium atoms in pure copper.

Electrical resistivity of ceramic insulators under irradiation using 14 MeV neutrons

Abstract Ceramic insulator materials used for various components such as diagnostic systems, in-vessel components, RF windows, etc. in fusion reactors are exposed to severe irradiation environments which are characterized by high energy neutrons with energies up to 14 MeV. The electrical resistivity of ceramic materials decreases due to radiation-induced conductivity (RIC) during irradiation. The RIC due to 14 MeV neutrons for Al 2 O 3 in the temperature range 300–570 K was measured in the neutron dose rate range 10 −2 to 10 0 Gy s −1 using the fusion neutronics source (FNS) at the Japan Atomic Energy Research Institute. The RIC of Al 2 O 3 was estimated for the more severe irradiation environment in ITER and a prototype fusion reactor (SSTR) by extrapolating the data due to 14 MeV neutrons in the present study and those due to gamma-ray in another study. The estimated electrical degradation due to the RIC is considered to be accommodated with appropriate fusion reactor designs.

Microstructure in vanadium irradiated by simultaneous multi-ion beam of hydrogen, helium and nickel ions

Abstract Pure vanadium was irradiated at 500 and 600 °C by either 5 MeV Ni ions (single beam) or Ni+H and He ions simultaneously. The pure vanadium was of nominal 99.8% purity. For the quantitative investigation of damage structure as a function of the depth, we utilized focused ion beam (FIB) microscopy. To preserve the surface of ion-irradiated metals, we deposited tungsten on the irradiated surface. The specimens were electro-polished to remove the damaged region by FIB. When only nickel ions were used, voids formed in the region from the surface to a depth of ∼0.5 μm when irradiated at 500 and 600 °C. However, in the region of the damage peak, voids were not observed. Needle-like precipitates of about 100 nm of length were observed for any specimen covering the full ion penetration depth. It is thought that the precipitate is a carbide. Moreover, in the specimen irradiated at 600 °C, the granular precipitates were over the region of the 1.0–1.5 μm depth. Void formation was observed over the whole ion penetration depth when the specimen was subjected to Ni+He simultaneous irradiation. Needle-like precipitates were observed.

Cross-sectional observation of damage structures in Al2O3 irradiated with multiple beams of H, He, and O ions and after annealing at 1273 K

Abstract Damage structures in a single crystal α-Al2O3 sample irradiated with triple (0.25 MeV H+, 0.6 MeV He+ and 2.4 MeV O2+) ion beams at 923 K to a peak dose of 3.6 dpa were characterized by dislocation loops distributed from the region close to the incident surface to the region of 1.8 μm in depth, with cavities being formed in the restricted region of depths from 1.2 to 1.75 μm. A similar depth profile of damage structures was formed in a sample irradiated with dual (0.25 MeV H+ and 2.4 Me V O+) ion beams to a peak dose of 10.6 dpa, except that tiny cavities were formed down to the smaller depth of 0.5 μm. Depth profiles of implanted hydrogen atoms measured through a nuclear reaction analysis revaled that point defect mobilities were largely enhanced in the presence of hydrogen without helium. Upon annealing for 1 h at 1273 K, cavities grown to 70–80 nm at the maximum size were formed around peak damage regions in both the irradiated samples. Resultant cavity swellings were 6.2% and 18%, the ratio of which was found to be equal to that of accumulated damage.

A study on irradiation-induced structural change of lithium orthosilicate by infrared spectroscopy analysis with MNDO calculation

Properties of lithium orthosilicate, Li4SiO4, as a tritium breeding material will be substantially affected by structural change due to irradiation. An infrared absorption at 1040 cm−1 appeared in the FT-IR PAS spectra of Li4SiO4 irradiated with high energy oxygen ions. The semi-empirical molecular orbital method, MNDO, was applied to the interpretation of the changes of FT-IR PAS spectra of irradiated Li4SiO4. Vibrational frequencies and force constants have been calculated for the silicate clusters model: SiO4 isolated tetrahedron, Si2O7 dimers, Si2O6 chain links, Si2O5 sheet units, and Si2O4 three-dimensional (3D) framework units. The calculations show that the SiOSi stretching vibrations are dependent on the degree of polymerization of the tetrahedron and connection of Li atoms. By comparing the FT-IR PAS spectra obtained experimentally in this study with the calculated spectra, the absorption at 1040 cm−1 due to the irradiation was assigned to the SiOSi stretching vibrations in the Si2O5 sheet and Si2O4 3D-framework structures.

Damage structure evolution in Al2O3 irradiated with multiple ion beams of H, He and O and after annealing

Damage structures in a single crystal Al 2 O 3 sample irradiated at 923 K with simultaneous triple beams of H-, He-and O-ions with respective energies of 0.25, 0.9 and 4.7 MeV up to a fluence of 2.7, 3.5 and 5.5 x 10 20 m 2 , were examined by cross-section electron microscopy. High density dislocation loops are formed to a depth of 2.4 μm and the loop density increases with increasing depth. Cavities are observed in bands with the swelling peaked at depths of 1.4, 1.9 and 2.3 μm for H-, He- and O-ion implanted regions, respectively. The growth of cavities is most pronounced in the O-ion damaged region with a maximum size of 13 nm and a density of 2-3 x 10 23 m 3 . In a sample annealed for I h at 1273 K after the irradiation, cavities grown to an average size 40 and 60 nm with a maximum of 70 nm were observed in He- and O-ion implanted regions, respectively, and the density was decreased by about two orders.

Microstructural evolution of single crystalline Al2O3 irradiated with single and triple ion beams

Abstract The radiation-induced microstructural changes have been studied by cross-sectional transmission electron microscopy for single-crystal α-Al2O3 samples irradiated with triple ion beams (0.25 MeV H+, 0.6 MeV He+ and 2.4 MeV O2+; ‘Triple (A)’), (0.33 MeV H+, 0.45 MeV He+ and 1.3 MeV O+; ‘Triple (B)’) and three consecutive single ion beams (0.3 MeV H+ ion followed by 0.6 MeV He+ and then 0.8 MeV O+ ions) at 650°C to doses in the range 0.1–8.4 dpa at the damage peak. In the specimen irradiated with Triple (A), having the same average projected range to a total peak dose of 3.7 dpa, cavities with an average diameter of 13 nm were formed between 1.2 and 1.75 μm in depth causing a swelling of 0.1% at the peak, which is larger than those of the specimens irradiated with other conditions. The extent of the cavity-introduced region is some 40% smaller than observed in the damage region due to the He+ and the O+ ions and due to the H+ ions in the sample irradiated with Triple (B). Cavities formed in the sample irradiated with H+ ion beams were found to be inhomogeneously distributed in the vicinity of dislocation loops which were grown to loop sizes up to 80 nm.

In-situ tritium release behavior from Li2TiO3 pebble-bed

Abstract The engineering data on neutron irradiation performance of fusion blanket are indispensable to design the fusion blanket. In this study, an in-situ irradiation test of the Li 2 TiO 3 pebble bed was carried out at the J apan M aterials T esting R eactor (JMTR), and the effects of various parameters, i.e. sweep gas flow rate, irradiation temperature and hydrogen content in sweep gas on tritium release were evaluated.