Kenichi Ito

Effect of grain size on recrystallization in high burnup fuel pellets

The effect of grain size on recrystallized structure formation in the outer region of high burnup UO2 fuel pellets was studied by optical microscopy, SEM, EPMA, XRD and TEM. Specimens were prepared from three kinds of fuels with different grain size (the standard pellet (grain size: 9 μm), the undoped large-grained pellet (51 μm) and the alumino-silicate-doped large-grained pellet (45 μm)), irradiated up to an average pellet burnup of 60 GWd/t in the Halden Reactor. The TEM observations showed that recrystallized structures were formed in a region from the middle to the edge (relative radius: r/r0= 0.7–1.0) of all fuel pellets, though they were less likely to form in the undoped large-grained pellet and the alumino-silicate-doped large-grained pellet than in the standard one. This result agreed qualitatively with the results obtained from optical microscope observations of the whole pellet region, SEM fractographs, and measurements of Xe concentration in the fuel matrix by EPMA. The effects of grain size and irradiation temperature on recrystallized structure formation were discussed in connection with fission damage accumulation.

The effect of cladding restraint on fission gas release behavior

Abstract This paper describes the results of a study on FP (fission product) gas release behavior under PCI (pellet-cladding interaction) restraint. The test data on the small gap fuel showed that an intense PCI caused FP gas to be released in a different manner from the well-known diffusion dependency. The comparison of the irradiation data of different pellet end shape fuels also clarified that the extent of PCI could vary release behavior. The chamfer pellet fuel exhibited in its gas release a linear time response, for which an interpretation was tried by a simple model comprised of bubble growth under compressive stress and gas release in a pressure gradient.

Magnetic properties and recording performances of patterned media fabricated by nitrogen ion implantation

Nitrogen ion implantation was performed on CoCrPt–SiO2 perpendicular media with a resist mask to fabricate patterned media. Signal amplitude and autocorrelation signal-to-noise ratio of the preamble pattern were measured to evaluate the quality of the pattern fabricated by the ion implantation. The signal-to-noise ratio and the jitter were closely related to the saturation magnetization of the ion-implanted area. The remained magnetization of the ion-implanted area probably affects edge roughness of the magnetic pattern. Larger reduction of the saturation magnetization at the ion-implanted area is important for obtaining higher signal quality. Off-track profiles and 747 curves were measured for 76-nm-pitch discrete track fabricated by ion implantation. The results show that discrete track recording had advantages over recording on continuous magnetic film, which indicates that the fabricated patterns were successfully isolated. The nitrogen ion implantation was effective in fabricating isolated magnetic t...

Effects of lateral straggling of ions on patterned media fabricated by nitrogen ion implantation

Patterned media were fabricated by nitrogen ion implantation and how ion lateral straggling affects the pattern size was analyzed to confirm the feasibility for high-density recording. N2+ ions were implanted with ion energies of 6, 10, 14, and 19 keV through an ion-depth-control layer with suitable thicknesses for the ion energies to make the same ion depth profile. The dosage was 1.8 × 1016 ions/cm2. The saturation magnetization in the ion-implanted area was decreased from 0.56 to 0.05 T for all the media. Preamble pattern width was obtained by analyzing the readback signal. It was also estimated using calculated ion lateral straggling width and measured mask width. The reduction ratio of the pattern width against the ion energy was almost the same for the signal analysis and calculation results. This indicates that the pattern width was decreased just by the width of the ion lateral straggling. Therefore, high-density small patterns can be fabricated by reducing ion energy and suppressing ion lateral s...

Fabrication of Discrete Track Media with Patterned Servo-areas and Tracks on Disk Substrate

Discrete track media (DTM) with patterned servo-areas and recording tracks were fabricated on disk substrates in a two-step RIE process, creating a fine pattern transfer from an imprinted resist mask to a substrate surface without variations in groove depth. The fabricated DTM with perpendicular magnetic recording layers flew stably during read out signal detection.