Tatsuo Shikama

Deposition of TiB2 films by a co-sputtering method

Abstract We deposited Ti 1− x B x films onto molybdenum by the co-sputtering deposition method. The sputter target is composed of a titanium disk 150 mm in diameter on which a suitable amount of boron particles are placed. The coating temperature is 870 K. The deposited Ti 1− x B x films are then characterized by conventional metallurgical techniques. It is recognized that the present coating method can produce stoichiometric TiB 2 (titanium diboride) films having the same chemical bonding state as sintered TiB 2 which is formed under thermal equilibrium conditions. The deposited stoichiometric TiB 2 films, however, do not crystallize well when they are deposited without a negative bias voltage applied to the substrates. The application of a small negative bias voltage to the substrate is found to enhance crystallization of the deposited TiB 2 films. The crystallized TiB 2 films have a preferred (001) orientation.

Radiation induced conductivity of ceramic insulators measured in a fission reactor

In-reactor measurements of the long-term change in the electrical conductivity of α-alumina were carried out in the JMTR fission reactor. Special attention was focussed on the effect of applied voltage on electrical degradation. Two experiments were carried out for 96 and 48 reactor full power days at 600–630 K and 770–800 K, with an applied electric field of 500 V/m AC and 500 kV/m DC, respectively. A long-term increase in electrical conductivity was observed, which is thought to be radiation induced electrical degradation, RIED.

Hydrogen solubility and diffusivity in neutron-irradiated graphite

Hydrogen solubility measurements and the analysis of absorption kinetics have been studied on graphite irradiated with neutrons at various fluences up to 5.4×10 24 n/m 2 . The absorption of hydrogen could be expressed as a diffusion-controlled process. The rate constant of hydrogen absorption was different from that for desorption. This difference may be ascribed to the effects of trapping sites in graphite. After neutron irradiation at 1.9×10 24 n/m 2 (~ 0.2 dpa), the hydrogen solubility was 20–50 times larger than that of unirradiated samples. The increase of hydrogen solubility was saturated above the damage level of ~ 0.3 dpa. The diffusivity of hydrogen was decreased by neutron irradiation up to 1.9×10 24 n/m 2 , and then increased above this fluence. This behavior can be ascribed to the production of the trapping sites for hydrogen, and the elongation of the distance between the basal planes by neutron irradiation.

Radiation-induced thermoelectric sensitivity (RITES) in ITER prototype magnetic sensors

This report summarizes the results of in situ measurements of noninductive voltages developed across prototype International Thermonuclear Experimental Reactor (ITER) magnetic coils performed at the Japan Materials Test Reactor. The voltages appear to be mainly thermoelectric in origin, with the thermal sensitivity developing gradually through irradiation and the thermal gradients supplied by nuclear heating. Possible mechanisms and circuits within the coils that can contribute to this radiation-induced thermoelectric sensitivity are described, and schemes to improve the ITER coil design are outlined.

Properties and structure of carbon excess TixC1−x deposited onto molybdenum by magnetron sputtering

Abstract There has been little concern about the properties and structure of the carbon excess Ti x C 1− x ( x x C 1− x ( x >0.5) deposits. It is found that carbon excess Ti x C 1− x deposits prepared by magnetron sputter coating have a higher microhardness than the stoichiometric deposits and show a strongly (111)-preferred orientation. The results of C Kα X-ray spectroscopy of these films strongly suggest that carbon excess Ti x C 1− x exists as a single phase of titanium carbide without the formation of a secondary graphite phase. The abnormal diffusion of substrate molybdenum atoms into the carbon excess Ti x C 1− x deposits suggests that they contain a high population of the titanium vacancies.

Properties of TixC1 − x films coated on molybdenum by magnetron sputtering

Abstract Properties of Ti x C 1 − x (0.3 ⩽ x ⩽ 0.6) films deposited onto molybdenum have been examined as functions of their chemical composition. Ti x C 1 − x films about 4–6 μm thick were deposited onto a sintered molybdenum substrate at 600 °C by magnetron sputtering. The near-stoichiometric Ti x C 1 − x ( x ≈ 0.5) was found to be the most thermally stable. Titanium excess Ti x C 1 − x ( x > 0.5) loses titanium by evaporation and changes its surface morphology above 1200 °C. However, excess titanium will improve the ductility of deposited films. Strong compressive stress was observed in carbon excess Ti x C 1 − x ( x x C 1 − x is more resistant against thermal evaporation than the stoichiometric compound. However, enhanced diffusion of molybdenum atoms into the carbon excess Ti x C 1 − x takes place, with the formation of Mo 2 C. This Mo 2 C formation nearly prohibits the use of carbon excess Ti x C 1 − x films coated on molybdenum as the first-wall material in fusion devices.

Radiation-induced electrical degradation experiments in the Japan materials testing reactor

Abstract An experiment to measure radiation-induced electrical degradation (RIED) in a sapphire sample and in three MgO-insulated cables was conducted at the JMTR light water reactor. The materials were irradiated at about 260°C to a fluence of 3 × 1024 n/m 2 ( E > 1 MeV) with an applied DC electric field between 100 kV/m and 500 kV/m. Even though the results for the sapphire sample are somewhat ambiguous because of an unexplained offset current of about 0.6 μA substantial degradation was not observed in the sapphire: instead, radiation-induced conductivity (RIC) seemed to decrease slightly during the experiment. Substantial increase in leakage current, that increased with applied electric field, occurred in the MgO-insulated cables. This increased conductivity disappeared when the reactor was shut down and sample temperature returned to ambient. However, the physical degradation apparently remained in the material while the reactor was off because restarting the irradiation brought the conductivity back to its previous, degraded, reactor-on value. This effect is different from the RIED effect reported by Hodgson but is similar to previous results reported by Shikama et al. Considerable data were taken to determine the sample temperature and leakage currents during the irradiation.

Behavior of developed radiation-resistant silica-core optical fibers under fission reactor irradiation

Abstract Oxyhydrate (OH) doped and fluorine (F) doped fused-silica core optical fibers were irradiated in a fission reactor at 370–380 K with a fast neutron flux of about 5×1016n/m2s and a gamma-ray dose rate of about 500 Gy/s for about 24 days. Oxyhydrate and fluorine dopings improved radiation resistance of optical fibers especially in the visible range. A fluorine doped and heat-treated optical fiber showed best radiation resistance and its radiation induced loss in a visible range is about 20 dB/m after it was irradiated up to a fast neutron fluence of 1×1023 n/m2. Results are implying that fused silica core optical fibers can be used nearer to a burning plasma for plasma diagnostics and remote sensing.

Trapping and detrapping of hydrogen in carbon-based materials exposed to hydrogen gas

Abstract Measurements of hydrogen solubility have been performed for several unirradiated and neutron-irradiated graphite (and CFC) samples at temperatures between 700 and 1050°C under a ⋍ 10 kPa hydrogen atmosphere. The hydrogen dissolution process has been studied and it is discussed here. The values of hydrogen solubility vary substantially among the samples by up to a factor of about 16. A strong correlation has been observed between the values of hydrogen solubility and the degrees of graphitization determined by the X-ray diffraction technique. The relation can be extended even for the neutron-irradiated samples. Hydrogen dissolution into graphite can be explained by the trapping of hydrogen at defect sites (e.g. dangling carbon bonds) considering an equilibrium reaction between hydrogen molecules and the trapping sites. The migration of hydrogen in graphite is speculated to result from a sequence of detrapping and retrapping events with high-energy activation processes.

Behavior of optical fibers under heavy irradiation

Abstract Several kinds of optical diagnostics are planned in a fusion reactor. Complicated optical systems such as periscopes are thought to be primary candidates for optical measurements, especially for visible wavelengths. However, optical fibers have several advantages over such optical systems. Also, the optical fibers could be a far better transmission line for signals under a high electromagnetic field. However, they have been considered vulnerable to heavy irradiation. In this study, several kinds of optical fibers were irradiated in the Japan Material Testing Reactor (JMTR). The optical transmissivity in fibers was measured in situ during fast neutron and gamma irradiation, up to doses of 2×1024 n m−2 and 5×109 Gy, respectively. The irradiation temperature ranged from 300 to 700 K. For pure ionizing irradiation environments, some methods for improving the radiation resistance of optical fibers were indicated. The results showed that effects of the irradiation associated with fast neutrons would be different from the effects of pure ionizing irradiation. Some fibers were found to withstand the heavy irradiation, especially in an infrared wavelength range.