Tsutomu Sagawa

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.

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.

Optical properties in fibers during irradiation in a fission reactor

Abstract The effects of irradiation on the performance of optical fibers with silica core were examined during irradiation in a fission reactor, JMTR. The fibers performed well in terms of their radiation characteristics up to the fast neutron fluence of 1.06 × 10 20 n/cm 2 and the gamma ray dose rate of 4.3 × 10 9 Gy at 460 K. Three kinds of optical radiation induced by the reactor irradiation were observed during irradiation. Origins of these optical radiations were studied. Broad optical radiations centered at 0.45 and 0.73 μm are thought to be Cherenkov radiation. The sharp optical radiation at 1.27 μm was thought to be generated in the silica core by the gamma-ray irradiations in the reactor.

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.

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.

Behavior of radiation-resistant optical fibers under irradiation in a fission reactor

Abstract Two kinds of optical fibers were irradiated in a fission reactor, JMTR (Japan Materials Testing Reactor), up to a 1.55 × 10 19 n/cm 2 fast neutron fluence and a 3.3 × 10 9 Gy ionizing dose at 370 K. Optical transmission spectra were measured in the wavelength range of 450–1750 nm, in situ. Growth of strong optical absorption bands and a peak were observed in the range of wavelength shorter than 750 nm. In the meantime, the fibers showed good radiation-resistance in the range of wavelength larger than 750 nm. Some optical radiation from the fibers was observed. The main cause of the optical radiation is thought to be so-called Cherenkov radiation.

Development of controlled temperature-cycle irradiation technique in JMTR

Abstract To investigate the effects of cyclic temperature changes during neutron irradiation upon radiation induced microstructure evolution and resulting property changes of materials is very important from both fundamental and engineering viewpoints. Therefore, a technique that allows us to do the controlled temperature-cycle irradiation was developed in the Japan Materials Testing Reactor (JMTR). The technique meets the following requirements: (1) the temperature-cycle irradiation is to be performed under three different conditions by changing lower and upper temperatures; 200⇓400°C, 300⇓400°C and 300⇓450°C; (2) the number and period of the temperature-cycles are to be six for 24-day full irradiation and approximately 44 h/44 h at the lower/upper temperatures; (3) the temperatures of each specimen assembly are to be maintained at the lower temperatures before start-up of the reactor and at the upper temperatures during shutdown until the complete absence of reactor power. In this paper, the details of the irradiation rig, successful results and several problems to be overcome for future improvement are presented.

Study of radiation induced electrical degradation of alumina in a dynamic pumping condition in a fission reactor

Abstract An electrical resistivity of polycrystal α-alumina was measured at about 680 K in a dynamic pumping condition in a fission reactor, JMTR. In a good vacuum, we observed smaller RIC (radiation-induced conductivity) than our previous results. We observed RIED (radiation-induced electrical degradation) -like behavior. The results suggested that RIED would take off faster at a higher ionizing dose rate. In the mean-time, the take-off occurred at about the same displacement damage of about 0.03–0.05 dpa in the range of fast neutron flux of (3.4–15.1) × 10 13 n/cm 2 s.

Radiation induced electromotive force in mineral insulated cable under reactor irradiation

Abstract A radiation induced electromotive force (RIEMF) was measured in mineral insulated cables (MI-cables) under fission reactor irradiation. A negative electromotive force of about 10 V was generated in the center lead by the reactor irradiation. The RIEMF increased with increasing irradiation dose with about − 14 V measured under the reactor full power irradiation at 573 K with a total dose of 1.1 × 1010 Gy ionizing irradiation and 1.1 × 1024 n/m2 fast neutron irradiation. The measured voltage of the RIEMF was found to depend linearly on the reciprocal temperature. The observed RIEMF was a so-called current driven source and the measured current was linearly dependent on the reactor power and was about 50 nA for the 0.7 m long MI-cables at 573 K. Under full power, irradiation conditions were 6 W/g of a gamma flux (6 × 103 Gy/s ionizing dose rate) for iron and 5 × 1017 n/m2 and 1.1 × 1018 n/m2 fast and thermal neutron fluxes, respectively.

Development of rig for systematic irradiation tests of fusion reactor materials in a fission reactor

Acquisition of systematic irradiation data are essential for understanding fundamental processes of irradiation effects and for establishment of a reliable database for irradiation effects in fusion reactor materials. It will take several years with expensive several different irradiation rigs in a fission reactor irradiation. There, it will take a long time to carry out the needed iterations between irradiation tests and evaluation and materials development. An irradiation rig was developed to carry out irradiation under multiple temperatures and irradiation fluences. Irradiation tests of fusion reactor materials were successfully carried out using the rig in Japan Materials Testing Reactor.