Tsunemi Kakuta

Irradiation test of diagnostic components for ITER application in the Japan Materials Testing Reactor

Radiation effects in components and materials will be one of the most serious technological issues in nuclear fusion systems realizing burning-plasmas. Especially, diagnostic components, which should play a crucial role in controlling plasmas and understanding the physics of burning-plasmas, will be exposed to high-flux neutrons and gamma rays. Dynamic radiation effects will affect the performance of components substantially from the beginning of exposure to radiation environments, and accumulated radiation effects will gradually degrade their functioning abilities in the course of their service. High-power-density fission reactors will be the only realistic tools to simulate the radiation environments expected to occur in burning-plasma fusion machines such as the International Thermonuclear Experimental Reactor (ITER), at present. Some key diagnostic components, namely magnetic coils, bolometers, and optical fibres, were irradiation-tested in a fission reactor, to evaluate their performances in heavy radiation environments. Results indicate that ITER-relevant radiation-resistant diagnostic components could be developed in time, although there are still some technological problems to be overcome.

Round-robin irradiation test of radiation resistant optical fibers for ITER diagnostic application

Fused silica core optical fibers are expected to play crucial roles especially in the size-reduced International Thermonuclear Experimental Reactor (ITER-FEAT). Several radiation resistant optical fibers have been developed in Japan and the Russian Federation. The task force on radiation effects in diagnostic components in the ITER-EDA (engineering and design activity) promoted international round-robin irradiation experiments on the developed optical fibers. Ten different optical fibers were tested in a cobalt-60 gamma-ray irradiation facility and in the Japan Materials Testing Reactor. The paper reports results obtained on five different optical fibers, which include purified, hydrogen loaded, and fluorine doped ones. Results show that the developed optical fibers could be deployed in remote handling and out-of-vessel applications. But, for the in-vessel diagnostics in the visible range optical spectroscopy, further improvement of the radiation resistance of optical fibers will be needed.

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.

Consequences of radiation effects on pure-silica-core optical fibers used for Raman-scattering-based temperature measurements

Two types of pure-silica-core fibers (one low-OH, Al-jacketed, one medium-OH, polyimide jacketed) suitable for use as sensing fibers for Raman-scattering-based temperature measurements in nuclear environments have been subjected to gamma and fission reactor irradiation tests. Spectral attenuation measurements were performed between 500 and 1500 nm with samples kept at room temperature, 80 and 300/spl deg/C. The Al-jacketed fiber was developed for use under ionizing radiation and showed lower loss compared with the polyimide-jacketed fiber at room temperature under gamma irradiation. Both fiber types showed similar spectra at room temperature with the main part of the loss originating from a band tail extending from the ultraviolet. Thermal bleaching of the radiation-induced defects was found to be effective in both fiber types. At 80/spl deg/C the loss in both fibers was compounded of a band at 625 nm together with the band tail from the ultraviolet, which now had a strength several times lower compared with room-temperature irradiations. At 300/spl deg/C, both fibers exhibited similar low-loss spectra, except for the band at 625 nm which reached levels of approximately /spl sim/2000 dB/km at an accumulated dose of 2.8/spl times/10/sup 4/ Gy(SiO/sub 2/). In light of the experimental spectral findings, selection of suitable Raman-distributed temperature sensors for nuclear plants can be made.

Japanese contribution to ITER task of irradiation tests on diagnostics components

Abstract As an R&D task of the ITER engineering design activities (EDA), we carried out the irradiation tests on the basic materials of transmission components such as ceramics, windows, fiber optics and mirrors, and in-vessel diagnostics sensors such as magnetic probes and bolometers. Ceramics will be used as general insulating materials. Here we investigated the radiation induced conductivity (RIC) of ceramics for 14 MeV neutrons. Optical elements are the most sensitive to radiation damages among the transmission components. Radiation induce transmission loss of window materials and fiber optics have been measured in the Japan material testing reactor (JMTR). Off line irradiation tests were carried out for molybdenum corner cube reflector (CCR) and gold coated mirrors in JMTR also. Fourteen MeV-neutron-induce luminescence of window material was evaluated in the fusion neutronics source (FNS). Bolometers should be installed inside the vacuum vessel to measure the radiation losses in the infra red to soft X-ray range. We demonstrated the performance of the JT 60 type bolometer under 60Co gamma-rays.

Radiation resistance characteristics of optical fibers

In order to fabricate an optical fiber with an excellent radiation resistance characteristic, we have investigated the radiation resistance characteristics of various types of optical fibers. As a result, two kinds of fiber were found to have an excellent radiation resistance characteristic which can be sufficiently used in the practical application under large doses exceeding 1 \times 10^{8} R.

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.

Distributed Raman temperature measurement system for monitoring of nuclear power plant coolant loops

A distributed temperature sensor based on Raman scattering in optical fibers has been tested for use as coolant loop monitor in nuclear power plants. Different types of pure- silica-core, polyimide-coated fibers have been subjected to 60Co-gamma-ray and fission-reactor irradiation at varying temperatures. 60Co-gamma-ray irradiations at dose rates from 4.8 kR/h up to 1 MR/h were done. Simultaneous gamma-ray and high temperature experiments up to 300 degrees Celsius have also been performed. The induced loss of the tested fibers was found to saturate with increasing dose at the anti-Stokes and Stokes wavelengths. This feature was then made use of to develop a model for radiation induced loss which was used to make system lifetime predictions. It has also been demonstrated that the induced loss of the optical fibers is favorably affected by high-temperature use. A 10-fold decrease in the radiation- induced loss levels when the system was operated at 300 degrees Celsius was observed, as compared with room- temperature operation. The experiments have shown that with a pure-silica-core, polyimide-coated fiber the temperature sensing capabilities of the RDTS will not be degraded excessively if used at primary coolant loops with an expected upper radiation level of 200 R/hr.