T. Shikama

Impact of irradiation effects on design solutions for ITER diagnostics

An overview of the results of the irradiation tests on diagnostic components under the ITER technology R&D tasks and the solutions for the present diagnostic design are given in the light of these results. A comprehensive irradiation database of diagnostic components has been accumulated and permits conclusions to be drawn on the application of these components in ITER. Under the ITER technology R&D tasks, not only has work been shared among four home teams, but also several bilateral collaborations and round-robin experiments have been performed to enhance the R&D activities. 2000 Elsevier Science B.V. All rights reserved.

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.

Current status of nanostructured tungsten-based materials development

Nanostructured tungsten (W)-based materials offer many advantages for use as plasma facing materials and components exposed to heavy thermal loads combined with irradiation with high-energy neutron and low-energy ion. This paper first presents the recent progress in nanostructured toughened, fine grained, recrystallized W materials. Thermal desorption spectrometry apparatus equipped with an ion gun has been installed in the radiation controlled area in our Center at Tohoku University to systematically investigate the effects of displacement damage due to high-energy neutron irradiation on hydrogen isotope retention in connection with the nano- or micro-structures in W-based materials. In this paper, the effects of high-energy heavy ion irradiation on deuterium retention in W with different microstructures are described as a preliminary work with the prospective view of neutron irradiation effects.

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.

Helium and hydrogen trapping in W and Mo single-crystals irradiated by He ions

Abstract Retention of He and accumulation of H in the near surface layer of W and Mo single-crystals were studied during and after the implantation of He ions with 2–10 keV at 295 and 820 K. The He retention was saturated at a concentration of a He/metal ratio of about 0.25, depending on the implantation temperature. Subsequent He implantation caused H accumulation in the He saturated layer, up to a maximum concentration about equal to that of He. The initial H uptake rate just after the He irradiation was comparable to the impingement rate of the H2 or H2O molecule at the crystal surface from the residual gas. For the He irradiation at 820 K, blisters and exfoliation with large sizes were observed on the crystal surface, where impurities other than H and He were also enriched.

Electrical properties of ceramics during reactor irradiation

Twelve different types of polycrystal and single crystal Al 2 O 3 (alumina and sapphire) specimens of varying grades of purity were irradiated for three reactor cycles in a removable beryllium position in a High Flux Isotopes Reactor (HFIR) at Oak Ridge National Laboratory at a temperature of 720-760 K up to a maximum dose of 3 dpa while a dc electric field of 200 V/mm was applied. The recently completed Temperature Regulated In Situ Test (TRIST) facility in the HFIR was used to perform in situ measurements of electrical conductivity. In addition, three Al 2 O 3 specimens were simultaneously irradiated without a continuously applied dc electric field. In situ electrical conductivity measurements were performed on the specimens before, during and following each irradiation cycle. Behavior of electrical conduction in Al 2 O 3 was studied, with special emphasis on detection of any long-term increase of the electrical conductivity.

Fission-reactor-radiation-tests of MI-cables and magnetic coils for fusion burning plasma diagnostics

Magnetic probes for plasma diagnostics in the International Thermonuclear Experimental Reactor (ITER) were developed and tested in a fission reactor, the Japan Materials Testing Reactor. Performance of coils made of mineral insulating cables could be analyzed by a proposed equivalent electrical circuit but were disturbed by electrical drift, presumably caused by a phenomenon called radiation induced electromotive force. However, results were promising and development of an ITER-relevant magnetic probe is within view.

Ion beam analysis of helium and its irradiation effect on hydrogen trapping in W single crystals

Abstract Retention of He implanted into W single crystals and the He irradiation effects on H behavior were studied by ion beam analysis techniques. During implantation of 4 He+ with 2–10 keV at 295 K, an accumulation of H started in the He implanted layer when the retained He concentration saturated. For the crystal irradiated by 10 keV He+ at 820 K, a remarkable increase of H was found in the He saturated layer, after stopping the implantation and cooling down the crystal below 400 K. Though blisters and exfoliation were observed for the surface irradiated at 820 K, less lattice disorder was found in the implanted layer and the thermal release of H occurred at lower temperature, in comparison with the crystal implanted at 295 K.

Radiation Hardness Test of Mica Bolometers for Iter in JMTR

Maximum radiation loss without disruption is essential to extend the maintenance intervals of ITER and following fusion reactors. The strong radiation distribution at the divertor must be known in fine detail to control such discharges. The corresponding reference diagnostic uses metal resistor bolometer cameras for detailed tomographic reconstructions of the radiation distribution2. The radiation hardness of the detector had been identified as the main question concerning the reference design. We report on the results of a Japanese/European collaboration (under ITER task T492) achieving the first in situ neutron irradiation tests of present day bolometers. A neutron dose equivalent to 0.1 dpa was reached, corresponding to the fluence between two maintenance periods at the bolometer locations behind the first wall elements or in the divertor under full load conditions of ITER-FEAT.