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History, present status and future of fusion reactor materials research in Japan
Section
1
INVITED PAPERS
Journal of Nuclear North-Holland
Materials
179-181
3
(1991) 3-8
History, present status and future of fusion reactor materials research in Japan S. Ishino ‘, T. Kondo 2 and M. Okada 3 I Department of Nuclear Engineering, University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113, Japan ‘Department of Fuels and Materials Research, Tokai Research Establishment, Japan Atomic Energy Tokai-mura, Naka-gun, Ibaraki-ken 319-I I, Japan ’ Tsukuba Branch, National Research Instituie for Metals, Sengen, Tsukuba, Ibaraki-ken 305, Japan
R&D programs on fusion reactor materials in Japan have been promoted mainly historical development and highlights of the research of each of these sectors will be and planning activities. The topics include the foundation of the National Institute Energy Selective Neutron Irradiation Testing Facility (ESNIT), construction of International Cooperation Programs. Industrial involvement is also growing larger and NRIM. Future research trend and coordination of the research among these organizations and roles of these organizations for the national fusion program.
1. Introduction Almost all new technologies in history have been related to the development of new materials. The importance of materials in the development of a new energy system such as fusion has been recognized from the relatively early stage of fusion research and development in Japan. “Zairyo” in Japanese is not merely substance but has a meaning of substance on which a certain objective is imposed or a certain role is expected
Research Institute,
by universities, JAERI and NRIM. The reported together with on-going programs of Fusion Science (NIFS), planning of an SUBNANOTRON facility, and current in cooperation with universities, JAERI will be discussed
based on the objectives
each of these sectors. In the following sections, the current status of fusion materials research within each sector will be described with the future program. Then, in the section to follow some of the recent highlights will be presented. 2. Present status and future program of fusion materials research 2. I. General remarks
VI.’ Fusion materials research was initiated very early in Japan, say, mid-1950s. However, systematic studies began in early-1970s. The first comprehensive national planning was made in 1977-1978 [2] by the Subcommittee on Fusion Reactor Materials set under the Nuclear Fusion Council. Also in 1983-1984, a similar Subcommittee on Irradiation Studies of Fusion Reactor Materials was set to discuss the national strategy of irradiation studies including discussion and recommendations for FMIT [3]. Materials research and development is also included in the comprehensive fusion technology program, which is a part of the Long Term Program for Nuclear Power Development and Utilization set up by the Atomic Energy Commission of Japan in July, 1987 [4]. Implementation of these programs will be discussed in the later sections. In fusion materials research, both the device-focused short term component and long term generic aspects with theoretical background are necessary [5]. The authors feel that these tasks have been adequately shared by the three major research sectors in Japan, namely universities, Japan Atomic Energy Research Institute (JAERI) and National Research Institute for Metals (NRIM). It should also be mentioned that industrial participation has been growing, in cooperation with 0022-3115/91/$03,50
0 1991 - Elsevier Science Publishers
As stated earlier, research and development on fusion reactor materials have been promoted mainly by three sectors, namely, by universities supported by Monbusho (Ministry of Education, Science and Culture) and JAERI and NRIM under the auspices of Science and Technology Agency (STA). In the long term national fusion program, JAERI is pursuing scientific feasibility with JT-60, enhancing the capability of tokamak type devices, thus being expected to play a primary role in the R&D of the next step machine. Universities and other national research institutes are expected to carry out basic and original research on various confinement schemes as well as reactor technologies [4,6]. Universities are also responsible for providing young scientists and engineers to the community. A similar sharing of tasks on fusion materials research and development has existed among various sectors: JAERI has been mainly responsible for fundamental and applied research and the development of materials directly applicable to fusion devices. NRIM has been responsible for developing new materials and processes with fundamental research to assist the development. Universities have been mainly involved in fundamental research on the basic factors for materials development and on the materials behaviour under fu-
B.V. (North-Holland)
sion reactor environment. Close communication and cooperation have been maintained among researchers belonging to each sector. Although such a sharing of tasks is still kept among three sectors, a new trend has become grdwingly evident to place more emphasis on basic technology studies. This seems to be a reflection of realistic cognizance on the status of fusion energy development, which would require continuing effort well into the middle of the next century. This trend will be described in the following sections. This trend raises a problem of the need for redefining the share of tasks and resources concerning the basic research among three sectors. Some sort of steering organization is required to better coordinate the national fusion materials program. 2.2. Umversities 2.2.1. Special research project on nuclear fusion (FY 1980-l 989) Before 1970, fusion research in universities was mainly confined to a plasma physics area. University activities on the fusion materials area became active and gradually organized in the 197Os, and the Special Research Project on Nuclear Fusion supported by Monbusho is the first large-scale coordinated joint research among various universities in the fusion technology area including materials. Six major research areas have been supported under this project, namely: (1) reactor materials and plasma-wall interactions; (2) science, technology and biological effects of tritium; (3) fundamentals of reactor plasma control; (4) technology of the superconducting magnet; (5) fusion reactor blanket engineering; and (6) design and evaluation of a fusion reactor. Materials research has been one of the most important elements of this special project. For structural materials, major emphasis has been placed on radiation effects. The problems have been studied from the standpoint of the microstructural evolution under irradiation on the one hand and of alloy development for better mechanical properties after irradiation on the other. Microstrural evolution is a key to clarify irradiation correlation from a fundamental view point; i.e. from an understanding of the basic defect processes in the material under irradiation. To this aim pure metals, pure binary and ternary alloys, model alloys and commercial alloys have been purposely utilized using various irradiation environments as RTNS-II, Octavian, KUR, JMTR, JOY0 and FFTF. Austenitic and ferritic steels, vanadium alloys and developmental low activation alloys have been the major structural materials studied. For fundamental studies of alloy development, microstructural as well as mechanical and other properties measurements are important. The role of alloying elements on these properties and the phase stability under irradiation have been studied. For plasma-wall
interaction studies, sputtering and hydrogen recycling were the major subjects in early stage of the project and then, plasma-surface interactions with low energy hydrogen and those using actual plasma devices were studied. Recently, comprehensive studies on the characteristics of various kinds of graphite have been carried out. Special emphasis has been placed on radiation damage in ceramics for the past four years. The research has been fundamental in nature, obtaining displacement energies for various ceramics materials and studying the effects of electronic excitation on radiation damage. A large amount of interesting results have come out from the research outlined above. The majority of the contributions to the present and the former Conferences of this series from Universities have been supported by the Special Research Project on Fusion and we shall not reiterate the results here. Since the Special Research Project on Nuclear Fusion is approaching its completion in March, 1990. the planning and proposal for the follow-up studies during the coming six years have been made. The research will be on fundamental studies of complex systems of fusion plasma and the blanket. 2.2.2. New research institute at Toki-site The Institute of Plasma Physics, Nagoya University was reorganized and a new institute named the National Institute for Fusion Science has been established since May, 1989 directly under Monbusho. A large helical device (LHD) with a major radius of 4 m and with superconducting helical and poloidal coils will be constructed. Design activities are going on by 120 scientists and engineers. Materials related activities as identifying issues relevant to helical devices and plasma-surface interactions will be initiated soon. 2.2.3. International cooperation Japan/USA Joint Project on Utilization of RTNS-II was completed successfully in 1987, and the post-irradiation experiments have yielded extremely useful data on radiation damage produced by 14 MeV neutrons. In particular, the understanding of the nature of cascade damage produced by high energy recoils has been progressed and the method has been established to analyse the neutron radiation damage in terms of recoil energy spectra. This provides a basis for irradiation correlation. Since 1987, the Japan/USA Collaboration in Fundamental Studies of Irradiation Effects in Fusion Materials Utilizing Fission Reactors (FFTF/MOTA Collaboration) has started as an eight-year project. The basic philosophy is to extend the neutron fluence to a high damage region using fission reactors, always considering the correlation of the results to the case of fusion neutron irradiation as schematically illustrated in fig. 1 [7]. Already some of the early results have come
5
S. Ishino et al. / History, present status and future of fusion reactor materials research in Japan
national or international project of an intense neutron source.
as the implementation
2.3. Japan Atomic Energy Research Institute
2.3. I. Introduction The fusion materials program at JAERI has been carried out in a few department sectors and the phases of research are distributed over the basic to strictly applied. The materials science oriented portions cover the fields of the structural, the plasma-facing high heat tritium breeding and functional flux component, ceramics materials. Among those activities, a large fraction of neutron irradiation relevant works have been carried out in the international collaborations under the US-Japan bilateral and the multilateral IEA agreements. In FY1988, a new plan of the Base Nuclear Materials Program was brought into implementation in the Science and Technology Agency, STA. In JAERI, a strategy was proposed to integrate the disciplinary part of the base materials research to form a matrix with nuclear systems development projects as shown in table 1. The fusion part of the base technology program includes five tasks on materials development and critical studies on new phenomena, two tasks on the irradiation test facility and test technique development and two tasks on computerized database development. The operation of those matrix type research works is in progress.
Fig. 1. Strategy of approaching high fluence irradiation effect in a fusion reactor environment by utilizing existing reactors (ref. [7]).
out from the FFTF/MOTA Collaboration, to be presented in this Conference. It should be mentioned that to establish irradiation correlation the experiments are so designed as to use the same specimens in different irradiation environments as RTNS-II, FFTF as well as domestic facilities as JMTR, JOY0 and accelerators, that the effect of spectrum, dose, dose rate and other irradiation parameters can be studied less ambiguously. 2.2.4. Future program A future program will be grouped into four categories: (1) follow-up studies of the Special Research Project on Nuclear Fusion, which are the coordinated studies by individual research groups; (2) Joint research at the FFTF/MOTA or JOY0 utilization, involving the increased capacity of hot cells; (3) Collaboration with large plasma devices; and (4) Taking part in a large
Table 1 Strategic matrix
of fusion material
Base technology
program
Database
‘) Committments
0:
2.3.2.1. Structural materials. R&D programs are going on in a modified stainless steel as the candidate material for the next generation machine and in other stainless steels, with regard to the control of minor elements and to the welding technique
at JAERI Fusion materials
program
‘)
Structural
Plasma
facing
T-breeding
Functional 0 0 0 0 _
0 0
0 0 _
ESNIT/MODULAB Small specimen (SSTT)
0 0
0 _
0 _
0 0
Performance (JMPD) Character (NETWORK)
0 0
0 0
0 0
0 0
I Low activation Radiation corrosion
Facility and methodology
2.3.2. Current research activities
0 0 _ _ _
Design and innovation Irradiation resistance Corrosion resistance
Materials and phenomena
research
direct,
0:
indirect.
(JAERI)
0 0 0
0 0
(SPM)
which is indispensable as the structural material. As the evaluation study of these materials, irradiation testing including spectral and isotopic tailoring experiments are in progress through the Japan/US collaboration using HFlR/ORR. Post-irradiation mechanical properties and dimensional changes are being analysed and evaluated. Recently, new testing programs on irradiation corrosion and on welding of irradiated materials have been initiated. R&D of low activation materials is focused on ferritic steels and ceramic materials. To obtain better materials, alloy design methodologies are being applied and new thermomechanical treatment procedures are being devised. Basic studies on composite materials and on the development of functionalgradient materials as stress free materials are being performed. 2.3.2.2. Plasma-facing and high heat flux component materials. Fundamental data on the chemical sputtering of carbon and graphite have been compiled. Performance data have been obtained for these materials in an integrity evaluation testing using JT-60 and JFT-2M. In the near future, development evaluation testing will be carried out on the composite materials with composition gradient, which is resistant to chemical sputtering to be used in a commercial reactor first wall without armour. Integrity of armour and metallic structural materials under high heat flux encountered in plasma disruption and arcing has been evaluated using an electron beam irradiation facility and neutral beam injection facility for JT-60 (PBEF; now a dedicated facility for high flux testing). Data have been compiled for critical heat flux for melting and the integrity has been evaluated in some candidate materials for the divertor and limiter such as MO, W, Cu and its composite materials, graphite and C-C composites. Particularly in MO, the formation of cracks in the solidified layer and of voids can be suppressed by using the single crystal. Disruption characteristics have been evaluated in ferritic, dual phase and austenitic stainless steels. In austenitic alloys, the role of minor elements on crack resistance and the shape of the resolidified surface has been defined. Evaluation testing will be extended to use the specimen with an actual component shape and to study mechanical properties including fatigue characteristics after high heat loading. 2.3.2.3. Tritium breeding materials. The radiation damage of tritium breeding materials is important because it affects the tritium recovery characteristics and their durability. Radiation induced defects have been identified and their recovery kinetics have been elucidated by ion irradiation experiments using an accelerator. The ionic conductivity of lithium oxide depends on the mobility of Li ions. which determines the transport properties. The in-situ measurement of ionic conductiv-
ity has been carried out during and after irradiation. Evaluation of tritium recovery characteristics is important from the standpoint of selection of breeding materials and tritium inventory. In-situ measurements of tritium release have been performed for various candidate breeding materials using the JRR-2 reactor. The experiment is called “VOM”. the special features of which are that tritium release from two different capsules can be measured comparatively at the same time and that the effect of piping can be minimized. The effect of irradiation on tritium release under simulated blanket conditions will be studied by the international collaborative experiment. BEATRIX-II using the FFTF reactor. Experiments are also going on to establish mass production technology of breeding materials. 2.3.2.4. Special purpose materials and ceramics. High performance solid electrolyte of ytterbium-stabilized zirconia (ZrO,-Yb,O,) has been developed. This can be operated at 600°C for the tritium steam electrolysis in a fuel gas purification process, now being widely applied in the world to high efficiency electrolytic devices to treat tritium-containing water. In the field of electric insulation materials. Si,N, with MgO as a sintering aid has been developed for a high strength rf window. Currently, studies are oriented to the in-situ electrical properties and optical property degradation in a radiation environment. Fluoride-glass materials containing BeFz are the potential radiation resistant light conductive materials. By adding BeFz AlF,-based glass with excellent mechanical properties and high vitrification stability can be produced. Future trend are to create high performance new materials by computer simulation such as applying molecular dynamics method. 2.3.2.5. Database. The JMPD (JAERI Material Performance Database) has been developed since FY1985. This is a database comprising data on the development of materials for reactor and nuclear installations, materials characteristics necessary for performance evaluation, structural design and safety analyses. Taking advantage of this experience, data collection has been initiated in collaboration with universities, and the data items are being extracted particularly for first wall, tritium breeding and plasma-facing component materials. At the moment, it is considered that the data system on a personal computer level may be appropriate from the stand point of flexibility and feasibility. 2.3.2.6. Facility and test methodology development. AS a necessary facility for efficient experimental studies for the creation of materials compatible to a high energy neutron environment, preliminary studies are going on to build the “Energy Selective Neutron Irradiation Test Facility” (ESNIT) and a modular type post irradiation testing facility (MODULAB) and the ancillary facility
S. Ishino et al. / History, present status and future of fusion reactor materials research in Japan
multi-functional cooperative refor multi-purpose, search. The ESNIT is a linear accelerator based neutron source capable of producing a variable spectrum of neutrons, thus providing a means to carry out irradiation experiment with high precision which allows detailed analysis of the results [8]. The MODULAB is a facility with radiation shielding, in which encapsulated experimental modules can be inserted so that various detailed post-irradiation experiments can be performed efficiently. R&D work will continue until FY1992, followed by general evaluation before construction. Development of small specimen testing techniques has been in progress in JAERI as well as in universities and NRIM. Those techniques such as shear punch, microtensile, four-point bending and microcorrosion have been developed. These are based on using a TEM specimen. Development of disc bend, shear punch, microfatigue and microhardness techniques are on the program. To study the effect of transmutation produced helium and hydrogen in metals and ceramics, the irradiation facility capable of doing dual and triple beam irradiations are being planned for construction in FY1992.
2.4. National Research
Institute for Metals (NRIM)
Organization of NRIM has been modified since April 1988. The Second Group, formerly the Division of Reactor Materials, comprises four subgroups with tasks as follows: (1) characterization of radiation damage; (2) development of radiation resistant materials; (3) development of low activation materials; and (4) plasma facing and high heat flux materials. For irradiation studies, light ions such as protons and alpha particles have been mainly used. An irradiation creep experiments using light ions from a compact cyclotron were started from the beginning of 1986. Torsion and tensile types of irradiation creep testings are possible: The stress and temperature dependences of irradiation creep rate were experimentally determined for annealed and cold-worked stainless steel and compared with calculation based on point defect kinetics and dynamically competing mechanisms, namely, SIPN, SIPA, climb and glide enabled by SIPA [91. The effects of helium on the high temperature tensile properties, creep and fatigue strength and on micro structure have been investigated using helium implantation techniques for 316 and JPCA steels. A remarkable improvement of high temperature properties has been obtained for JPCA in which Tic particles are very finely dispersed by appropriate thermomechanical treatment to trap helium effectively within the matrix. Two kinds of low activation materials are being developed: one is a modification of 9Cr-1W and 9Cr3W steels with minor additions of V, Ta and B which
I
resulted in markedly increased creep rupture strength without significant loss of toughness [lo]. The other is a class of materials with much lower retained radioactivity: C fiber/Sic composite with high purity better than 99.98% has been obtained by a chemical vapor infiltration process. The composite with woven carbon yam showed high strength and toughness [ll]. For high heat flux materials in a divertor. a thermal shock test by an electron beam has been conducted on various commercial carbon materials and on synthesized carbon/boron materials. Thermal stress cracking and surface erosion were compared among those materials and relations between resistance to high heat flux damage and physical properties were analysed: the results are reported in the present conference [12]. The irradiation behaviour of molybdenum single crystal produced by secondary recrystallization was investigated under the collaboration with JAERI. DBTT was lowered by single-crystallization but the increase of DBTT by neutron irradiation was slightly larger than that of polycrystals. Small island crystals surviving after secondary recrystallization had a severe effect on ductility and toughness. A new facility called “SUBNANOTRON” for in-situ and dynamic analysis of microstructural aspects of materials under dual-ion irradiations is now under construction. This consists of a 1 MV transmission electron microscope with several analytical tools and two ion accelerators. The status of the facility and preliminary work using a TEM/ion accelerator interface were reported in the Poster Session of this Conference [13].
2.5. Other activities There are many researchers belonging to other research institutes than the above mentioned three sectors, who are actively involved in research in fusion materials or in related areas. Among various such organizations, the following institutes should be listed: Electrotechnical Laboratory (ETL), Government Industrial Research Institute, Nagoya both funded by the Ministry of International Trade and Industry (MITI), National Institute for Research in Inorganic Materials funded by STA, and the Institute of Physical and Chemical Research mostly funded by STA. Power Reactor and Nuclear Fuel Development Corporation (PNC) is operating a 100 MW fast experimental reactor, JOYO, at the Oarai site, which has been used for several years by the university research groups for irradiation studies of fusion reactor materials. It should also be mentioned that industries are taking part in fusion materials development by cooperation with JAERI, NRIM and universities. Such industrial firms include major steel makers and reactor manufacturers. For graphite and other functional materials, various other industries are taking part.
3. Discussion As stated earlier. both facility-oriented short-term research and generic long term research are needed. Research for the major fusion facilities is important because it provides the thrust to the whole fusion research and development activity. In this respect. further progress in plasma confinement and control is required. It has become more and more evident that such a progress is strongly dependent on the choice of plasmafacing component materials. For example. the edge plasma condition is determined by the selection of the plasma-facing component materials and vice versa, the service condition of the plasma-facing component is determined by the edge plasma condition. These interactions among plasma, materials and design are the central problems of plasma-surface interactions. Without stronger and closer collaboration among plasma physicists, materials scientists and facility designers, currently facing problems will not be overcome. In this respect. active participation in the TEXTOR project as w-e11as utilization of Heiiotron-E, JIPP T-II, JFT-2M, TRIAM and other plasma-machines by materials scientists have been promoted. Similarly, participation in the experiments using a large tokamak machine as JT-60 and in reactor design activity such as ITER should be extended. Task group, to identify materials problems related to the Large Helical Device (LHD) wiil be initiated
soon.
For resolving long-term and generic problems such as the irradiation effects of structural materials to a very high fluence. we have at present no intense neutron sources relevant to a D-T fusion reactor. Full utilization of existing irradiation facilities as fast and thermal fission reactors as JOYO, FFTF. JMTR, KUR, HFIR and so on, accelerator based neutron sources, RTNS-II, OKTAVIAN, LAMPF, heavy and/or light ion accelerators, HVEMs has had to be sought for. Various novel techniques to tailor radiation environments or the specimen itself to produce fusion relevant irradiation effects such as spectral or isotopic tailoring, dynamic helium charging experiments, dual ion beam irradiation techniques and so on are being designed or have been applied, providing very useful results. However, it should be emphasized that establishing irradiation effect causality based on a fundamental understanding of radiation damage processes is important, not merely seeking for phenomenological correlation among radiation effects obtained in different irradiation environments. This has been the strategy taken by the Japanese university groups and we believe this will be particularly effective in solving the long-term problems. The discussions given above may apply not only for local Japanese programs but may be common to worldwide fusion materials programs. In this respect, further promotion of international collaboration on the com-
mon programs on materials will be fruitful progress of fusion research and development.
for the
4. Conclusions
(1) The history,
current status and future trends on fusion materials research in universities, JAERI, NRIM and other organizations are reviewed. (2) Japanese program seems to be well balanced between near term and long term issues. However, some sort of organizational mechanism will be required to assure better coordination of the national fusion materials program. among (31 It is pointed out that increased collaboration plasma physicists, materials scientists and reactor designers will be necessary to solve the near term problems. involving irradiation (4) For the long term problems effects, both designing fusion relevant simulation techniques and establishing physics-based irradialion causality are necessary. (5) The Japanese current activities and future trends seem to contain many of the world-wide common problems. Further promotion of international collaboration on these common problems will be fruitful in the future research and development of fusion.
References 1’1 S. Ishino, Radiation
Damage, Nuclear Engineering Monograph. Series 8 (University of Tokyo Press. Tokyo. 1979) p. 195 (in Japanese). to the 121 Report of the Fusion Materials Subcommittee Fusion Council, July 1978 (in Japanese). 131 Report of the Fusion Materials, Irradiation Research Subcommittee to the Fusion Council, November 1984 (in Japanese). of Japan, The Long Term 141 Atomic Energy Commission Program for Nuclear Power I>evelopment and Utilization (June 1987). 151 S. Amelinckx. J. Nucl. Mater. 155-157 (198X) 3. [cl S. Mori. Fusion Engrg. Des. 8 (1989) 9. 171 S. Ishino, Bull. Japan. Inst. Met. 26 (1987) 1044 (in Japanese). 1x1 T. Kondo, H. Ohno, M. Mizumoto and M. Odera, J. Fusion Energy 8 (1989) 229. N. Yamamoto and H. Shiraishi. in these 191 J. Nagakawa. Proceedings (ICFRM-4). J. Nucl. Mater. 179-181 (1991). 1101 F. Abe, T. Noda, H. Araki and S. Nakazawa. in these Proceedings (ICFRM-4). J. Nucl. Mater, 1799181 (1991). 1111 7‘. Noda, H. Araki. F. Abe and M. Okada. in these Proceedings (ICFRM-4), J. Nucl. Mater. 179-181 (1991). 1121 M. Fujitsuka, H. Shinno. T. Tanabe and H. Shiraishi, in these Proceedings (ICFRM-4). J. Nucl. Mater. 1799181 (1991). 1131 K. Furuya, T. Kimoto, T. Noda, H. Shiraishi and M. Okada. presented at ICFRM-4. paper 7-109.
1
INVITED PAPERS
Journal of Nuclear North-Holland
Materials
179-181
3
(1991) 3-8
History, present status and future of fusion reactor materials research in Japan S. Ishino ‘, T. Kondo 2 and M. Okada 3 I Department of Nuclear Engineering, University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113, Japan ‘Department of Fuels and Materials Research, Tokai Research Establishment, Japan Atomic Energy Tokai-mura, Naka-gun, Ibaraki-ken 319-I I, Japan ’ Tsukuba Branch, National Research Instituie for Metals, Sengen, Tsukuba, Ibaraki-ken 305, Japan
R&D programs on fusion reactor materials in Japan have been promoted mainly historical development and highlights of the research of each of these sectors will be and planning activities. The topics include the foundation of the National Institute Energy Selective Neutron Irradiation Testing Facility (ESNIT), construction of International Cooperation Programs. Industrial involvement is also growing larger and NRIM. Future research trend and coordination of the research among these organizations and roles of these organizations for the national fusion program.
1. Introduction Almost all new technologies in history have been related to the development of new materials. The importance of materials in the development of a new energy system such as fusion has been recognized from the relatively early stage of fusion research and development in Japan. “Zairyo” in Japanese is not merely substance but has a meaning of substance on which a certain objective is imposed or a certain role is expected
Research Institute,
by universities, JAERI and NRIM. The reported together with on-going programs of Fusion Science (NIFS), planning of an SUBNANOTRON facility, and current in cooperation with universities, JAERI will be discussed
based on the objectives
each of these sectors. In the following sections, the current status of fusion materials research within each sector will be described with the future program. Then, in the section to follow some of the recent highlights will be presented. 2. Present status and future program of fusion materials research 2. I. General remarks
VI.’ Fusion materials research was initiated very early in Japan, say, mid-1950s. However, systematic studies began in early-1970s. The first comprehensive national planning was made in 1977-1978 [2] by the Subcommittee on Fusion Reactor Materials set under the Nuclear Fusion Council. Also in 1983-1984, a similar Subcommittee on Irradiation Studies of Fusion Reactor Materials was set to discuss the national strategy of irradiation studies including discussion and recommendations for FMIT [3]. Materials research and development is also included in the comprehensive fusion technology program, which is a part of the Long Term Program for Nuclear Power Development and Utilization set up by the Atomic Energy Commission of Japan in July, 1987 [4]. Implementation of these programs will be discussed in the later sections. In fusion materials research, both the device-focused short term component and long term generic aspects with theoretical background are necessary [5]. The authors feel that these tasks have been adequately shared by the three major research sectors in Japan, namely universities, Japan Atomic Energy Research Institute (JAERI) and National Research Institute for Metals (NRIM). It should also be mentioned that industrial participation has been growing, in cooperation with 0022-3115/91/$03,50
0 1991 - Elsevier Science Publishers
As stated earlier, research and development on fusion reactor materials have been promoted mainly by three sectors, namely, by universities supported by Monbusho (Ministry of Education, Science and Culture) and JAERI and NRIM under the auspices of Science and Technology Agency (STA). In the long term national fusion program, JAERI is pursuing scientific feasibility with JT-60, enhancing the capability of tokamak type devices, thus being expected to play a primary role in the R&D of the next step machine. Universities and other national research institutes are expected to carry out basic and original research on various confinement schemes as well as reactor technologies [4,6]. Universities are also responsible for providing young scientists and engineers to the community. A similar sharing of tasks on fusion materials research and development has existed among various sectors: JAERI has been mainly responsible for fundamental and applied research and the development of materials directly applicable to fusion devices. NRIM has been responsible for developing new materials and processes with fundamental research to assist the development. Universities have been mainly involved in fundamental research on the basic factors for materials development and on the materials behaviour under fu-
B.V. (North-Holland)
sion reactor environment. Close communication and cooperation have been maintained among researchers belonging to each sector. Although such a sharing of tasks is still kept among three sectors, a new trend has become grdwingly evident to place more emphasis on basic technology studies. This seems to be a reflection of realistic cognizance on the status of fusion energy development, which would require continuing effort well into the middle of the next century. This trend will be described in the following sections. This trend raises a problem of the need for redefining the share of tasks and resources concerning the basic research among three sectors. Some sort of steering organization is required to better coordinate the national fusion materials program. 2.2. Umversities 2.2.1. Special research project on nuclear fusion (FY 1980-l 989) Before 1970, fusion research in universities was mainly confined to a plasma physics area. University activities on the fusion materials area became active and gradually organized in the 197Os, and the Special Research Project on Nuclear Fusion supported by Monbusho is the first large-scale coordinated joint research among various universities in the fusion technology area including materials. Six major research areas have been supported under this project, namely: (1) reactor materials and plasma-wall interactions; (2) science, technology and biological effects of tritium; (3) fundamentals of reactor plasma control; (4) technology of the superconducting magnet; (5) fusion reactor blanket engineering; and (6) design and evaluation of a fusion reactor. Materials research has been one of the most important elements of this special project. For structural materials, major emphasis has been placed on radiation effects. The problems have been studied from the standpoint of the microstructural evolution under irradiation on the one hand and of alloy development for better mechanical properties after irradiation on the other. Microstrural evolution is a key to clarify irradiation correlation from a fundamental view point; i.e. from an understanding of the basic defect processes in the material under irradiation. To this aim pure metals, pure binary and ternary alloys, model alloys and commercial alloys have been purposely utilized using various irradiation environments as RTNS-II, Octavian, KUR, JMTR, JOY0 and FFTF. Austenitic and ferritic steels, vanadium alloys and developmental low activation alloys have been the major structural materials studied. For fundamental studies of alloy development, microstructural as well as mechanical and other properties measurements are important. The role of alloying elements on these properties and the phase stability under irradiation have been studied. For plasma-wall
interaction studies, sputtering and hydrogen recycling were the major subjects in early stage of the project and then, plasma-surface interactions with low energy hydrogen and those using actual plasma devices were studied. Recently, comprehensive studies on the characteristics of various kinds of graphite have been carried out. Special emphasis has been placed on radiation damage in ceramics for the past four years. The research has been fundamental in nature, obtaining displacement energies for various ceramics materials and studying the effects of electronic excitation on radiation damage. A large amount of interesting results have come out from the research outlined above. The majority of the contributions to the present and the former Conferences of this series from Universities have been supported by the Special Research Project on Fusion and we shall not reiterate the results here. Since the Special Research Project on Nuclear Fusion is approaching its completion in March, 1990. the planning and proposal for the follow-up studies during the coming six years have been made. The research will be on fundamental studies of complex systems of fusion plasma and the blanket. 2.2.2. New research institute at Toki-site The Institute of Plasma Physics, Nagoya University was reorganized and a new institute named the National Institute for Fusion Science has been established since May, 1989 directly under Monbusho. A large helical device (LHD) with a major radius of 4 m and with superconducting helical and poloidal coils will be constructed. Design activities are going on by 120 scientists and engineers. Materials related activities as identifying issues relevant to helical devices and plasma-surface interactions will be initiated soon. 2.2.3. International cooperation Japan/USA Joint Project on Utilization of RTNS-II was completed successfully in 1987, and the post-irradiation experiments have yielded extremely useful data on radiation damage produced by 14 MeV neutrons. In particular, the understanding of the nature of cascade damage produced by high energy recoils has been progressed and the method has been established to analyse the neutron radiation damage in terms of recoil energy spectra. This provides a basis for irradiation correlation. Since 1987, the Japan/USA Collaboration in Fundamental Studies of Irradiation Effects in Fusion Materials Utilizing Fission Reactors (FFTF/MOTA Collaboration) has started as an eight-year project. The basic philosophy is to extend the neutron fluence to a high damage region using fission reactors, always considering the correlation of the results to the case of fusion neutron irradiation as schematically illustrated in fig. 1 [7]. Already some of the early results have come
5
S. Ishino et al. / History, present status and future of fusion reactor materials research in Japan
national or international project of an intense neutron source.
as the implementation
2.3. Japan Atomic Energy Research Institute
2.3. I. Introduction The fusion materials program at JAERI has been carried out in a few department sectors and the phases of research are distributed over the basic to strictly applied. The materials science oriented portions cover the fields of the structural, the plasma-facing high heat tritium breeding and functional flux component, ceramics materials. Among those activities, a large fraction of neutron irradiation relevant works have been carried out in the international collaborations under the US-Japan bilateral and the multilateral IEA agreements. In FY1988, a new plan of the Base Nuclear Materials Program was brought into implementation in the Science and Technology Agency, STA. In JAERI, a strategy was proposed to integrate the disciplinary part of the base materials research to form a matrix with nuclear systems development projects as shown in table 1. The fusion part of the base technology program includes five tasks on materials development and critical studies on new phenomena, two tasks on the irradiation test facility and test technique development and two tasks on computerized database development. The operation of those matrix type research works is in progress.
Fig. 1. Strategy of approaching high fluence irradiation effect in a fusion reactor environment by utilizing existing reactors (ref. [7]).
out from the FFTF/MOTA Collaboration, to be presented in this Conference. It should be mentioned that to establish irradiation correlation the experiments are so designed as to use the same specimens in different irradiation environments as RTNS-II, FFTF as well as domestic facilities as JMTR, JOY0 and accelerators, that the effect of spectrum, dose, dose rate and other irradiation parameters can be studied less ambiguously. 2.2.4. Future program A future program will be grouped into four categories: (1) follow-up studies of the Special Research Project on Nuclear Fusion, which are the coordinated studies by individual research groups; (2) Joint research at the FFTF/MOTA or JOY0 utilization, involving the increased capacity of hot cells; (3) Collaboration with large plasma devices; and (4) Taking part in a large
Table 1 Strategic matrix
of fusion material
Base technology
program
Database
‘) Committments
0:
2.3.2.1. Structural materials. R&D programs are going on in a modified stainless steel as the candidate material for the next generation machine and in other stainless steels, with regard to the control of minor elements and to the welding technique
at JAERI Fusion materials
program
‘)
Structural
Plasma
facing
T-breeding
Functional 0 0 0 0 _
0 0
0 0 _
ESNIT/MODULAB Small specimen (SSTT)
0 0
0 _
0 _
0 0
Performance (JMPD) Character (NETWORK)
0 0
0 0
0 0
0 0
I Low activation Radiation corrosion
Facility and methodology
2.3.2. Current research activities
0 0 _ _ _
Design and innovation Irradiation resistance Corrosion resistance
Materials and phenomena
research
direct,
0:
indirect.
(JAERI)
0 0 0
0 0
(SPM)
which is indispensable as the structural material. As the evaluation study of these materials, irradiation testing including spectral and isotopic tailoring experiments are in progress through the Japan/US collaboration using HFlR/ORR. Post-irradiation mechanical properties and dimensional changes are being analysed and evaluated. Recently, new testing programs on irradiation corrosion and on welding of irradiated materials have been initiated. R&D of low activation materials is focused on ferritic steels and ceramic materials. To obtain better materials, alloy design methodologies are being applied and new thermomechanical treatment procedures are being devised. Basic studies on composite materials and on the development of functionalgradient materials as stress free materials are being performed. 2.3.2.2. Plasma-facing and high heat flux component materials. Fundamental data on the chemical sputtering of carbon and graphite have been compiled. Performance data have been obtained for these materials in an integrity evaluation testing using JT-60 and JFT-2M. In the near future, development evaluation testing will be carried out on the composite materials with composition gradient, which is resistant to chemical sputtering to be used in a commercial reactor first wall without armour. Integrity of armour and metallic structural materials under high heat flux encountered in plasma disruption and arcing has been evaluated using an electron beam irradiation facility and neutral beam injection facility for JT-60 (PBEF; now a dedicated facility for high flux testing). Data have been compiled for critical heat flux for melting and the integrity has been evaluated in some candidate materials for the divertor and limiter such as MO, W, Cu and its composite materials, graphite and C-C composites. Particularly in MO, the formation of cracks in the solidified layer and of voids can be suppressed by using the single crystal. Disruption characteristics have been evaluated in ferritic, dual phase and austenitic stainless steels. In austenitic alloys, the role of minor elements on crack resistance and the shape of the resolidified surface has been defined. Evaluation testing will be extended to use the specimen with an actual component shape and to study mechanical properties including fatigue characteristics after high heat loading. 2.3.2.3. Tritium breeding materials. The radiation damage of tritium breeding materials is important because it affects the tritium recovery characteristics and their durability. Radiation induced defects have been identified and their recovery kinetics have been elucidated by ion irradiation experiments using an accelerator. The ionic conductivity of lithium oxide depends on the mobility of Li ions. which determines the transport properties. The in-situ measurement of ionic conductiv-
ity has been carried out during and after irradiation. Evaluation of tritium recovery characteristics is important from the standpoint of selection of breeding materials and tritium inventory. In-situ measurements of tritium release have been performed for various candidate breeding materials using the JRR-2 reactor. The experiment is called “VOM”. the special features of which are that tritium release from two different capsules can be measured comparatively at the same time and that the effect of piping can be minimized. The effect of irradiation on tritium release under simulated blanket conditions will be studied by the international collaborative experiment. BEATRIX-II using the FFTF reactor. Experiments are also going on to establish mass production technology of breeding materials. 2.3.2.4. Special purpose materials and ceramics. High performance solid electrolyte of ytterbium-stabilized zirconia (ZrO,-Yb,O,) has been developed. This can be operated at 600°C for the tritium steam electrolysis in a fuel gas purification process, now being widely applied in the world to high efficiency electrolytic devices to treat tritium-containing water. In the field of electric insulation materials. Si,N, with MgO as a sintering aid has been developed for a high strength rf window. Currently, studies are oriented to the in-situ electrical properties and optical property degradation in a radiation environment. Fluoride-glass materials containing BeFz are the potential radiation resistant light conductive materials. By adding BeFz AlF,-based glass with excellent mechanical properties and high vitrification stability can be produced. Future trend are to create high performance new materials by computer simulation such as applying molecular dynamics method. 2.3.2.5. Database. The JMPD (JAERI Material Performance Database) has been developed since FY1985. This is a database comprising data on the development of materials for reactor and nuclear installations, materials characteristics necessary for performance evaluation, structural design and safety analyses. Taking advantage of this experience, data collection has been initiated in collaboration with universities, and the data items are being extracted particularly for first wall, tritium breeding and plasma-facing component materials. At the moment, it is considered that the data system on a personal computer level may be appropriate from the stand point of flexibility and feasibility. 2.3.2.6. Facility and test methodology development. AS a necessary facility for efficient experimental studies for the creation of materials compatible to a high energy neutron environment, preliminary studies are going on to build the “Energy Selective Neutron Irradiation Test Facility” (ESNIT) and a modular type post irradiation testing facility (MODULAB) and the ancillary facility
S. Ishino et al. / History, present status and future of fusion reactor materials research in Japan
multi-functional cooperative refor multi-purpose, search. The ESNIT is a linear accelerator based neutron source capable of producing a variable spectrum of neutrons, thus providing a means to carry out irradiation experiment with high precision which allows detailed analysis of the results [8]. The MODULAB is a facility with radiation shielding, in which encapsulated experimental modules can be inserted so that various detailed post-irradiation experiments can be performed efficiently. R&D work will continue until FY1992, followed by general evaluation before construction. Development of small specimen testing techniques has been in progress in JAERI as well as in universities and NRIM. Those techniques such as shear punch, microtensile, four-point bending and microcorrosion have been developed. These are based on using a TEM specimen. Development of disc bend, shear punch, microfatigue and microhardness techniques are on the program. To study the effect of transmutation produced helium and hydrogen in metals and ceramics, the irradiation facility capable of doing dual and triple beam irradiations are being planned for construction in FY1992.
2.4. National Research
Institute for Metals (NRIM)
Organization of NRIM has been modified since April 1988. The Second Group, formerly the Division of Reactor Materials, comprises four subgroups with tasks as follows: (1) characterization of radiation damage; (2) development of radiation resistant materials; (3) development of low activation materials; and (4) plasma facing and high heat flux materials. For irradiation studies, light ions such as protons and alpha particles have been mainly used. An irradiation creep experiments using light ions from a compact cyclotron were started from the beginning of 1986. Torsion and tensile types of irradiation creep testings are possible: The stress and temperature dependences of irradiation creep rate were experimentally determined for annealed and cold-worked stainless steel and compared with calculation based on point defect kinetics and dynamically competing mechanisms, namely, SIPN, SIPA, climb and glide enabled by SIPA [91. The effects of helium on the high temperature tensile properties, creep and fatigue strength and on micro structure have been investigated using helium implantation techniques for 316 and JPCA steels. A remarkable improvement of high temperature properties has been obtained for JPCA in which Tic particles are very finely dispersed by appropriate thermomechanical treatment to trap helium effectively within the matrix. Two kinds of low activation materials are being developed: one is a modification of 9Cr-1W and 9Cr3W steels with minor additions of V, Ta and B which
I
resulted in markedly increased creep rupture strength without significant loss of toughness [lo]. The other is a class of materials with much lower retained radioactivity: C fiber/Sic composite with high purity better than 99.98% has been obtained by a chemical vapor infiltration process. The composite with woven carbon yam showed high strength and toughness [ll]. For high heat flux materials in a divertor. a thermal shock test by an electron beam has been conducted on various commercial carbon materials and on synthesized carbon/boron materials. Thermal stress cracking and surface erosion were compared among those materials and relations between resistance to high heat flux damage and physical properties were analysed: the results are reported in the present conference [12]. The irradiation behaviour of molybdenum single crystal produced by secondary recrystallization was investigated under the collaboration with JAERI. DBTT was lowered by single-crystallization but the increase of DBTT by neutron irradiation was slightly larger than that of polycrystals. Small island crystals surviving after secondary recrystallization had a severe effect on ductility and toughness. A new facility called “SUBNANOTRON” for in-situ and dynamic analysis of microstructural aspects of materials under dual-ion irradiations is now under construction. This consists of a 1 MV transmission electron microscope with several analytical tools and two ion accelerators. The status of the facility and preliminary work using a TEM/ion accelerator interface were reported in the Poster Session of this Conference [13].
2.5. Other activities There are many researchers belonging to other research institutes than the above mentioned three sectors, who are actively involved in research in fusion materials or in related areas. Among various such organizations, the following institutes should be listed: Electrotechnical Laboratory (ETL), Government Industrial Research Institute, Nagoya both funded by the Ministry of International Trade and Industry (MITI), National Institute for Research in Inorganic Materials funded by STA, and the Institute of Physical and Chemical Research mostly funded by STA. Power Reactor and Nuclear Fuel Development Corporation (PNC) is operating a 100 MW fast experimental reactor, JOYO, at the Oarai site, which has been used for several years by the university research groups for irradiation studies of fusion reactor materials. It should also be mentioned that industries are taking part in fusion materials development by cooperation with JAERI, NRIM and universities. Such industrial firms include major steel makers and reactor manufacturers. For graphite and other functional materials, various other industries are taking part.
3. Discussion As stated earlier. both facility-oriented short-term research and generic long term research are needed. Research for the major fusion facilities is important because it provides the thrust to the whole fusion research and development activity. In this respect. further progress in plasma confinement and control is required. It has become more and more evident that such a progress is strongly dependent on the choice of plasmafacing component materials. For example. the edge plasma condition is determined by the selection of the plasma-facing component materials and vice versa, the service condition of the plasma-facing component is determined by the edge plasma condition. These interactions among plasma, materials and design are the central problems of plasma-surface interactions. Without stronger and closer collaboration among plasma physicists, materials scientists and facility designers, currently facing problems will not be overcome. In this respect. active participation in the TEXTOR project as w-e11as utilization of Heiiotron-E, JIPP T-II, JFT-2M, TRIAM and other plasma-machines by materials scientists have been promoted. Similarly, participation in the experiments using a large tokamak machine as JT-60 and in reactor design activity such as ITER should be extended. Task group, to identify materials problems related to the Large Helical Device (LHD) wiil be initiated
soon.
For resolving long-term and generic problems such as the irradiation effects of structural materials to a very high fluence. we have at present no intense neutron sources relevant to a D-T fusion reactor. Full utilization of existing irradiation facilities as fast and thermal fission reactors as JOYO, FFTF. JMTR, KUR, HFIR and so on, accelerator based neutron sources, RTNS-II, OKTAVIAN, LAMPF, heavy and/or light ion accelerators, HVEMs has had to be sought for. Various novel techniques to tailor radiation environments or the specimen itself to produce fusion relevant irradiation effects such as spectral or isotopic tailoring, dynamic helium charging experiments, dual ion beam irradiation techniques and so on are being designed or have been applied, providing very useful results. However, it should be emphasized that establishing irradiation effect causality based on a fundamental understanding of radiation damage processes is important, not merely seeking for phenomenological correlation among radiation effects obtained in different irradiation environments. This has been the strategy taken by the Japanese university groups and we believe this will be particularly effective in solving the long-term problems. The discussions given above may apply not only for local Japanese programs but may be common to worldwide fusion materials programs. In this respect, further promotion of international collaboration on the com-
mon programs on materials will be fruitful progress of fusion research and development.
for the
4. Conclusions
(1) The history,
current status and future trends on fusion materials research in universities, JAERI, NRIM and other organizations are reviewed. (2) Japanese program seems to be well balanced between near term and long term issues. However, some sort of organizational mechanism will be required to assure better coordination of the national fusion materials program. among (31 It is pointed out that increased collaboration plasma physicists, materials scientists and reactor designers will be necessary to solve the near term problems. involving irradiation (4) For the long term problems effects, both designing fusion relevant simulation techniques and establishing physics-based irradialion causality are necessary. (5) The Japanese current activities and future trends seem to contain many of the world-wide common problems. Further promotion of international collaboration on these common problems will be fruitful in the future research and development of fusion.
References 1’1 S. Ishino, Radiation
Damage, Nuclear Engineering Monograph. Series 8 (University of Tokyo Press. Tokyo. 1979) p. 195 (in Japanese). to the 121 Report of the Fusion Materials Subcommittee Fusion Council, July 1978 (in Japanese). 131 Report of the Fusion Materials, Irradiation Research Subcommittee to the Fusion Council, November 1984 (in Japanese). of Japan, The Long Term 141 Atomic Energy Commission Program for Nuclear Power I>evelopment and Utilization (June 1987). 151 S. Amelinckx. J. Nucl. Mater. 155-157 (198X) 3. [cl S. Mori. Fusion Engrg. Des. 8 (1989) 9. 171 S. Ishino, Bull. Japan. Inst. Met. 26 (1987) 1044 (in Japanese). 1x1 T. Kondo, H. Ohno, M. Mizumoto and M. Odera, J. Fusion Energy 8 (1989) 229. N. Yamamoto and H. Shiraishi. in these 191 J. Nagakawa. Proceedings (ICFRM-4). J. Nucl. Mater. 179-181 (1991). 1101 F. Abe, T. Noda, H. Araki and S. Nakazawa. in these Proceedings (ICFRM-4). J. Nucl. Mater, 1799181 (1991). 1111 7‘. Noda, H. Araki. F. Abe and M. Okada. in these Proceedings (ICFRM-4), J. Nucl. Mater. 179-181 (1991). 1121 M. Fujitsuka, H. Shinno. T. Tanabe and H. Shiraishi, in these Proceedings (ICFRM-4). J. Nucl. Mater. 1799181 (1991). 1131 K. Furuya, T. Kimoto, T. Noda, H. Shiraishi and M. Okada. presented at ICFRM-4. paper 7-109.