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Journal Articles

Recent advances in modeling and simulation of the exposure and response of tungsten to fusion energy conditions

Marian, J.*; Becquart, C. S.*; Domain, C.*; Dudarev, S. L.*; Gilbert, M. R.*; Kurtz, R. J.*; Mason, D. R.*; Nordlund, K.*; Sand, A. E.*; Snead, L. L.*; et al.

Nuclear Fusion, 57(9), p.092008_1 - 092008_26, 2017/06

 Times Cited Count:101 Percentile:99.27(Physics, Fluids & Plasmas)

Under the anticipated operating conditions for demonstration magnetic fusion reactors beyond ITER, structural materials will be exposed to unprecedented conditions of irradiation, heat flux, and temperature. While such extreme environments remain inaccessible experimentally, computational modeling and simulation can provide qualitative and quantitative insights into materials response and complement the available experimental measurements. For plasma facing components such as the first wall and the divertor, tungsten (W) has been selected as the best candidate material due to its superior high-temperature and irradiation properties. In this paper we provide a review of recent efforts in computational modeling of W both as a plasma-facing material as well as a bulk structural material subjected to fast neutron irradiation. We highlight several of the most salient findings obtained via computational modeling and point out a number of remaining future challenges.

Journal Articles

Physical properties of F82H for fusion blanket design

Hirose, Takanori; Nozawa, Takashi; Stoller, R. E.*; Hamaguchi, Dai; Sakasegawa, Hideo; Tanigawa, Hisashi; Tanigawa, Hiroyasu; Enoeda, Mikio; Kato, Yutai*; Snead, L. L.*

Fusion Engineering and Design, 89(7-8), p.1595 - 1599, 2014/10

 Times Cited Count:41 Percentile:96.55(Nuclear Science & Technology)

The material properties, focusing on the properties used for design analysis were re-assessed and newly investigated for various heats including F82H-IEA. Moreover, irradiation effects on those properties were studied in this work. As for thermal properties, thermal conductivity that has significant impacts on the thermo-hydraulic properties of the blanket was investigated on several heats of F82H including F82H-IEA. According to the measurements, the thermal conductivity falls in the range 28.3$$pm$$1.1 W/m/K at 293 K. Although this is comparable with that of the other ferritic/martensitic steels, it is 20% lower than the published value for F82H-IEA. The re-assessment on the published value revealed that the thermal diffusivity was over-estimated. As for irradiation effects on the physical properties, electric resistivity was measured after irradiation up to 6 dpa at 573 K and 673 K. The reduction of resistivity in F82H and its welds were 3% and 6%, respectively.

Journal Articles

R&D and irradiation plans for new nuclear grade graphites for application to VHTR

Takizawa, Kentaro*; Kakehashi, Kazuyuki*; Fukuda, Toshiaki*; Kida, Toru*; Sawa, Kazuhiro; Sumita, Junya; Kato, Yutai*; Snead, L. L.*

Ceramic Materials for Energy Applications; Ceramic Engineering and Science Proceedings, Vol.32, No.9, p.13 - 19, 2011/11

Fine-grained isotropic graphite shows high strength making it a promising material for the graphite component of High Temperature Gas-cooled Reactor (HTGR) and Very High Temperature Reactor (VHTR). Service life of the graphite component is determined primarily by the residual strength after neutron irradiation in the reactor core. It is expected that development of a new nuclear grade graphite possessing higher strength will contribute toward added design margins and an extension of the service life of components, which likely improve the reactor economy very significantly. Tokai Carbon Co. LTD. has started the development of nuclear grade graphite for the graphite component of VHTR. G347S and G458S grades are fine-grained isotropic graphites having high tensile strength greater than 30 MPa. It is planned to carry out the neutron irradiation tests using High Flux Isotope Reactor at Oak Ridge National Laboratory up to the neutron fluence of 30 dpa and the irradiation temperatures of 300-900$$^{circ}$$C. The dimensional changes, elastic modulus, coefficient of thermal expansion, etc., will be studied. It is also planned to evaluate the non-irradiated mechanical/thermal properties and the irradiation effects in collaboration with Japan Atomic Energy Agency. This paper introduces our technical R&D plan for G347S and G458S. The initial results of the properties and the irradiation test plan are also shown.

Journal Articles

Recent advances and issues in development of silicon carbide composites for fusion applications

Nozawa, Takashi; Hinoki, Tatsuya*; Hasegawa, Akira*; Koyama, Akira*; Kato, Yutai*; Snead, L. L.*; Henager, C. H. Jr.*; Hegeman, J. B. J.*

Journal of Nuclear Materials, 386-388, p.622 - 627, 2009/04

 Times Cited Count:112 Percentile:99.19(Materials Science, Multidisciplinary)

A new class SiC/SiC composite have been developed for fusion. While the development efforts has resulted in a radiation-resistant SiC/SiC, development continues to improve on engineering properties of this composite. With the completion of the "proof-of-principal" phase, the R&D on SiC/SiC is now shifting to the more pragmatic phase of material data-basing. Critical application issues still remain, including (1) heavy irradiation effect with considerations of He/H synergistic effects and irradiation creep, (2) erosion-corrosion behavior, (3) nuclear transmutation other than He/H, and (4) joining. Understanding the irradiation effect on electrical conductivity is another important issue for the FCI application. Along with the review in material development, characterization, and irradiation effect studies, this paper provides an introductive work toward standardization of the test methodology.

Journal Articles

Effect of Al and Be ions pre-implantation on formation and growth of helium bubbles in SiC/SiC composites

Taguchi, Tomitsugu; Igawa, Naoki; Wakai, Eiichi; Jitsukawa, Shiro; Snead, L. L.*; Hasegawa, Akira*

Nuclear Instruments and Methods in Physics Research B, 256(2), p.669 - 674, 2007/03

 Times Cited Count:1 Percentile:14.19(Instruments & Instrumentation)

The effect of Al and Be ions pre-implantation on microstructural change and, the formation and growth of He bubbles in SiC/SiC composite was investigated. Four kinds of ion implanted specimens were prepared with 100 appm Al, 1000 appm Al, 100 appm Be and 1000 appm Be implanted. No microstructural change was observed in the matrices and fibers of SiC/SiC composites implanted with Al or Be ions up to 1000 appm. The un-implanted and Al or Be pre-implanted SiC/SiC composites were simultaneously irradiated to 10 dpa using triple ion-beams (6.0-MeV Si$$^{2+}$$, 1.0-MeV He$$^{+}$$ and 340-keV H$$^{+}$$) at 1000 $$^{circ}$$C. Helium bubbles were formed in every matrix and fiber irradiated by triple ion-beams. The size of He bubbles in the matrix was increased by implanting Al or Be ions and increased with increasing amount of implanted Al or Be ions. The size of He bubbles in the fiber was slightly increased by implanting Al or Be ions. These results suggest that Al or Be as transmutation products and impurities may accelerate the growth of He bubbles in SiC/SiC composites under fusion reactor conditions.

Journal Articles

Fabrication of SiC fiber reinforced SiC composite by chemical vapor infiltration for excellent mechanical properties

Igawa, Naoki; Taguchi, Tomitsugu; Nozawa, Takashi*; Snead, L. L.*; Hinoki, Tatsuya*; McLaughlin, J. C.*; Kato, Yutai*; Jitsukawa, Shiro; Koyama, Akira*

Journal of Physics and Chemistry of Solids, 66(2-4), p.551 - 554, 2005/02

 Times Cited Count:45 Percentile:82.52(Chemistry, Multidisciplinary)

Silicon carbide is an important engineering ceramic because of its high strength and stability at high temperature and low induced radioactivity after neutron irradiation. Though monolithic SiC is brittle and low toughness, SiC fiber reinforced SiC matrix composites significantly improve these properties and therefore are attractive candidate materials for fusion reactor structural applications. Recently, stoichiometric SiC fibers with superior mechanical properties have been produced. We carried out the optimization of interface and composite fabrication using the chemical vapor infiltration, which is the one of the best techniques to fabricate the SiC composite. The composite with higher density and homogeneous matrix was obtained by the optimization of materials and carrier gas flow rate. The porosity was decreased with increasing the fiber volume fraction. We adopted the carbon or carbon/SiC interface between fiber and matrix and we found that the dependence of interface thickness on the tensile properties was small in the interface thickness from 50 to 300 nm.

Journal Articles

Synergistic effects of implanted helium and hydrogen and the effect of irradiation temperature on the microstructure of SiC/SiC composites

Taguchi, Tomitsugu; Igawa, Naoki; Miwa, Shuhei*; Wakai, Eiichi; Jitsukawa, Shiro; Snead, L. L.*; Hasegawa, Akira*

Journal of Nuclear Materials, 335(3), p.508 - 514, 2004/12

 Times Cited Count:35 Percentile:89.27(Materials Science, Multidisciplinary)

The microstructure of near-stoichiometric fiber SiC/SiC composites implanted with He and H ions was studied at implantation temperatures of 1000 and 1300 $$^{circ}$$C. The average size of He bubbles in the CVI SiC matrix decreases with increasing concentration of implanted H ions. Moreover, the number density of He bubbles increases with increasing irradiation temperature and amount of implanted H. At the irradiation temperature of 1000 $$^{circ}$$C, He bubbles were mainly formed at grain boundary within the matrix. On the other hand, He bubbles were formed both at grain boundaries and within grains at the irradiation temperature of 1300 $$^{circ}$$C. The average size of He bubbles at grain boundaries was much larger than within the grain. The average size of He bubbles in the fiber was smaller than that in the matrix in all cases.

Journal Articles

Fabrication of advanced SiC fiber/F-CVI SiC matrix composites with SiC/C multi-layer interphase

Taguchi, Tomitsugu; Nozawa, Takashi*; Igawa, Naoki; Kato, Yutai*; Jitsukawa, Shiro; Koyama, Akira*; Hinoki, Tatsuya*; Snead, L. L.*

Journal of Nuclear Materials, 329-333(Part1), p.572 - 576, 2004/08

 Times Cited Count:46 Percentile:92.93(Materials Science, Multidisciplinary)

The SiC/SiC composite with SiC/C multi-layer interphase coated on advanced SiC fibers was fabricated by the forced thermal-gradient chemical vapor infiltration (F-CVI) process for improvement in mechanical properties. The SEM and TEM observation verified that SiC/C multi-layer interphase was formed on SiC fibers. The both flexural and tensile strengths of SiC/SiC composite with SiC/C multi-layer interphase were approximately 10 % higher than that with single carbon interphase. The SEM observation on the fracture surface of the composite with SiC/C multi-layer reveals that cylindrical steps around the fiber were formed. The several crack deflections occurred within SiC/C multi-layer interphase. The SiC/C multi-layer applied in this study operated efficiently to improve the mechanical properties.

Journal Articles

Effect of simultaneous ion irradiation on microstructural change of SiC/SiC composites at high temperature

Taguchi, Tomitsugu; Wakai, Eiichi; Igawa, Naoki; Nogami, Shuhei*; Snead, L. L.*; Hasegawa, Akira*; Jitsukawa, Shiro

Journal of Nuclear Materials, 307-311(Part2), p.1135 - 1140, 2002/12

 Times Cited Count:16 Percentile:71.55(Materials Science, Multidisciplinary)

no abstracts in English

Journal Articles

Optimizing the fabrication process for superior mechanical properties in the FCVI SiC matrix/stoichiometric SiC fiber composite system

Igawa, Naoki; Taguchi, Tomitsugu; Snead, L. L.*; Kato, Yudai*; Jitsukawa, Shiro; Koyama, Akira*; McLaughlin, J. C.*

Journal of Nuclear Materials, 307-311(Part2), p.1205 - 1209, 2002/12

 Times Cited Count:16 Percentile:69.89(Materials Science, Multidisciplinary)

no abstracts in English

Journal Articles

Optimizing the fabrication process for excellent mechanical properties in stoichiometric SiC fiber/FCVI SiC matrix composites

Taguchi, Tomitsugu; Igawa, Naoki; Jitsukawa, Shiro; Nozawa, Takashi*; Kato, Yudai*; Koyama, Akira*; Snead, L. L.*; McLaughlin, J. C.*

Advanced SiC/SiC Ceramic Composites: Developments and Applications in Energy Systems; Ceramic Transactions Vol. 144, p.69 - 76, 2002/00

Process optimization for Forced-thermal gradient Chemical Vapor Infiltration (FCVI) fabrication of 75 mm diameter size SiC composites with advanced SiC fibers; Hi-Nicalon Type S and Tyranno SA, was carried out. The SiC/SiC composites fabricated by FCVI exhibited significant reduction in porosity (15.1%) and more uniform pore distribution by decreasing the MTS and H$$_{2}$$ gases flow rates in the latter part of the FCVI process. The tensile strength of the both composites using Hi-Nicalon Type S or Tyranno SA fibers was slightly increased with increased thickness of carbon interphase in the range of 75-300 nm. In order to perform the comparative testing required to directly compare the thermomechanical property changes following neutron irradiation, larger composites with uniform microstructural property are required. From the results of process optimization for fabrication of 75 mm diameter size FCVI SiC/SiC composites, the definitive purpose in this study is the fabrication of the 300 mm diameter size SiC/SiC composite with the uniform microstructural properties.

Journal Articles

In-situ measurement of thermal conductivity of ceramics and ceramic matrix composites; TRIST-TC1 experiment

Yamada, Reiji; Snead, L. L.*; Kato, Yudai*

Proceedings of 4th International Energy Agency Workshop on SiCf/SiC Ceramic Composites for Fusion Structural Application, p.175 - 180, 2000/00

no abstracts in English

Oral presentation

Design and evaluation of fiber/matrix interface for advanced SiC/SiC composites

Nozawa, Takashi; Kato, Yutai*; Snead, L. L.*

no journal, , 

This study aims to identify the effect of neutron irradiation on the fiber/matrix interface of advanced SiC/SiC composites. A single fiber push-out test was applied to evaluate the shear properties at the interface. The results indicate that the interfacial shear strength depends significantly on the irradiation temperature and neutron dose up to 7.7 dpa at $$<$$1080$$^{circ}$$C. It is worth noting that no major deterioration of the bulk strength of the composites is expected by the slight decrease of interfacial shear properties. This paper will report the general guideline to design irradiation resistant fiber/matrix interface and critical issues for further development will be discussed.

Oral presentation

Interfacial property evaluation of SiC/SiC model composites

Ozawa, Kazumi; Nozawa, Takashi; Tanigawa, Hiroyasu; Kato, Yutai*; Snead, L. L.*

no journal, , 

An advanced SiC/SiC composite is a candidate material for fusion DEMO reactor. Four types of model unidirectional SiC/SiC composites (minicomposites) with variations in fiber (Hi-Nicalon Type-S (HNLS), Tyranno-SA3, Sylramic and Sylramic-iBN) by chemical vapor infiltration method were evaluated for tensile and fiber/matrix interfacial properties. The ultimate tensile strength (UTS) increased with the interfacial sliding stress ($$tau$$) obtained by the hysteresis loop analysis for Tyranno-SA3, Sylramic and Sylramic-iBN. However, this was not the case for the HNLS composite. The composite achieved high UTS due to very low $$tau$$, arising from the larger residual radial tensile stress and smooth fiber surface. In contrast, the fracture behavior of the other composites may have been strongly affected by the clamping stress produced by the relatively rough fiber surfaces.

Oral presentation

Evaluation of damage tolerance of neutron-irradiated SiC/SiC composites

Ozawa, Kazumi; Kato, Yutai*; Nozawa, Takashi; Snead, L. L.*

no journal, , 

Advanced SiC/SiC composites are candidates for the advanced blanket material for fusion DEMO reactor. In order to investigate the effects of neutron irradiation on fracture resistance, the composites after neutron irradiation to $$sim$$5.9 $$times$$ 10$$^{25}$$ n/m$$^{2}$$ (${it E}$ $$>$$ 0.1 MeV) at 800 and 1300$$^{circ}$$C were evaluated by three-point single edge notched bend miniature test. Taking all results into consideration for the global energy balance analysis based on the non-linear fracture mechanics, the previous tensile tests, and the hysteresis loop analysis for interfacial property evaluation, it was concluded that the effects of neutron irradiation in conditions studied on fracture resistance of the composites appeared insignificant.

Oral presentation

Strength and interfacial properties of single fiber-tow CVI SiC/SiC minicomposites

Ozawa, Kazumi; Kato, Yutai*; Nozawa, Takashi; Tanigawa, Hiroyasu; Snead, L. L.*

no journal, , 

An advanced SiC/SiC composite is very attractive as an advanced blanket material for fusion DEMO reactor. In this study, tensile and fiber/matrix interfacial properties of unirradiated SiC/SiC minicomposites were evaluated in order to examine the effects of difference of fiber and interphase thickness. The experiments revealed that fiber surface roughness directly correlates with interfacial sliding stress and that the relationship can affects ultimate tensile stress. Additionally the dependence of interphase thickness on tensile and interfacial strength was also confirmed.

Oral presentation

Tensile and interfacial properties of unidirectional advanced SiC/SiC minicomposites

Ozawa, Kazumi; Nozawa, Takashi; Tanigawa, Hiroyasu; Kato, Yutai*; Snead, L. L.*

no journal, , 

A silicon carbide (SiC) matrix composite is a promising candidate for nuclear fusion energy applications. Unloading-reloading cyclic tensile tests were conducted to estimate interfacial properties for unidirectional SiC/SiC minicomposites reinforced by Hi-Nicalon Type-S (HNLS) or Tyranno-SA3 SiC fibers via CVI process were conducted. The interfacial properties were also evaluated by fiber push-out test. According to these results, it is implied that both fiber surface roughness and interfacial layter thickness can impact the tensile and interfacial properties.

Oral presentation

Changes of microstructure and mechanical properties of Hi-Nicalon Type-S SiC composites irradiated to 100 dpa

Ozawa, Kazumi; Koyanagi, Takaaki*; Nozawa, Takashi; Kato, Yutai*; Kondo, Sosuke*; Tanigawa, Hiroyasu; Snead, L. L.*

no journal, , 

A silicon carbide fiber-reinforced silicon carbide matrix (SiC/SiC) composite is a promising candidate material for an advanced fusion DEMO blanket. High-dose irradiation experiments were performed with our special focuses on understanding; (1) integrity of the Hi-Nicalon Type-S (HNLS) composites, (2) functionality of thin pyrocarbon (PyC) /SiC multilayer, and (3) clarifying the mechanism underlying degradation, as feedback to R&D on SiC/SiC composites. The materials used in this study were plain-weave HNLS composites produced via the chemical vapor infiltration process. Neutron irradiation was conducted in the HFIR at ORNL. The peak neutron fluence was ~1.0$$times$$10$$^{26}$$ n/m$$^{2}$$ (E $$>$$ 0.1 MeV, equivalent to ~100 dpa) at nominal irradiation temperatures of 300, 500, and 800$$^{circ}$$C. Results of post irradiation experiments including 1/4-four-point flexural tests, SEM, and TEM observation were reported.

Oral presentation

Mechanical properties and microstructures of nuclear-grade SiC/SiC composites after high dose irradiation

Ozawa, Kazumi; Koyanagi, Takaaki*; Nozawa, Takashi; Kato, Yutai*; Kondo, Sosuke*; Tanigawa, Hiroyasu; Snead, L. L.*

no journal, , 

A silicon carbide fiber-reinforced silicon carbide matrix (SiC/SiC) composite is a promising candidate material for an advanced fusion DEMO blanket because of the excellent thermo-mechanical / -chemical properties and irradiation tolerance of SiC itself. The irradiation response of highly-crystalline and near-stoichiometric SiC fiber has been assumed to be the same as that of high purity monolithic $$beta$$-SiC. Unfortunately, based on recent data this assumption appears not to be correct. This study mainly aims to investigate mechanical and microstructural changes of the SiC/SiC composite after neutron irradiation to higher dose. In the post neutron irradiation experiments to 100 dpa, ~50% degradation of proportional limit stress and ultimate flexural strength was observed for the composite irradiated at 629$$^{circ}$$C, while the one irradiated at 319$$^{circ}$$C exhibited brittle behavior. In the ion-experiment results, shrinkage of the HNLS fiber was observed for specimens irradiated at 300 and 600$$^{circ}$$C to 100 dpa. The FE-TEM results, EELS analysis and previous our works suggest that the shrinkage would be responsible for the transport of excess carbon atoms from intergranular phase into SiC grains.

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