Japanese Fusion Materials Development Path to DEMO

This paper reports Japanese strategy for developing blanket structural materials for DEMO. In the Japanese program, the candidate materials are categorized into Primary Option (RAFM) and Advanced Options (V-alloy, SiC/SiC, ODS-RAFM etc.). A staged development is planned corresponding to the three decision-making points (DPs), DP1: Intermediate check and review (C&R), DP2: Decision of transition of research and development (R&D) focus to DEMO, and DP3: Decision of DEMO construction. The near-term D-Li neutron source (A-FNS) and IFMIF are regarded as key facilities for the development. The strategy emphasizes “standardization” as an important step toward DEMO design qualification and licensing. The procedure to standard materials specifications by way of establishing structural design criteria and materials property requirements, and the procedure’s interaction with the schedule of irradiation data acquisition are discussed

Effects of Isotopically Controlled Boron Addition on Microstructure of Nickel Irradiated at the Below Core Canister of FFTF

Nickel specimens doped with several levels of ^B were irradiated in the Fast Flux Test Facility(FFTF) for the purpose of examining the applicability of boron addition to the study of helium production effects. It was found that the boron has its chemical and transmutative effetcs, the former suppressing and the latter enhancing void nucleation. It seems that reliable estimation of helium effects is possible by well designed experiments seprating some side effects of boron from its transmutation effects

Creep mechanism of highly purified V-4Cr-4Ti alloys during thermal creep in a vacuum

Pressurized thermal creep tubes of highly purified V-4Cr-4Ti, the NIFS-Heat2 alloy have been examined following testing in the range 700 to 850°C. It was found that the creep stress exponent of the NIFS-Heat2 alloy is about 5 and that the characteristic creep mechanism was the dislocation creep usually observed in pure metals. The apparent activation energy of creep deformation is about 210kJ/mol in the temperature range 700 to 850°C. Creep deformation was considered to be controlled by climb-controlled dislocation glide at 850°C, where sub-grain boundary structure predominates and consists of dislocation dipole structures and pile-ups of dislocations

Overview of Fusion Engineering Research in Japan Focusing on Activities in NIFS and Universities

his paper provides an overview of Japanese fusion engineering research activities focusing on those being carried out by the National Institute for Fusion Science (NIFS) and Japanese universities (Universities). NIFS is promoting the Fusion Engineering Research Project (FERP) as one of three research projects. The majority of the activity in FERP is being carried out by collaboration with Universities. Utilizing the core facilities installed in NIFS and the unique infrastructures of Universities, collaboration between NIFS and Universities is performed for the superconducting magnet, the liquid breeder blanket, advanced materials, high heat flux components, and tritium safety. NIFS also carries out international collaboration programs such as Japan-China–based, Japan-U.S.–based, and International Energy Agency–based collaborations, promoting participation of University researchers. Division of responsibilities with the National Institutes for Quantum and Radiological Science and Technology (QST), contributions to the ITER Broader Approach, and the Action Plan Toward DEMO Development are also reported

Users′ perspective on D-Li neutron sources (A-FNS and IFMIF-DONES) for DEMO and beyond

A-FNS and IFMIF-DONES neutron source program in Japan and in the EU, respectively, are advancing targeting on the start of operation in ∼2030. These facilities will play a central function in realizing DEMO. This paper will discuss the role of the materials scientists, as the users, in A-FNS and IFMIF-DONES programs in order to encourage them to enhance their commitment to the programs. The past collaborations by the materials scientists and the designers in the IFMIF project are introduced first. Then the roles of the neutron sources in the DEMO-roadmap are discussed, for which materials scientists are requested to take leadership. This paper also emphasizes the need for development of advanced materials and for obtaining fundamental understanding on fusion neutron radiation effects by application of the neutron sources

Transformation super plasticity deformation of reduced activation ferritic/martensitic steel

Reduced Activation Ferritic/Martensitic (RAFM) steel is a promising candidate for the blanket structural materials of fusion reactors. One of the key issues in the use of RAFM for the blanket is its low workability. As a solution of this issue, innovative processing technology based on the transformation super plasticity (TSP) was proposed. In general, TSP is known as high temperature creep deformation which is induced by phase transformation. In this study, tensile tests under constant load were carried out with cyclic temperature variation for a RAFM steel to investigate the effect of cyclic temperature variations on elongation. The result of the tensile test under constant load for the RAFM steel with cyclic temperature variations exhibited a macro-elongation to 172%. The results of the tensile tests with varying loads showed the possibility of reducing the necessary time and enhancing the controllability for the formation of RAFM steel products using TSP

Super-saturated hydrogen effects on radiation damages in tungsten under the high-flux divertor plasma irradiation

Tungsten is a prime candidate as the divertor material of the ITER and DEMO reactors, which would be exposed to unprecedentedly high-flux plasmas as well as neutrons. For a better characterization of radiation damages in the tungsten under the divertor condition, we examine influences of super-saturated hydrogen on vacancies in the tungsten. The present calculations based on density functional theory (DFT) reveal unusual phenomena predicted at a super-saturated hydrogen concentration: (1) strongly enhanced vacancy concentration with the super-saturated hydrogen concentration is predicted by a thermodynamics model assuming multiple-hydrogen trapping, i.e. hydrogen clusters formation, in the vacancies; and (2) DFT molecular dynamics revealed that hydrogen clusters can prevent a vacancy from recombining with the neighboring crowdion-type self-interstitial-atom. This suggests that neutron damage effects will be increased in the presence of the hydrogen clusters

Effect of atmospheric control during MA-HIP process on mechanical properties of oxide dispersion-strengthened Cu alloy

In this study, mechanical properties of Dispersion Strengthened (DS)-Cu-Al (aluminum) and Zr (zirconium) alloys, which were fabricated by an MA-HIP method, were investigated for application to the heat sink materials of fusion reactors. The effect of air exposure before the HIP process was studied using a NIFS-Sealing Device. Cu–Al specimen with air exposure before HIP was broken during wire-electrical discharge formation. Cu–Al specimen without air exposure exhibited high fracture strength, but without yielding. Cu–Zr specimen, both with and without the exposure, yielded and exhibits elongation. An increase in yield and tensile strength by approximately 61% and 45%, respectively, were obtained for Cu–Zr specimen by avoiding air exposure. The results showed that Cu–Zr specimen is less susceptible to the atmosphere of the MA-HIP process than Cu–Al

Moderation of Negative Oxygen Effects by Small Yttrium Addition to Low Activation Vanadium Alloys

In order to improve irradiation embrittlement of vanadium alloys for fusion reactors, yttrium (Y) has been added reducing the interstitial oxygen impurity. However Y addition can also degrade high-temperature strength, because Y could scavenge oxygen in solid solution, which is a strong hardening agent in vanadium alloys. In this study, the effect of Y addition and oxygen level on the mechanical properties was investigated from the view points of both the high-temperature strength and low temperature ductility. Y addition was suggested to moderate the hardening and embrittlement induced by oxygen impurity sustaining the high-temperature strength within an acceptable level

Effect of HIP temperature and cooling rate on microstructure and hardness of joints for ODS-RAFM steels and JLF-1 steel

Dissimilar-metal joints between ODS-RAFM (oxide-dispersion-strengthened reduced activation ferritic/ martensitic) steels and JLF-1 steel were fabricated by hot isostatic pressing (HIP) at 1000 - 1100 °C with a cooling rate of 5 °C/min. After the HIP, it was always quenched martensite for JLF-1 steel. However, coarse precipitates were found in 9Cr-ODS. Additional annealing experiments to simulate HIP conditions were conducted for 9Cr-ODS with cooling rate ranged from 0.5 to 36 °C/min at 800 - 1100 °C. The results showed that, to form quenched martensite for 9Cr-ODS, the HIP temperature should be above 1000 °C with cooling rate no less than 25 °C/min. When the cooling rate is increased to 36 °C/min, the microstructure of 9Cr-ODS is quenched martensite with precipitate size similar as that before HIP. If the limitation of precipitate size in 9Cr-ODS is 0.2 μm, HIP temperature above 1050 °C with cooling rate no less than 30 °C/min is needed. In this case, post-weld heat treatment (PWHT) with only tempering is necessary to recover the microstructure of 9Cr-ODS to tempered martensite. For 12Cr-ODS, the HIP temperature and cooling rate has no effect on hardness and precipitate size. PWHT is not necessary for the single-metal joint of 12Cr-ODS from the view point of precipitation control. However, for the dissimilar-metal joints between ODS-RAFM steels and JLF-1 steel, the PWHT condition should be comprehensively determined by considering microstructural evolution of each part in the joints after HIP