Hiroyasu Tanigawa

Swelling behavior of F82H steel irradiated by triple/dual ion beams

Abstract Irradiations for spallation target vessels and structural materials of fusion reactors were simulated using simultaneous triple/dual ion beams consisting of Fe 3+ , He + and H + ions or Fe 3+ and He + ions at temperatures between 470 and 600 °C to 50 dpa. The swelling of F82H (Fe–8Cr–2W–0.2V–0.04Ta–0.1C) martensitic steel was enhanced by a synergistic effect of displacement damage and the implantation of helium and hydrogen. The maximum swelling of F82H steel was 3.2% at 470 °C under a simulation of structural materials of fusion reactors, and was higher than 1.2%, which applied to a simulation of spallation target vessel. The swelling under a simulation of fusion reactor decreased with increasing irradiation temperature, however the swelling under a simulation of spallation target vessel was again increased at 600 °C by the high helium concentration. From the microstructural analysis of taking account of cavity growth process, the cause of the enhancement of swelling under a simulation of fusion reactor is thought to be gas pressure of hydrogen and helium in cavities during irradiation. The effects of 50% cold-working and carbon implantation on swelling behavior were also examined. The swelling was reduced from 3.2% to 1.4% by 50% cold-working, and to 0.5% by carbon implantation.

Multimodal options for materials research to advance the basis for fusion energy in the ITER era

Well-coordinated international fusion materials research on multiple fundamental feasibility issues can serve an important role during the next ten years. Due to differences in national timelines and fusion device concepts, a parallel-track (multimodal) approach is currently being used for developing fusion energy. An overview is given of the current state-of-the-art of major candidate materials systems for next-step fusion reactors, including a summary of existing knowledge regarding operating temperature and neutron irradiation fluence limits due to high-temperature strength and radiation damage considerations, coolant compatibility information, and current industrial manufacturing capabilities. There are two inter-related overarching objectives of fusion materials research to be performed in the next decade: (1) understanding materials science phenomena in the demanding DT fusion energy environment, and (2) application of this knowledge to develop and qualify materials to provide the basis for next-step facility construction authorization by funding agencies and public safety licensing authorities. The critical issues and prospects for development of high-performance fusion materials are discussed along with recent research results and planned activities of the international materials research community.

Response of reduced activation ferritic steels to high-fluence ion-irradiation

Abstract Effects of high-fluence irradiation in fusion-relevant helium production condition on defect cluster formation and swelling of reduced activation ferritic/martensitic steels (RAFs), JLF-1 (Fe–9Cr–2W–V–Ta) and F82H (Fe–8Cr–2W–V–Ta), have been investigated. Dual-ion (nickel plus helium ions) irradiation using electrostatic accelerators was adopted to simulate fusion neutron environment. The irradiation has been carried out up to a damage level of 100 displacement per atom (dpa) at around 723 K, at the HIT facility in the University of Tokyo. Thin foils for transmission electron microscopy (TEM) were prepared with a focused ion beam (FIB) microsampling system. The system enabled not only the broad cross-sectional TEM observation, but also the detailed study of irradiated microstructure, since unfavorable effects of ferromagnetism of a ferritic steel specimen were completely suppressed with this system by sampling a small volume in interests from the irradiated material.

Effect of specimen size on fatigue properties of reduced activation ferritic/martensitic steels

Abstract Small specimen testing technology (SSTT) and related remote-control testing techniques are indispensable for the effective use of the limited volumes of materials test reactor and proposed intense neutron sources. As a part of this work, a new fatigue test machine with a laser extensometer for hot-cell usage has been developed. Materials used in this work were Japanese reduced activation ferritic/martensitic steel (RAFs), JLF-1 (Fe–9Cr–2W–V–Ta) and its weldment (WM). Correlations between fatigue life characteristics and fracture mechanisms were investigated for full- and mini-sized hourglass type specimens to clarify the effect of specimen size on fatigue properties. These tests revealed that there was not a significant difference in the number of cycles to failure in both specimens, except for the case of very low cycle fatigue.

Charpy Impact Properties of Reduced-Activation Ferritic/Martensitic Steels Irradiated in HFIR up to 20 dpa

ABSTRACT The effects of irradiation up to 20 dpa on the Charpy impact properties of reduced-activation ferritic/martensitic steels (RAFs) were investigated. The ductile-brittle transition temperature (DBTT) of F82H-IEA shifted up to around 323K. TIG weldments of F82H showed a fairly small variation on their impact properties. A finer prior austenite grain size in F82H-IEA after a different heat treatment resulted in a 20K lower DBTT compared to F82H-IEA after the standard heat treatment, and that effect was maintained even after irradiation. Helium effects were investigated utilizing Ni-doped F82H, but no obvious evidence of helium effects was obtained. ORNL9Cr-2WVTa and JLF-1 steels showed smaller DBTT shifts compared to F82H-IEA.

Radiation effects on low cycle fatigue properties of reduced activation ferritic/martensitic steels

The reduced activation ferritic/martensitic steel, RAFs F82H IEA heat has been fatigue-tested at ambient temperature under diametral strain controlled conditions. In order to evaluate the effects of radiation damage and transmutation damage on fatigue characteristics, post-neutron irradiation and post-helium ion implantation fatigue tests were carried out. Fracture surfaces and fatigue crack initiation on the specimen surface were observed by SEM. Low-temperature irradiation caused an increase in stress amplitude and a reduction in fatigue lifetime corresponding to radiation hardening and loss of ductility. Neutron irradiated samples showed brittle fracture surface, and it was significant for large strain tests. On the other hand, helium implantation caused delay of cyclic softening. However, brittle crack initiation and propagation did not depend on the helium concentration profiles.

R&D of a Li2TiO3 pebble bed for a test blanket module in JAEA

At JAEA, a test blanket module (TBM) with a water-cooled solid breeder is being developed. This paper presents recent achievements of research activities for the TBM, particularly addressing the pebble bed of the tritium breeder materials and tritium behaviour. For the breeder material, the chemical stability of Li2TiO3 was improved using Li2O additives. To analyse the pebble bed behaviour, thermomechanical properties of the Li2TiO3 pebble bed were assessed experimentally. To verify the pebble beds nuclear properties, the activation foil method was proposed and a preliminary experiment was conducted. To reduce the tritium permeation, the chemical densified coating method was developed and the coating was attached to F82H steel. For tritium behaviour, the tritium recovery system was modified in consideration of the design change of the TBM.

Deformation Microstructure of a Reduced-Activation Ferritic/Martensitic Steel Irradiated in HFIR

Abstract In order to determine the contributions of different microstructural features to strength and to deformation mode, microstructure of deformed flat tensile specimens of irradiated reduced activation F82H (IEA heat) base metal (BM) and its tungsten inert-gas (TIG) weldments (weld metal and weld joint) were investigated by transmission electron microscopy (TEM), following fracture surface examination by scanning electron microscopy (SEM). After irradiation, the fracture surfaces of F82H BM and TIG weldment showed a martensitic mixed quasi-cleavage and ductile-dimple fracture. The microstructure of the deformed region of irradiated F82H BM contained dislocation channels. This suggests that dislocation channeling could be the dominant deformation mechanism in this steel, resulting in the loss of strain-hardening capacity. While, the necked region of the irradiated F82H TIG, where showed less hardening than F82H BM, showed deformation bands only. From these results, it is suggested that the pre-irradiation microstructure, especially the dislocation density, could affect the post-irradiation deformation mode.

Void swelling in reduced activation ferritic/martensitic steels under ion-beam irradiation to high fluences

Abstract Swelling behavior of a 9Cr–2.0W–V,Ta reduced activation ferritic/martensitic steel (JLF-1) and a tungsten-enriched 9Cr–2.5W–V,Ta steel (JLS-1) under ion-beam irradiation was studied. A technique of dual-beam ion irradiation utilizing a pair of electrostatic accelerators was employed to achieve high fluence levels and a helium production rate that are relevant to fusion power reactor blanket environments. Under single ion-beam irradiation at 743 K, a cavity structure was formed only in regions of more than 40 dpa. Cavities were in the martensite lath structure. Cavities did not form at the displacement peak about 1400 nm. Under dual ion-beam irradiation at 743 K, a cavity structure was formed at dual ion-beam irradiated regions. A bi-modal size distribution of faceted voids and spherical helium bubbles of diameter up to 2 nm were observed.

Effects of helium implantation on hardness of pure iron and a reduced activation ferritic–martensitic steel

Helium was implanted into high purity Fe and F82H at room temperature up to 2000 appm to investigate helium effects on hardening. Ultra micro-indentation tests were performed on the specimens before and after helium implantation with loads that penetrate in 300 nm depth. After the indentation tests, the specimens were prepared with a focused ion beam (FIB) processing system for transmission electron microscopy (TEM) of the deformed regions. Results of the indentation tests indicated clearly that helium implantation caused hardening for both pure Fe and F82H. For pure Fe, it was also observed by TEM that the propagation of the plastic deformation zone formed during the indentation was limited to the helium-implanted layer, ranging from 600 to 800 nm from the incident surface.