Y. Kohno

R&D of low activation ferritic steels for fusion in japanese universities

Abstract Following the brief review of the R&D of low activation ferritic steels in Japanese universities, the status of 9Cr-2W type ferritic steels development is presented. The main emphasis is on mechanical property changes by fast neutron irradiation in FFTF. Bend test, tensile test, CVN test and in-reactor creep results are provided including some data about low activation ferritic steels with Cr variation from 2.25 to 12%. The 9Cr-2W ferritic steel, denoted as JLF-1, showed excellent mechanical properties under fast neutron irradiation as high as 60 dpa. As potential materials for DEMO and beyond, innovative oxide dispersion strengthened (ODS) quasi-amorphous low activation ferritic steels are introduced. The baseline properties, microstructural evolution under ion irradiation and the recent progress of new processes are provided.

Mechanical property changes of low activation ferritic/martensitic steels after neutron irradiation

Mechanical property changes of Fe–XCr–2W–0.2V,Ta (X: 2.25–12) low activation ferritic/martensitic steels including Japanese Low Activation Ferritic/martensitic (JLF) steels and F82H after neutron irradiation were investigated with emphasis on Charpy impact property, tensile property and irradiation creep properties. Dose dependence of ductile-to-brittle transition temperature (DBTT) in JLF-1 (9Cr steel) irradiated at 646–700 K increased with irradiation up to 20 dpa and then decreased with further irradiation showing highest DBTT of 260 K at 20 dpa. F82H showed similar dose dependence in DBTT to JLF-1 with higher transition temperature than that of JLF-1 at the same displacement damage. Yield strength in JLF steels and F82H showed similar dose dependence to that of DBTT. Yield strength increased with irradiation up to 15–20 dpa and then decreased to saturate above about 40 dpa. Irradiation hardening in 7–9%Cr steels (JLF-1, JLF-3, F82H) were observed to be smaller than those in steels with 2.25%Cr (JLF-4) or 12%Cr (JLF-5). Dependences of creep strain on applied hoop stress and neutron fluence were measured to be 1.5 and 1, respectively. Temperature dependence of creep coefficient showed a maximum at about 700 K which was caused by irradiation induced void formation or irradiation enhanced creep deformation. Creep coefficient of F82H was larger than those of JLF steels above 750 K. This was considered to be caused by the differences in N and Ta concentration between F82H and JLF steels.

Production of low activation steel; JLF-1, large heats : Current status and future plan

Abstract Based on the excellent basic properties prior to neutron irradiation and under neutron irradiation up to 100 dpa, the 9Cr–2W type low activation ferritic steel (LAF). JLF-1, had been selected as one of the reference materials to the IEA low activation ferritic steels R&D activity. This paper provides the general information about the second large heat of JLF-1 steel. The basic property of thick plates (24 and 12 mm in thickness) and their welded joints together with the information about microstructure are also provided.

Microstructural evolution of an oxide dispersion strengthened steel under charged particle irradiation

Abstract As a future material for a fusion first wall, ferritic steels have come to appear more attractive than austenitic steels for their higher swelling resistance. To reduce the degradation in mechanical properties at elevated temperatures, which makes ferritic steels inferior to austenitic steels, the application of stable oxide as strengthening dispersoid has been proposed. In this work, Feue5f814Crue5f81Ti steel strengthened with 0.25% Y 2 O 3 (MA957) was dual- and single-ion bombarded at High-fluence Irradiation Test (HIT) facility of the University of Tokyo to 150 dpa at 723 and 923 K. Dual-ion irradiation with 1 appm He/dpa produced only small cavities with average diameter less than 3 nm. In 15 appm He/dpa irradiated samples, small bubbles as well as clustered voids were formed, the latter sited in regions where the titanium content was higher than the matrix. The dislocation structure and dispersion was stable, especially under single-ion irradiation, while recovery was observed in dual-ion irradiated specimens.

New dual-ion irradiation station at the University of Tokyo

Abstract The High-fluence Irradiation Test (HIT) facility of the University of Tokyo, under construction since 1983, was completed in the fall of 1985 with the assembly of the target station for dual-ion irradiation. The HIT facility was composed of two types of accelerators, viz., a Van de Graaff accelerator for light ions and a Tandem accelerator (Tandetron) for heavy ions. The target stations has some new and advantageous features for the measurement of irradiation parameters, e.g., ion-beam profile monitoring. The constitution of the dual-ion irradiation system of HIT is described together with a brief indication of the test results of a dual-ion experiment.

The influence of He/dpa ratio and displacement rate on microstructural evolution: a comparison of theory and experiment☆

Abstract A kinetic model was developed to investigate the influence of the displacement rate and helium generation rate on microstructural evolution in austenitic stainless steels. The model integrates the rate equations describing the evolution of point defects, small point defect clusters, helium-vacancy clusters, and the larger cavity size distribution that is responsible for observable swelling. Cavity (bubble) nucleation is accounted for by the helium-vacancy cluster evolution, while void formation occurs when bubbles grow beyond a critical size in the larger cavity distribution. A series of ion irradiation experiments were used to both calibrate the model and to provide a comparison between model predictions and experimental observations. The experiments involved single and dual-beam irradiations of solution annealed AISI-316 stainless steel at 873 K. The displacement rates were in the range of 2 × 10−3 to 1 × 10−2 dpa/s and the helium-to-dpa ratios were in the range of 0 to 50 appm He/dpa. The maximum displacement dose was 25 dpa. The experiments revealed a significant effect of helium on both the dislocation structure and the cavity distribution. The model predictions of helium effects over a broad range of He/dpa ratios and displacement rates were consistent with experimental observations.

Irradiation response of a reduced activation Fe-8Cr-2W martensitic steel (F82H) after FFTF irradiation☆

Abstract The tungsten-stabilized martensitic steel F82H (Fe-8Cr-2W-0.1C-V,Ta,B) was examined after FFTF irradiation to fluences as high as 37 dpa over the temperature range of 365 to 750°C. TEM microstructural observations showed that the tempered martensitic structure was quite stable below 405°C. At 550°C, slight recovery of martensite lath structure was recognizable, concurrent with the coarsening of carbide particles. Above 670°C, the martensite lath structure was eliminated. Cavity formation was observed only at or below 405°C irradiation. The maximum swelling was measured to be as high as 0.14% at 405°C. Analysis of precipitate extraction replicas showed superior phase stability for F82H during irradiation. Precipitates identified in the irradiated specimens were M23C6 and Laves, the same phases that were identified following thermal aging. Radiation-induced precipitates other than M23C6 or Laves were not observed. Based on these microstructural observations, superior mechanical properties may be expected for F82H after irradiation.

Irradiation creep of low-activation ferritic steels in FFTF/MOTA☆

Abstract Irradiation creep behavior of low-activation steels, developed as structural materials for fusion reactors (JLF series steels), was investigated to obtain a fundamental understanding of these alloys under fast neutron irradiation in FFTF. (2.25–8)Cr(1–2)W bainitic steels and 12Cr-2W ferritic steels showed superior creep resistance to type-316 stainless steels under fast neutron irradiation up to 520°C. At temperatures below 460°C the creep strain increased with increasing Cr content up to 7 Cr, and further increments of Cr content up to 12% reduced the creep strain. At temperatures between 460 and 600°C, 7–8 Cr ferritic steels showed the largest creep strain. Swelling-enhanced creep, near the peak swelling temperature of 410°C, was also observed. The 9Cr-2W ferritic steel JLF-1 presented excellent properties, suggesting it as a leading candidate alloy for structural components of fusion reactors.

Effects of neutron irradiation on microstructural evolution in candidate low activation ferritic steels

Abstract Fe-(2.25-12)Cr-2W-V, Ta low activation ferritic steels (JLF series steels) were developed in the fusion materials development program of Japanese universities. Microstructural observations, including precipitation response, were performed after neutron irradiation in the FFTF/MOTA. The preirradiation microstructure was stable after irradiation at low temperature (

Effect of solute addition on swelling of vanadium after FFTF irradiation

Abstract Effects of solute addition on the swelling of vanadium have been investigated by transmission electron microscopy following neutron irradiation in FFTF to 31.5, 31.9 and 34.2 dpa at 410, 520 and 600°C, respectively. Addition of iron drastically enhances the swelling as much as 160% at 600°C, while molybdenum suppresses the swelling to 0.07%. The present results are interpreted in terms of the atomic size effect of the alloying elements; smaller sized solute atoms yield larger swelling.