Koreyuki Shiba

Embrittlement of reduced-activation ferritic/martensitic steels irradiated in HFIR at 300°C and 400°C

Abstract Miniature tensile and Charpy specimens of four ferritic/martensitic steels were irradiated at 300°C and 400°C in the high flux isotope reactor (HFIR) to a maximum dose of ≈12 dpa. The steels were standard F82H (F82H-Std), a modified F82H (F82H-Mod), ORNL 9Cr–2WVTa, and 9Cr–2WVTa–2Ni, the 9Cr–2WVTa containing 2% Ni to produce helium by (n,α) reactions with thermal neutrons. More helium was produced in the F82H-Std than the F82H-Mod because of the presence of boron. Irradiation embrittlement in the form of an increase in the ductile–brittle transition temperature (ΔDBTT) and a decrease in the upper-shelf energy (USE) occurred for all the steels. The two F82H steels had similar ΔDBTTs after irradiation at 300°C, but after irradiation at 400°C, the ΔDBTT for F82H-Std was less than for F82H-Mod. Under these irradiation conditions, little effect of the extra helium in the F82H-Std could be discerned. Less embrittlement was observed for 9Cr–2WVTa steel irradiated at 400°C than for the two F82H steels. The 9Cr–2WVTa–2Ni steel with ≈115 appm He had a larger ΔDBTT than the 9Cr–2WVTa with ≈5 appm He, indicating a possible helium effect.

Fracture toughness and tensile behavior of ferritic-martensitic steels irradiated at low temperatures

Abstract Disk compact tension and sheet tensile specimens of the ferritic-martensitic steels F82H and Sandvik HT-9 were irradiated in the High Flux Isotope Reactor (HFIR) at 90°C and 250°C to neutron doses of 1.5–2.5 dpa. For both steels, radiation hardening was accompanied by a reduction in strain hardening capacity (SHC). When combined with other literature data it is apparent that severe loss of SHC occurs in F82H for irradiation temperatures below ∼400°C and in HT-9 for irradiation temperatures below ∼250°C. For both alloys, severe loss of SHC does not correlate with brittle behavior during fracture toughness testing.

Effects of low temperature neutron irradiation on deformation behavior of austenitic stainless steels

Abstract Two experiments have been conducted to quantify the effects of neutron irradiation on the deformation and fracture behavior of solution annealed austenitic stainless steels irradiated to doses ranging from 3 to 19 dpa at temperatures from 60 to 400°C. For all alloys, yield strength increases rapidly with dose in the 60–300°C regime. Radiation hardening is accompanied by changes in the flow properties with the appearance of an initial yield drop and a significant reduction in strain hardening capacity. The magnitude of the changes is dependent upon both neutron dose and irradiation temperature, with reductions in strain hardening capacity occurring most rapidly in the range 250–350°C. It is shown that for neutron doses up to about 3 dpa, although the changes in deformation mode reduce the fracture thoughness, the toughness remains satisfactorily high.

Irradiation response on mechanical properties of neutron irradiated F82H

Abstract Tensile and Charpy impact properties of neutron irradiated F82H (Fe8Cr2WVTa) with and without boron have been investigated to obtain the basic irradiation response on mechanical properties in low damage regime less than 1 dpa at the temperature ranging from 300° to 590°C. Boron-doped steel was used for the helium effect due to (n, α) reaction. Typical irradiation hardening was observed at 300°C. The irradiation above 520°C did not reveal increase in yield stress, but the specimen irradiated at 590°C showed some reduction in elongation in room temperature tensile testing. Slight difference in the tensile properties between boron-doped and boron-free were observed at 590°C. No changes in ductile brittle transition temperature (DBTT) occurred at a temperature between 335° and 460°C by Charpy impact testing.

Microstructure of welded and thermal-aged low activation steel F82H IEA heat

Abstract F82H(8Cr–2WVTa steel) IEA heat was used to prepare tungsten-inert-gas (TIG) and electron-beam (EB) weld joints, followed by heat treatment at 720°C for 1 h. Hardening in the weld metal and softening in the heat-affected zone (HAZ) were detected in TIG weld joints. In EB weld joints, hardening in the weld metal was more clearly observed but HAZ softening was hardly observed. Hardness of TIG weld metal was reduced after 550°C thermal-aging, but softening of the base metal was only observed after 650°C thermal-aging. M 23 C 6 phase was the major precipitate in aged base metal and weld joints. The amount of precipitates in aged weld metal was lower than that of normalized and tempered base metal. W-rich Laves phase was also detected in aged weld metal, HAZ and base metal.

Lithium isotope separation by laser

A lithium isotope separation was performed using a laser isotope separation method. It was found that the lithium atoms with a natural isotopic abundance enhanced its6Li concentration up to over 90% by tuning the laser wavelength to the2P1/2 of6Li. Too high power, however, leads to a loss of enrichment due to the power broadening effect which was analysed by the equation of motion of density matrices.

Recent progress in reduced activation ferritic steels R&D in Japan

The Japanese reduced activation ferritic steels (RAFSs) R&D road map towards DEMO is shown. The important steps include high-dose irradiation in fission reactors such as the high flux isotope reactor at Oak Ridge National Laboratory, irradiation tests with 14 MeV neutrons in the International Fusion Materials Irradiation Facility and application to ITER test blanket modules to provide an adequate database of RAFSs for the design of DEMO. The current status of RAFS development is also introduced. The major properties of concern are well-known, and process technologies are mostly ready for fusion application. RAFSs are now certainly ready to proceed to the next stage. A materials database is already in hand, and further progress is anticipated with the design of the ITER test blanket. Oxide dispersion strengthening steels are quite promising for high temperature operation of the blanket system, with potential improvements in radiation resistance and in corrosion resistance.

Chemical form of the solid fission products in (Th, U) O2 simulating high burnup

The chemical form of the solid fission products has been studied for (Th0.81U0.19) O2 simulating 21.5% FIMA in an HTGR environment. Experiments have been performed with X-ray diffraction, electron-probe microanalysis, ceramography and hardness measurement. The results showed that fission-product phases of two types, Mo-Ru-Pd and (Ba, Sr)(Zr, Ce) O3, are present in the simulated fuel pellet. The fuel matrix comprises (Th1-xUx, Zr, Ce, RE) O2 with an x value of 0.067(RE = Nd, La, Pr, Y, Sm). Dissolution of the rare earths (RE) and the residual Zr plus Ce in (Th0.933U0.067) O2 was accompanied by contraction of the unit cell of the oxide matrix. Reaction behavior in the selected fission product system BaO-ZrO2-Nd2O3 was also investigated. The results showed that in the presence of BaO, Nd2Zr2O7 is converted to barium zirconate: at 1630°C, Nd2Zr2O7 + 2 BaO → 2 BaZrO3 + Nd2O3. This fact, combined with thermochemical assessment, confirms the relative stability of (Ba, Sr)(Zr, Ce) O3 against Nd2Zr2O7 in the simulated (Th0.81U0.19) O2. From these results and fission product inventories, it is inferred that the chemical state of high-burnup ThO2 is very similar to that of (Th0.81U0.19) O2.

Microstructural study of irradiated isotopically tailored F82H steel

Abstract The synergistic effect of displacement damage and hydrogen or helium atoms on microstructures in F82H steel irradiated at 250–400 °C to 2.8–51 dpa in HFIR has been examined using isotopes of 54 Fe or 10 B . Hydrogen atoms increased slightly the formation of dislocation loops and changed the Burgers vector for some parts of dislocation loops, and they also affected on the formation of cavity at 250 °C to 2.8 dpa. Helium atoms also influenced them at around 300 °C, and the effect of helium atoms was enhanced at 400 °C. Furthermore, the relations between microstructures and radiation-hardening or ductile to brittle transition temperature (DBTT) shift in F82H steel were discussed. The cause of the shift increase of DBTT is thought to be due to the hardening of dislocation loops and the formation of α′-precipitates on dislocation loops.

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.