K. Fukaya

Low cycle fatigue properties of 8Cr–2WVTa ferritic steel at elevated temperatures

The effects of test temperature and tension holding on the fatigue properties of reduced activation ferritic/martensitic 8Cr–2WVTa (F-82H) steel were investigated by conducting low cycle fatigue tests at temperatures ranging from RT to 650°C under the axial strain-controlled condition with strains ranging from 0.5% to 2.0%. Fatigue life data were formulated as strain-life equations. A large reduction in the fatigue life was recognized at temperatures above 600°C. Softening without showing a saturated region was observed in the fatigue softening curves at temperatures above 600°C. Tension holding during fatigue tests reduced the fatigue life at 400°C, 500°C and 600°C. The microstructural examination showed that the large softening during cycle was associated with carbide (M23C6) coarsening and Laves phase (Fe2W) precipitation at 600°C.

Evaluation of toughness degradation by small punch (SP) tests for neutron-irradiated 214Cr-1Mo steel

Mechanical properties correlations between the small punch (SP) test and conventional tensile, Charpy impact and fracture toughness tests were investigated on neutron irradiated 214Cr-1Mo ferritic steel. Estimation of radiation induced changes on tensile strength, elastic-plastic fracture toughness (JIC) and ductile-to-brittle transition temperature (DBTT) are thought to be basically possible by mechanical properties correlations when based on sufficient pre-irradiation data.

RADIATION AND ANNEAL HARDENING IN VANADIUM.

Abstract The tensile properties at room temperature in vanadium irradiated to a fast neutron fluence of 8.2 × 10 19 n/cm 2 at about 200 °C were investigated in terms of changes in microstructure with the irradiation and subsequent heat treatment. Dislocation channeling was observed in the deformed specimens that were irradiated and subsequently annealed for 1 h at temperatures of 400 °C or below; the specimens contained small dislocation loops of the order of 10 16 /cm 3 in density. Hardening due to postirradiation annealing was found to occur in the temperature range of 180 to 600 °C. The radiation anneal hardening was considered to arise from interstitial impurity clusters. The radiation-induced hardening recovered almost completely with a l h anneal at 700 °C.

Mechanical properties of HIP bonded W and Cu-alloys joint for plasma facing components

Abstract Hot isostatic pressing (HIP) bonding technology of W (tungsten) and Cu-alloys have been developed to fabricate plasma facing components of the fusion reactor. As regards W and oxygen free high conductivity copper (OFHC-Cu), the highest bonding strength was achieved at the HIP condition of 1273 K ×2 h ×147 MPa. On the other hand, W and dispersion strengthened copper (DS-Cu) were not bonded directly because of tungsten oxide production at the bonding interface. In this study, HIP bonding tests on W and DS-Cu with OFHC-Cu disk and/or Au-foil were performed. Bonding tests with OFHC-Cu disk were successfully bonded and it is shown that thickness of OFHC-Cu disk over 1.0 mm may be needed and the tensile strength are a little higher than that of HIP treated OFHC-Cu. Bonding tests with Au-foil were also performed and successfully bonded. Au-foil lead to an improvement in bonding strength and a lowering of bonding temperature.

Radiation and anneal hardening in neutron-irradiated vanadium

Abstract Vanadium samples were neutron irradiated at the reactor ambient temperature to fluences in the range from 2.0 × 10 7 to 1.0 × 10 20 n/cm 2 ( E n ⩾ 1 MeV ). The radiation hardening measured at the ambient temperature increased linearly with the square root of the neutron fluence, up to a fluence of about 2.5 × 10 19 n/cm 2 , to approximately 25 kg/mm 2 for the highest fluence. The radiation-anneal hardening phenomenon was clearly observed in samples irradiated at a low fluence (2.0 × 10 17 and 1.0 × 10 18 n/cm 2 ) and the hardening was accompanied by changes in the density and size distribution of the radiation-produced defect clusters. The radiation hardening induced during irradiation to 1.0 × 10 20 n/cm 2 recovered monotonically as the annealing temperature increased. Defect clusters invisible in the electron microscope played an important role in the radiation and anneal hardening except when radiation hardening was induced at the highest fluence.

Thermal shock tests on various materials of plasma facing components for FER/ITER

Development of plasma facing components and materials is a key element in the R&D program for the Fusion Experimental Reactor (FER), which has been designed at JAERI, and the International Thermonuclear Experimental Reactor (ITER), which has been designed under international collaboration. In these next-step tokamak devices, the plasma facing components and materials will be exposed to severe heat load and incident particle flux. The concern is especially acute that the extremely high thermal shock due to plasma disruption could cause material fracture. Efforts on developing the first wall and divertor have been energetically undertaken at JAERI. The present paper describes recent experimental and analytical results on thermal shock characteristics of various materials.

High heat flux experiment on B4C-overlaid C/C composites for plasma facing materials of JT-60U

High heat flux experiments (5–40 MW/m 2 , 5 s and 550 MW/m 2 , 5–10 ms) in the JAERI electron beam irradiation stand (JEBIS) have been carried out on three kinds (conversion, CVD and LLPS) of B 4 C-overlaid C/C composites, on which B 4 C is overlaid with a thickness of 100–250 μm. Measurements were made with respect to the weight loss, changes of the surface morphology and of the surface atomic composition, and the surface temperature. As a result of these experiments, it is found that B 4 C layers of all samples have no damages except small weight losses up to 12 MW/m 2 heat loads, which are estimated at the divertor tiles of JT-60U in normal plasma operation, and that the conversion method is the best of the three methods applied in the present tests, since no exfoliation has occurred even under the disruption conditions.

Mechanical properties and damage behavior of non-magnetic high manganese austenitic steels

Abstract Fe–Cr–Mn steels have been considered as materials of structural components for fusion reactor because of their low induced-radio-activity compared with SUS316 stainless steels. It has been expected to develop a non-magnetic steel with a high stability of the austenitic phase and a strong resistance to irradiation environments. For these objectives, a series of the Fe–Cr–Mn steels have been examined by tensile tests and simulation irradiation by electrons. The main alloying compositions of the steels developed are; C:0.02–0.2 wt%, Mn: 15 wt%, Cr: 15–16 wt%, N: 0.2 wt%. These steels were heat-treated at 1323 K for 1 h. The structure of the steels after the heat-treatment was austenite single phase. The yield stress of the steels was 350–450 MPa and the elongation were 55–60%. When the steels of high C and N was electron-irradiated at below 673 K, no voids were nucleated and only small dislocation loops were formed with high density. The austenite phase was also stable during irradiation below 673 K. Thus, newly developed high manganese steels have excellent mechanical proprieties and high irradiation resistance at relatively low temperature.

Quality evaluation of graphites and carbon/carbon composites during production of JT-60U plasma facing materials

Abstract The variations of physical and mechanical properties have been investigated for three grades of isotropic graphites and four grades of carbon/carbon (C/C) composites on the basis of the sample inspection data which have been obtained during production of the first wall and divertor plate materials for JAERI Tokamak-60 Upgrade (JT-60U). The evaluated properties are density, electrical resistivity, coefficient of thermal expansion (CTE), thermal conductivity, bending, tensile and compressive strengths. It is found that the maximum standard deviations normalized by the mean values are 22.7% for the C/C composites and 9.2% for the isotropic graphites. This scatter of the material quality should be considered in the design of the isotropic graphite and C/C composite armor tiles. Correlations between these properties are also observed for several materials.

Experimental study on melting and evaporation of metal exposed to intense hydrogen ion beam

Abstract This report describes experimental studies on melting and evaporation of metals, mainly stainless steels, subjected to high heat flux simulating a plasma disruption in a thermonuclear fusion reactor. The test pieces were heated by an intense hydrogen ion beam. The heated area was about 70 mm in diameter. The peak heat flux on the surface ranged from 68 to 261 MW/m 2 , and the heating duration from 40 to 250 ms. The melting and evaporating process was observed by using a high-speed video camera. The melt layer convected from the center of the piece to the periphery and the thickness of the piece was decreased not only by the evaporation but also by the convection in the melt layer.