Tatsuhiko Tanabe

Thermal shock experiments for carbon materials by electron beams

Abstract Thermal shock tests by electron beams were conducted on various commercial carbon materials including isotropic graphite, pyrolytic carbon coated graphite, carbon fiber composites and on synthesized carbon/boron materials. Thermal stress cracking and surface erosion were compared among the materials, and relations between resistance to high heat flux damage and physical properties were investigated. The synthesized carbon/boron material exhibited higher resistance to thermal stress fracture than most isotropic graphites. A carbon fiber composite (CX-2002U) with high thermal conductivity exhibited higher resistance to surface erosion than isotropic graphite (IG-110U). Resistance to surface erosion increased with increasing sample thickness and decreasing pore size. In pyrolytic carbon coated graphite the incubation period for erosion decreased with increasing coating thickness. The eroded surface of the carbon/boron material was smoother than that of graphite, which showed that erosion by particle emission was suppressed in the carbon/boron material.

Corrosion behavior of Ni-base superalloys at 1373 K in simulated HTGR impure helium gas environment

Abstract The present paper describes the corrosion behavior of recently developed Ni-base superalloys: an oxide dispersion strengthened (ODS) alloy, two kinds of Ni-Cr-W alloys and Hastelloy XR at 1373 K in a simulated HTGR impure helium gas. Corrosion tests were carried out using a transparent quartz retort at 1373 K for up to 3.6 Ms in circulating helium gas which contained a small amount of H2, CH4, CO, CO2 and H2O. The mass change versus exposure time relation revealed that the ODS alloy exhibited excellent oxidation resistance. One Ni-Cr-W alloy (113MA) and Hastelloy XR showed exfoliation of the oxide films after the tests. All the alloys were decarburized from the early exposure times. These results are discussed on the basis of phase stability diagrams for constituent elements in the alloys.

Hydrogen Permeation in Metals During Exposure to a Process Gas Environment

Hydrogen permeation of nickel-base heat-resistant alloys in a process gas environment is investigated in a high-temperature range up to 1273 K. Time-dependent permeation behavior of candidate alloys (R, NSC-1, SZ, KSN, 113M, and Hastelloy XR-51) for intermediate heat exchangers of a high-temperature gas-cooled reactor is examined in a reducing gas of 80% H/sub 2/ + 15% CO + 5% CO/sub 2/. The result in the reducing gas is compared to that of the permeation in pure hydrogen. For both measurements, a helium carrier gas method is used, simulating the practical configuration of the heat exchangers. The permeation rate decreased proportionally to the inverse of the square root of time in the reducing gas and had a square root dependence on hydrogen pressure at a constant thickness of the oxide layer. These results are discussed on the basis of a two-layer diffusion model.

Creep Rupture Properties of Superalloys Developed for Nuclear Steelmaking

Creep rupture tests on six candidate alloys for intermediate heat exchangers of high-temperature gascooled reactors were carried out at 1173 to 1323 K in helium with small amounts of H/sub 2/, CH/sub 4/, CO, and CO/sub 2/, and at 1173 K in H/sub 2/ + 15% CO + 5% CO/sub 2/. The creep rupture strengths of each alloy were scarcely different at 1173 K in both environments. At higher temperatures in helium environments, the degradation of the creep rupture strengths appeared in carbidestrengthened alloys because of decarburization. The alloy, which mainly uses ..cap alpha..-W as a strengthener, showed stable creep rupture strength up to 1323 K in spite of severe decarburization.

Phase transformation in the γ-TiAl alloy induced by Ar ions

Intermetallic γ-TiAl alloy specimens were irradiated with 25 keV Ar ions at room temperature. The defect formation and structural changes were investigated by conventional and high-resolution transmission electron microscopy. The defects observed in low-dose irradiated specimens are planar defects, which may be stacking faults or dislocation loops. A phase transformation was induced in high-dose irradiated specimens. The results suggest that the induced phase has a relation with the planar defects in the early stage of the ion irradiation. The induced phase was found to be of the same structure as the induced phase in Xe-irradiated γ-TiAl. The features of the ion irradiation induced structure changes are compared with reported results. The mechanism of the phase transformation is discussed.

High heat load test on tungsten and tungsten containing alloys

Abstract In view of the recent interest in high Z materials as plasma facing materials, high heat load tests were performed on tungsten and tungsten containing alloys such as WRe and TaW. The dimensions of the specimens were 20 mm Φ × 1 – 10 mm thickness. Heat loads were supplied by an electron beam testing apparatus. The low energy (40–50 eV) and high current (100–200 A) electron beam from the cathode hit the specimen surface in a duration of max. 40 s. After the tests, the damaged parts of the specimens were examined by stylus instrument and metallographically examined by using optical microscope and scanning electron microscope. Incubation time for erosion of the materials decreased with increase of their melting point. Crater-like damaged areas(eroded areas) were observed in all the materials, however, that of W—25% Re was the largest among them. In the damaged area, grains elongated from the fringe of the crater toward its center and also from the bottom of the crater toward the specimen bottom. There existed no visible cracks in the undamaged part of the specimens if their thicknesses were greater than 2 mm. The results were discussed in terms of microstructures and thermophysical properties of the materials.

Irradiation behavior of carbon-boron compounds and silicon carbide composites developed as fusion reactor materials

We are developing carbon-boron compounds for use as a plasma facing material and highly pure carbon-fiber silicon carbide composites as a low-activation first-wall material. To study the irradiation behavior of these materials, samples have been irradiated in a fission neutron spectrum (in JMTR). The present work will show basic properties of developed materials and some aspects of the irradiation behavior of these materials.

Applicability of creep damage rules to a nickel-base heat-resistant alloy Hastelloy XR

Abstract A series of constant load and temperature creep rupture tests and varying load and/or temperature creep rupture tests was carried out on a nickel-base heat-resistant alloy Hastelloy XR, which was developed for applications in the High-Temperature Engineering Test Reactor, at temperatures ranging from 850 to 1000°C in order to examine the applicability of the conventional creep damage rules, i.e., the life fraction, the strain fraction and their mixed rules. The life fraction rule showed the best applicability of these three criteria. The good applicability of the rule was considered to result from the fact that the creep strength of Hastelloy XR was not strongly affected by the change of the chemical composition and/or the microstructure during exposure to the high-temperature simulated HTGR helium environment. In conclusion the life fraction rule is applicable in engineering design of high-temperature components made of Hastelloy XR.

Induced radioactivity of commercial isotropic graphites for high heat flux tiles

It used as the plasma-facing material in the next-generation fusion devices, graphite will induce radioactivity in impurities in the graphite. This study was carried out to evaluate the amount of radiologically significant impurities in commercial isotropic graphite tiles. Special attention is given to the benefits of purification by halogen treatment. Graphite tiles from seven Japanese companies were irradiated in JMTR to neutron fluences up to 7.7 × 1024 n/m2 fast (E > 0.1 MeV) and 1 × 1025 n/m2 thermal (E < 0.683 eV) at about 450 K. Subsequent γ-ray spectroscopy revealed that major impurities contributing to the induced radioactivity are the IIId, IVa, Va elements and rare earth elements. The origins of these impurities are suggested and the effects of halogen treatment on the reduction of these impurities are analyzed.

Corrosion behaviors of Inconel 617 in hydrogen base gas mixture

The corrosion behavior of Inconel 617, a candidate for the structural material of heat exchanger in the high temperature gas-cooled reactor (HTGR), has been investigated at elevated temperatures in the hydrogen base gas mixture (80 pct H2 + 15 pct CO + 5 pct CO2). This gas mixture simulates the reducing gas in the direct steel making system that uses heat from HTGR in Japan. This gas has relatively high oxidizing and carburizing potentials. In the temperature range of 650 to 1000 °C Inconel 617 oxidized to form a Cr2O3 scale containing titanium oxide. The activation energy for this process is estimated to be 50 to 60 kcal/mol. The time dependence of the growth of the surface oxide scale was parabolic. The aluminum in Inconel 617 was internally oxidized. The time dependence of the internal oxidation was noticed to obey a 0.4 power rate law. Carburization was noticed at 650 and 900 °C. At 900 °C, carbides containing Si, Ti, and Mo precipitated beneath the oxide scale for gas exposure times up to 200 h. After 200 h, the formation and growth of the surface scale suppresses carburization. The thermodynamic analysis of gas atmosphere proposed by Gurry could be applied successfully to the experimental results. Some inconsistency existed mainly because of the scale formation and direct gas-metal interactions.