Heitaro Yoshida

Corrosion of inconel 617 in HTGR grade helium

Corrosion of the primary circuit materials is one of the serious problems in High Temperature Gas-cooled Reactors (HTGR). In the present work, the effect of gaseous impurities in the helium coolant on the corrosion behavior of Inconel 617 has been studied at 1000 °C and atmospheric pressure for 1000 h. The helium gases used contained several impurities; H2, H2O, CH4, CO, and CO2. The corrosion behavior of alloying elements (Cr, Mo, Si, C) was strongly affected by the impurity concentration in helium and the extent of corrosion could be explained on the basis of the oxygen and carbon potentials in the gases used.

The effect of grain size on the corrosion behaviour of inconel 600 in high-temperature steam

Abstract The effect of grain size on the corrosion behaviour of Inconel 600 has been studied in steam of 40 atm pressure at 800°C. A protective Cr 2 O 3 -rich scale is formed initially. After this introductory stage, nodular oxides nucleate at the positions where cracks have occurred in the Cr 2 O 3 -rich scale. The weight gain due to steam corrosion has a tendency to decrease with increasing grain size in the stage where the Cr 2 O 3 -rich scale is formed. This tendency is reversed in the nodular oxide-forming stage, where the weight gain increases with increasing grain size. The effect of grain size on the corrosion behaviour in the two stages is discussed.

Carburization and decarburization of superalloys in the simulated htgr helium

Abstract Carburization and decarburization of superalloys due to the impurities in the High Temperature Gas-cooled Reactor (HTGR) helium influences the mechanical properties of the superalloys. The objective of this study is to investigate the carburization and decarburization behavior thermodynamically. The four commercial superalloys were exposed to the three simulated HTGR helium gases and the change in carbon concentration was examined. The following results were obtained: 1. (1) The carburization and decarburization depend strongly on the superalloys and the helium gases. 2. (2) The extent of the carburization or decarburization exhibits a linear relation against its thermodynamic driving force. The results make it possible to predict the carburization and decarburization in the HTGR helium environment.

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.

Effect of low cycle fatigue on the ductile-brittle transition of molybdenum

Abstract An explicit ductile-brittle transition of molybdenum occurring in both tensile and low cycle fatigue tests was investigated. Tests were performed on several sorts of molybdenum and its alloy TZM, and effects of heat treatment, fabrication method and alloying on the transition behavior and fracture mode are described in detail. All the materials exhibited a brittle failure with degraded fatigue behavior at room temperature, while they became ductile as temperature increased up to 573 K. The tendency of fatigue results was qualitatively in accordance with that of reduction of area in tensile tests. Differences among the materials were minor on the ductile-brittle transition temperature (DBTT), but major on the fatigue life for the embrittled materials.

Polarization Measurements of Soft X‐ray Emitted from the Figure‐8 Undulator

A figure‐8 undulator is installed in the soft X‐ray beamline BL27SU at SPring‐8 as insertion device. It provides linearly polarized light whose direction of polarization is horizontal for the integer order harmonic light and vertical for the half‐integer order harmonic light. We have measured the degree of linear polarization of the monochromatic soft X‐ray radiation at the end station of the beamline. Photoelectron spectra of Ne emitted in the horizontal direction are recorded for both horizontal and vertical directions of polarization, for two different settings of the front‐end‐slits. The degrees of linear polarization and the anisotropy parameters and branching ratios of photoelectrons are extracted in a self‐consistent manner, at various photon energies. The resultant values for the degree of linear polarization are interpreted with the help of theoretical calculations.

A study of inner-valence Auger transitions in Ne+ induced by the resonant Auger decay of photoexcited Ne 1s-1np states

High-resolution electron spectrometry is used to investigate inner-valence Auger transitions in Ne+. The following series of transitions have been observed: Ne+ 2s12p5(1,3P)np → Ne2+ 2s22p4 3P, 1D and Ne+ 2s02p6(1S)np→ Ne2+ 2s12p5 1,3P. In addition, we have observed a new series of valence inter-multiplet Auger transitions Ne+ 2s12p5(1P)np → Ne2+ 2s12p5 3P. The initial states of these transitions were populated by the resonant Auger decay of photoexcited Ne 1s−1np resonances. Thus, the investigated transitions constitute the second step in an Auger cascade process. The photon energy was chosen to effectively populate Rydberg states of Ne+ with n = 4–10. The energy positions, quantum defects, relative intensity and angular distribution parameters of the Ne+ Auger electron emission have been determined. Experimental results are compared with multi-configuration Dirac–Fock calculations carried out as a part of this study.

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.

Deterioration of electromotive force of chromel-alumel thermocouples in reducing atmospheres at high temperatures

The deterioration of electromotive force (emf) of Chromel-Alumel (CA) thermocouples in 80 pct H2 + 15 pct CO + 5 pct CO2 has been analyzed in terms of the corrosion behavior of Chromel. Emf of the CA thermocouple deteriorated drastically in 80 pct H2 + 15 pct CO + 5 pct CO2. After exposure for about 1000 hours at 900 °C, the decrease of emf was about 16 mV. The deterioration process could be separated into three terms. The first term, which has the smallest time constant of about 20 hours, was attributed to carbon deposition on the Chromel surface in the temperature range of 600 to 700 °C. The second term, which has a time constant of about 100 hours, was attributed to the severe internal oxidation of chromium in the temperature range of 500 to 800 °C. The third term, having the largest time constant of several thousand hours, might be attributed to the moderate and gradual preferential oxidation of chromium in Chromel in the range 800 to 900 °C. Boron nitride (BN) coating on CA thermocouples could reduce this deterioration of emf; the decrease of emf was improved to about 3 °C during 700 hours test at 900 °C.