Kan Sakamoto

Observation of Hydrogen Distribution Around Non-Metallic Inclusions in Steels with Tritium Microautoradiography

Hydrogen distributions around non-metallic inclusions in steels are successfully characterized with high-resolution tritium autoradiography. The autoradiographs show that hydrogen accumulation characteristics around the inclusions depend on types of the inclusions. In the case of MnS, hydrogen was inhomogeneously distributed in the ferrite matrix surrounding the MnS inclusion, probably because hydrogen is trapped in defects formed around MnS. The inhomogeneous distribution of hydrogen may be originated from the asymmetric stress field produced by a contraction of the MnS phase in the heat treatment, i.e. the inhomogeneous volumetric change of MnS owing to its larger thermal expansion than that of the ferrite phase. In the case of Al2O3, hydrogen was intensely localized at boundary layers of the ferrite matrix surrounding the Al2O3 inclusion. This could be attributed to hydrogen trapping at defects introduced by a residual stress in the boundary layers of the ferrite matrix due to larger contraction of the ferrite phase than that of the Al2O3 phase on cooling. Similarly hydrogen was accumulated in the surrounding ferrite matrix but more widely distributed around Cr carbide probably because difference in the thermal expansion between the Cr carbide and ferrite phases is less than that between the Al2O3 and ferrite phases.

Chemical interaction between granular B4C and 304L-type stainless steel materials used in BWRs in Japan

Chemical reactions between stainless steel and boron carbide were investigated using the materials applied for control rods in BWRs in Japan, specifically 304L-type stainless steel and granular boron carbide. The reaction region consisted of 2–4 layers, in which the significant composition variation of each element was detected, especially for B and C. Assuming that the reaction layer growth obeys the parabolic law, the effective rate constant between 304L-type stainless steel and granular boron carbide was evaluated to be approximately one order of magnitude smaller than the previously reported values for boron carbide pellets or powers. This difference might originate from the loose contact between the stainless steel and the granular boron carbide in the present study. Regarding liquefaction progress, the stainless steel components were selectively dissolved in the melt; consequently, the unreacted boron carbide tended to remain.

Influence of coexisting hydrogen isotopes on diffusion of tritium in niobium

Abstract The tracer diffusion coefficient of tritium in Nb has been measured at 523 K under the condition that tritium coexists with protium or deuterium. These data suggest the trend that the tracer diffusion coefficient of tritium in NbH x T y is somewhat larger than that in NbD x T y , though a definite conclusion has been reserved because their difference is small and comparable to the scattering of the experimental data. The concentration dependence of tracer diffusion coefficient of tritium in NbH x T y and NbD x T y can be practically considered to be equal to that of NbT x . On the basis of these experimental data and the data reported by Fukai et al., it has been suggested that the tracer diffusion coefficient of lighter hydrogen isotopes is appreciably influenced by coexisting heavier hydrogen isotopes but that of heavier hydrogen isotopes is scarcely influenced by coexisting lighter hydrogen isotopes.

Hydrogen Concentration Dependence of Tritium Tracer Diffusion Coefficient in Alpha Phase of Niobium

In order to examine influences of coexistent hydrogen isotopes on diffusion behavior of tritium in niobium, tracer diffusion coefficients Dt of tritium in alpha phase of hydrogenated and deuterized niobium (α-NbHxTy and α-NbDxTy (x<0,8,y<<x)) have been measured at 473 K, 493 K and 553 K. The data on Dt show typical hydrogen concentration dependence: Dt of tritium for both α-NbHxTy, and α-NbDxTy, decreases with hydrogen concentration under all experimental conditions. The obtained concentration dependence of Dt of tritium differs from that of Dt of protium in α-NbHx or of deuterium in α-NbHxTy. On the other hand, no appreciable differences in the concentration dependence of Dt of tritium between α-NbHxTy, and α-NbDxTy, are observed: there are no definite isotope effects due to the coexisting hydrogen isotopes. This result suggests that Dt of tritium for a tritiated niobium (α-NbTx) is not very different from that for α-NbHxTy and α-NbDxTy. The chemical diffusion coefficient D* of tritium is also evaluated on the basis of the obtained Dt of tritium and of a literature value of a thermodynamic factor F for Nb-H and Nb-D systems.

Diffusion coefficient of tritium in Ni-based alloy

ABSTRACT Hydrogen diffusivity and solubility in Inconel 600 were determined with a gas-absorption method using tritiated hydrogen gas in a temperature range from 573 K to 973 K. The temperature dependence of diffusivity D and solubility S were obtained as, D (cm2s−1) = 1.3 x 10−2 exp(−50.6/(RT / kJ)), S (cm3(NTP)cm−3Pa−0.5) = 4.7 x 10−3 exp(−11.7 /(RT / kJ)).

Change of chemical states of niobium in the oxide layer of zirconium–niobium alloys with oxide growth

The change of chemical states of niobium with oxide growth was examined in the oxide layers of Zr–2.5Nb around the first kinetic transition by the conversion electron yield – X-ray absorption near-edge structure measurements. The detailed depth profiles of niobium chemical states were obtained in both the pre- and the post-transition oxide layers of Zr–2.5Nb formed in water at 663 K for 40–280 d. The depth profiling revealed that the inner oxide layer remained protective to oxidizing species even though in the post-transition region and this excellent stability of barrierness would be attributed the suppression of hydrogen pickup.

Measurement of Diffusion Coefficient of Tritium in Alpha-Phase of Zirconium Based on Glow Discharge Implantation Method

A tritium implantation method based on a glow discharge of tritium gas was applied to the measurement of diffusion coefficient of tritium in alpha-phase of zirconium. The diffusion experiment was carried out in a temperature region from 673 K to 873K, and the diffusion coefficient of tritium was determined as follows, D/(cm 2 /s) = 3.14x10 -3 exp? The present data were in good agreement with our previous data which were determined with an usual implantation method based on a nuclear reaction 6 Li(n,a)T.

Segregation of Tin Oxide in Oxide Layer of Zircaloy-type Alloys

Zicaloy-type alloys including 1.51 mass% and 0.23 mass% of tin were completely oxidized in 0.1 MPa steam at temperatures of673 K and 873 K. The chemical composition near surface of the fine fragments of the oxidized specimen was measured by X-ray photoelectron spectroscopy. The appreciable enrichment of tin oxide SnO2 near surface of fine fragments was observed, and it was attributed to the segregation of tin oxide SnO2 at the grain boundaries of zirconium oxide. It was also concluded that tin oxide SnO2 was not dissolved in ZrO2 in the initial stage of oxidation at temperatures below 873K, and that the solubility of SnO2 in ZrO2 at 1,173K was larger than 0.23mass%. The diffusion coefficient of tin in the monoclinic phase of ZrO2 at 1,173 K was evaluated to be in the order of magnitude of 10-19-10-18 cm2/s.

Dependence of vacancy concentration on morphology of helium bubbles in oxide ceramics

Since the formation of helium bubbles degrades swelling property and thermal conductivity of minor actinide-containing mixed oxide (MA-MOX) fuel, it is essential to understand the conditions of the bubble formation. In order to examine the dependence of vacancy concentration on morphology of helium bubbles, helium was infused into (Zr,Fe)O2−x. The oxygen vacancy concentration was controlled by addition of solute Fe3+ into ZrO2. Helium was infused by hot isostatic pressing. The helium-infused specimens were observed by field emission scanning electron microscopy (FE-SEM) and field emission transmission electron microscopy (FE-TEM). In addition, X-ray fluorescence, X-ray diffraction analysis, conversion electron yield–X-ray absorption near-edge structure and FE-SEM/EDX (energy dispersive X-ray) analyses were also made to interpret the results of microstructure observations. As a result of the helium infusion treatment, numerous 0.5–10 nm bubbles were observed and its number density clearly depended on oxygen vacancy concentration. On the other hand, sizes of the helium nano-bubbles in all specimens were almost constant.

Effect of Coexistent Hydrogen Isotopes on Tracer Diffusion of Tritium in Alpha Phase of Group-V Metal-Hydrogen Systems

Tracer diffusion coefficients of tritium in the alpha phase of group-V metal-hydrogen systems, α-MH(D)xTy (M = V and Ta; x >> y), were measured in order to clarify the effects of coexistent hydrogen isotopes on the tritium diffusion behavior. The hydrogen concentration dependence of such behavior and the effects of the coexistent hydrogen isotopes (protium and deuterium) were determined. The results obtained in the present (for V and Ta) and previous (for Nb) studies revealed that tritium diffusion was definitely dependent on hydrogen concentration but was not so sensitive to the kind of coexistent hydrogen isotopes. By summarizing those data, it was found that the hydrogen concentration dependence of the tracer diffusion coefficient of tritium in the alpha phase of group-V metals could be roughly expressed by a single empirical curve.