Tetsuo Tanabe

Hydrogen transport in stainless steels

Abstract The precise measurements of hydrogen permeability (φ) and diffusivity (D) were performed for some stainless steels and high temperature materials utilizing an extremely purified hydrogen under pressure of 0.001–0.1 MPa at temperatures between 500 and 1200 K. Most of the determined values of φ and D give good Arrhenius relationship against inverse temperature. The effect of alloying elements, particulary, of Cr is discussed by comparing the obtained values of φ and D.

Permeation and reemission of deuterium, implanted in first wall materials

Abstract The permeation rate and the reemission rate of deuterium during the implantation of 20 keV deuteron have been measured for samples of nickel and molybdenum with use of a quadrupole mass-spectrometer. The results have shown that the behavior of implanted hydrogen is largely affected by the radiation damage introduced by the bombardment in the temperature range of 300–500°C for nickel and above 600°C for molybdenum. Some of results, for example, a decrease of the permeation rate with prolonged implantation, can be explained by a diffusion model including the trapping of hydrogen in the damaged region.

Review of high Z materials for PSI applications

Application of carbon based low Z materials to PFM has significantly improved plasma parameters in large tokamaks. There are, however, serious concerns of erosion, neutron damage, etc. for application of low Z materials in the future D-T burning machine. To apply high Z metals to PFM, there are several issues to be solved: high Z impurity production by sputtering, their accumulation in the plasma center, and high radiation loss. Because of these concerns high Z metals are not widely employed nor planned to be used in the present large tokamaks. Since our efforts have been concentrated to optimize the low Z materials, little systematic investigations for high Z materials in tokamaks have been done, resulting in a poor data base especially for those materials concerning impacts on plasma core. In order to employ high Z material as PFM near future, material properties related to impurity production and hydrogen recycling are reviewed, and what is important and shall be done is also discussed.

Hydrogen behavior in graphite at elevated temperatures

Abstract We have examined deuterium retention in the graphite exposed to D 0 at elevated temperatures by means of reemission and thermal desorption techniques. The total retention increases with exposure times showing a t 1 2 dependence without any saturation behavior, which is interpreted as diffusion-limited formation and growth of a modified layer. On the other hand, desorption spectra obey second order desorption kinetics and their peak structure is hardly dependent on exposure time. The deuterium retention under D 0 exposure and succeeding thermal desorption are well interpreted by deuterium diffusion in the saturated layer and recombinational release in the saturated layer, respectively, and both the second order desorption rate and diffusion coefficient of deuterium in the saturated layer are determined

Annealing experiment of ion-irradiated graphite by laser Raman spectroscopy

The annealing behavior of highly oriented pyrolytic graphite foils irradiated at room temperature was systematically investigated in terms of irradiation dose by means of Raman spectroscopy, A remarkable decrease in the intensity ratio of the defects peak at 1355 cm −1 to the Raman-allowed graphite peak at 1580 cm −1 , I 1355 / I 1580 , could be seen on annealing at temperatures as low as −1 and I 1355 / I 1580 . One was considered to correspond to the accumulation of defects in the basal plane, which are unstable and easily annealed at low annealing temperatures, while the other to that of the disordered structure between the basal planes, which is stable and induces amorphization. The two linear relations were very similar to those observed in the irradiated graphites but the change was opposite in direction.

Isotope effect in dissociation of uranium hydride.

Isotope effect on dissociation behavior of uranium hydride has been studied. The equilibrium dissociation pressure of uranium protide(UH3), deuteride (UD3), and of equimolar mixture of both hydrides are measured. The dissociation pressure of deuteride is higher than that of the protide by a factor of about 2 at 400°C and mixed hydride of UH3 and UD3 has intermediate dissociation pressures. Enthalpy and entropy of formation as well as free energy of formation at 298 K obtained from the dissociation pressure are −29.9kcal/mol, −42.8 cal/K·mol and −17.7 kcal/mol for UH3, and −31.5 kcal/mol, −46.8 cal/K·mol and −17.5 kcal/mol for UD3, respectively, which are in good agreement with the values obtained from calorimetrie and heat capacity studies. Mole fractions of deuterium in the gas phase and the solid phase are determined when the mixed hydride is in dissociation equilibrium with gas mixture. In this case deuterium is found to be enriched in the gas phase and the separation factor defined by [D/H]g3S phase/[...

Hydrogen permeation and diffusion in molybdenum

In order to determine both coefficients of diffusion and permeation of hydrogen in Mo correctly, a precise hydrogen permeation measurement has been carried out at temperatures ranging from 500 to 1100 K using pure H2 (less than 1 ppm H2O) with an upstream hydrogen pressure of 1 to 105 Pa. The determined diffusion (D) and permeation (Φ) coefficients are D = 4.0×10−8 exp(−22.3 (kJ/mol)/RT), m2 s−1, Φ = 1.4×10−7 exp(−60.3(kJ/mol)/RT) mol(H2) m−1 s−1 Pa−1/2. And solubility determined by the relation of Φ = DS is S = 3.3 exp(−37.4(kJ/mol)/RT). mol(H2) m−3 Pa−1/2. An addition of water vapor into the pure H2 reduces both the permeability and diffusivity, and the permeability deviates from the square root dependence of the upstream hydrogen pressure. The reduction of the diffusivity is more appreciable with decreasing the pressure.

Reemission and permeation of deuterium implanted into metals

Abstract Focusing on the marked depression of deuterium permeation rate during the deuteron bombardment, implantation experiments coupled with gaseous permeation experiments are performed on pure Ni and Ni with evaporated MnO. It is concluded that the reemission of implanted deuterium is initially depressed, but it soon becomes enhanced with increase of fluence leading to a rapid decrease of permeation rate at the intermediate temperatures 600–1000 K, which is attributed to the formation of short diffusion paths from the projected range to the front surface.

On the estimation of tritium retention and permeation in metallic first wall

Abstract Recently, we have shown that the ion-driven permeation through metal is basically controlled by hydrogen diffusion in both the front and back sides. The maximum permeation rate Φpmax is well represented by Φpmax = rφ/(d − r), (r is the range of the incident ion, φ the incident flux and d the specimen thickness), which is much smaller than the estimation given by the recombination theory. In the present work, therefore, we have re-estimated the tritium retention and permeation in the metallic first wall according to the diffusion model. In addition, we also demonstrate that the effect of residual hydrogen gas on the retention and permeation cannot be neglected, especially at high temperatures. In order to reduce the hydrogen permeation, we have investigated the usefulness of a protective layer. A fairly thick protective metal layer with small permeability is found to be effective. Unfortunately, the layer does not work for reducing the retention.

Hydrogen reflection and Hα emission

In order to investigate the relation between H α emission and the kinetics of reflected particles at a solid surface, D α (656. 10 nm) and HeI (578.76 nm) spectra emitted from reflected particles under D + and He + bombardment are analyzed in terms of incident ion energy, target material and surface impurity. With increasing incident ion energy, both D α and HeI emissions show blue shifts and line broadenings, whereas the total photon intensity decreases. The intensity also increases with the atomic number (Z number) of the target materials, and the relation between the intensity and Z number is very similar to that found between the reflection coefficients calculated by a Monte Carlo simulation and Z number. From quantitative photon emission measurements, the yield of D α emitting particle normalized to the number of incident deuteron is found to be in the range 10 −2 to 10 −3 , depending on the incident energies and target materials. The emission yield is mainly dependent on the velocity of the reflected particles and the effect of electronic structure of target materials, as far as metals are concerned, seems to be small