Masao Matsuyama

Thermal desorption of hydrogen, deuterium and tritium from pyrolytic graphite

Abstract Thermal release of hydrogen, deuterium and tritium implanted into a pyrolytic graphite was studied by means of mass analyzed thermal desorption spectroscopy along with surface characterization by X-ray photoelectron spectroscopy. Hydrogen (or its isotopes) ions were implanted into the sample at room temperature with an applied voltage of 5 kV using a conventional ion gun. Subsequently, the sample was heated to 900 ° C with various temperature ramps to measure the thermal desorption spectra. The implanted hydrogen (or its isotopes) was predominantly desorbed as H 2 and in small amount as CH 4 . The desorption spectra of H 2 changed gradually while repeating the implantation-desorption cycles and became reproducible after the total dose amounting to 1 × 10 19 ion/cm 2 , indicating that the virgin graphite is modified due to formation/accumulation of radiation damage. For the modified graphite, three desorption peaks were observed. The first peak is attributed to the desorption of hydrogen atoms trapped on the carbon atoms in the normal graphite lattice. The others correspond to differently trapped hydrogen atoms in the graphite. The desorption of the first peak obeyed the second order kinetics with respect to the amount of the implantation, indicating that the rate determining step is the surface association reaction of the hydrogen atoms. The activation energy was estimated as 44 kcal/mol for three hydrogen isotopes. However, the isotope effect appeared on the frequency factor: their ratio was estimated as H 2 :D 2 :T 2 = 3:1.5:1 . The desorption of methane obeyed the pseudo-first order kinetics with an activation energy of 38 kcal/mol.

Tritium assay in materials by the bremsstrahlung counting method

Abstract To estimate depth profiles and the amount of tritium captured in materials, the applicability of a bremsstrahlung counting method was examined. Two kinds of materials were used as model samples. One was a tritium source made of an organic polymer and the other was a disk of ZrNi-alloy containing tritium. For both samples, bremsstrahlung X-ray spectra showed a single broad peak. The maximum intensity of each peak appeared at ∼7–8 keV. The intensities were 19 counts s −1 for the polymer source and 240 counts s −1 for the ZrNi disk. To evaluate the detection depth, the observed spectra were compared with those by numerical analysis. The spectral shape and peak position obtained by computer simulation agreed quite well with those of the spectrum observed for the polymer source, whereas for the ZrNi disk, a significant difference was observed at the higher energy side of the peak. The difference could be attributed to the superposition of bremsstrahlung and characteristic X-rays arising from nickel. The detection depth was determined to be ∼1 mm for a polymer source consisting mainly of carbon atoms, and ∼0.1 mm for the ZrNi disk. The present method is highly promising for non-destructively measuring tritium captured deep in materials and to estimate tritium concentration profiles.

ZrNi alloys as candidate getter materials for tritium processing

Abstract Reversible tritium getters are indispensable for the safe handling of a large amount of tritium in the fuel processing for thermonuclear fusion reactors. In ZrNi metallic compounds, we looked for the reversible getters showing the heat of deuteride formation around − 25 kcal/mol (D2) which will be appropriate to the storage-supply-recovery of tritium. Among five metallic compounds used in this study, we found that the first plateau for ZrNi and Zr9Ni11 showed ΔH 0 ⋍ −25 kcal / mol (D2). The second plateau for ZrNi gave ΔH0 = −19.2 kcal/mol (D2). This suggests that ZrNi is also applied as the source of tritium supply near one atmosphere at relatively low temperature (260°C). They were withstanding pulverization and nonflammable against air exposure. In addition, ZrNiD2.3 was reactivated quite easily with vacuum heat at 300°C even after air exposure. Those properties of ZrNi and/or Zr9Ni11, are quite promising as the reversible tritium getter in the fuel processing for thermonuclear fusion reactors.

Alloying effect on the activation processes of Zr‐alloy getters

Gettering materials, most of which are alloys, have wide applicability to tritium processing functions in fusion and fission reactors such as storage, supply, recovery, separation, and so on. However, required getter properties differ depending on unit process conditions. To develop suitable getters for each unit process, it is important to investigate fundamentals of alloying effects on their properties. Therefore, we studied the activation processes of three Zr‐alloy getters (Zr–Al, Zr–Ni, and Zr–V–Fe) by means of x‐ray photoelectron spectroscopy–secondary ion mass spectrometry and thermal desorption spectroscopy. It was observed that the formation of a metallic Zr surface is the principal process for activation, by which the getters show pumping action for various gases. A considerable alloying effect was observed on the activation processes of the three getters. The activation temperature varied with alloying elements: 800 °C for Zr–Al, 700 °C for Zr–Ni, and 400 °C for Zr–V–Fe. It was concluded that t...

Gas chromatographic separation of H2-D2 mixtures by Pd-Pt alloy near room temperature

Abstract A new type of chromatography for hydrogen isotope separation was developed by use of a Pd-Pt alloy as functional material. It could separate H 2 -D 2 mixtures (90% H 2 -10% D 2 and 50% H 2 -50% D 2 ) and HD to H 2 and D 2 in a temperature range from 274∼303 K in a short time with simple operation procedures without introducing any replacement gas. The new chromatography promises great potential for the application to an isotope separation system of thermonuclear fusion devices

Trapped states of deuterium implanted into graphite and the thermal desorption

Abstract Chemical sputtering of graphite and the retention of hydrogen are important subjects for thermonuclear fusion devices. We measured thermal desorption processes of deuterium implanted into graphite at room temperature by means of the flash desorption spectroscopy along with surface characterization by XPS and SIMS. Chemical shift of the Cls peak was observed by XPS due to the deuterium-ion implantation. In SIMS spectra, CD± and C2D− signals appeared after the implantation. Based on these observations, it was concluded that C-D and/or C2-D species were formed on the surface due to the implantation. The implanted deuterium desorbed as D2 and CD4 by flashing the sample above 500°C. The fraction of the desorbed amount of CD4 strongly depended on the flashing mode. Moreover, no methane appeared after the irradiated sample was annealed above 500°C. It was concluded that a part of the C-D and/or C2-D species associated each other in the graphite during the flash desorption to form molecular deuterium which presumably adsorbed on carbon atoms.

Nondestructive measurement of surface tritium by β-ray induced X-ray spectrometry (BIXS)

Abstract Applicability of a newly developed β-ray induced X-ray spectrometry (BIXS) has been examined to measure nondestructively tritium retained on/in the graphite samples. Examination was carried out by using the graphite plates irradiated with tritium ions and an ALT-II limiter tile exposed to D-plasmas in TEXTOR. For the former samples, a sharp intense peak and a broad weak peak appeared clearly in the spectra; the former peak was attributed to the characteristic X-rays from argon used as a working gas, and the latter peak was assigned to the bremsstrahlung X-rays from sub-surface layers of graphite. On the other hand, for the latter sample, a rather weak characteristic X-ray peak was observed along with a diminutive bremsstrahlung X-ray peak. Although the intensities of those X-rays differed from spot to spot, the tritium levels retained on the limiter tile were determined to be 58–132 Bq / cm 2 . It was concluded, therefore, that valuable information on the amount and the distribution of tritium retained on/in the wall materials can be nondestructively obtained by using the BIXS.

Absorption of hydrogen isotopes by Pd-Pt alloys

Abstract In the search for more profitable working materials for the recently developed gas chromatographic hydrogen isotope separation operated around room temperature without any replacement gas, thermodynamic properties of Pd–Pt alloys for hydrogen absorption and hydride formation were studied in the composition range 0–10 at.% Pt, the temperature range 273–493 K, and with a H/M from 2×10 −4 to 0.65. The heat of absorption at infinite dilution and hydride formation were determined for H 2 and D 2 . The results showed that both the heat of absorption and hydride formation decreased with increasing Pt content in the alloy and the isotope effect defined by the ratio of equilibrium pressures did not change much with alloy composition. It is implied that any alloy used in the present study could be selected as a working material depending on the required operation conditions without significant change in the separation efficiency.

Catalytic inactivities of Ni alloys expressed by surface and bulk compositions

The surface compositions of three kinds of powdered alloys with different structures, CuNi (fcc-fcc), FeNi (bcc-fcc), and CoNi (hcp-fcc), were determined by the penetration of β-rays of 63Ni mixed previously in the alloy, then the effect of the respective alloy structure both of the surface and of the bulk on the catalytic hydrogenation of ethylene was studied. The surfaces of these alloys were found to be Ni-poor for the wide range of alloy composition. The catalytic activity expressed by each surface composition was difficult to explain by the physicochemical properties of the surface. The activity expressed by the bulk composition of CuNi of fcc structure was high in the region of unfilled d-band, and so were that of FeNi and that of CoNi in the region of fcc structure. The activity of FeNi for the dimerization of 14C-ethylene was studied, of whose activity the similar tendency was observed as found in the hydrogenation reaction. These could be interpreted as the results of the positive contribution of the dissolved hydrogen in bulk to the catalytic reactions.

Quantitative Measurement of Surface Tritium by β-Ray-Induced X-Ray Spectrometry (BIXS)

ABSTRACT To make a nondestructive measurement of tritium retained on/in materials surfaces, conversion efficiency of β-rays to characteristic X-rays in an argon atmosphere has been examined. For this purpose, various tritium-containing graphite plates were prepared at first by ion implantation. After the tritium implantation, measurements of an X-ray spectrum from the graphite plates were carried out in the argon atmosphere. A good linear relation was observed between the intensity of Ar(Kα) characteristic X-rays and the total amount of tritium deter-mined by full-combustion. The apparent conversion efficiency was determined as 4.15x10−6 counts/s/Bq. To determine the intrinsic conversion efficiency for argon atoms, relevant correction factors such as geometrical efficiency, absorption of X-rays, effects of a tritium depth profile and a photoelectric effect were experimentally evaluated through numerical calculations. Taking into account these correction factors, the intrinsic conversion efficiency was determined to be 3.1x10−4 photons/β-particle.