Masanobu Miyake

Absorption and desorption of deuterium on graphite at elevated temperatures

Abstract The absorption and desorption behavior of deuterium has been studied on graphite exposed to a deuterium gas atmosphere at elevated temperatures. Thermal desorption measurements have been carried out at a constant heating rate of 10°C/min in vacuum. The solubility can be expressed by S(STP cm 3 /g) = 1.9 × 10 −4 p 1 2 (Pa)exp[19(kJ/mol)/RT] . Deuterium desorption curves on graphite seem to have three peaks at approximately 140°C, 480°C and 930°C. The second peak may be attributed to pore diffusion of deuterium expressed by D(cm2/s) =1800 exp[−121(kJ/mol)/RT]. The third peak may be attributed to bulk diffusion in graphite filler grains, and this can be expressed by D(cm2/s) = 1.69 exp[−251(kJ/mol)/RT].

A study of hydrogen absorption and desorption by titanium

Abstract The hydrogen absorption and desorption behavior of titanium in a constant volume system has been investigated in the temperature range of 450 to 800°C. A method of the reaction rate analysis was proposed and examined for determining the rate constant. The activation energy values obtained by this analysis for the absorption and the desorption were 74 and 25 kJ/mol, respectively. When the absorption and the desorption was repeated, the absorption rate constant for the second run was found to be appreciably larger than that for the first run whereas the desorption rate constant did not show any marked change.

Thermal desorption of deuterium and helium from ion irradiated graphite

In order to estimate the characteristics of Isotropic graphite under a fusion reactor environment, surface erosion caused by ion bombardment (20 keV D2+ or He+ ) and thermal desorption behavior of D2 and He implanted by means of two different procedures, ion bombardment and isothermal gas exposure, have been studied. The SEM observations show that the erosion due to He+ bombardment at a dose of 5.0 × 1018 ions/cm2 was more severe than that for D2+ bombardment at a similar dose. Thermal desorption of deuterium from bombarded graphite gave a quite different behavior from that of the thermally exposed graphite, which might be due to a chemical reaction, whereas in the case of helium, no irradiation effect on desorption behavior could be observed.

Chemical vapor deposition of niobium on graphite

Abstract Chemical vapor deposition of niobium on a graphite rod has been carried out at temperatures exceeding 1200 °C using NbBr 5 as a feed material. The properties of the deposits have been studied by X-ray diffraction analysis, surface hardness measurements, metallographic and scanning electron microscopy observations. It was found that coatings deposited at a low temperature (below 1300 °C) have a smooth surface and a fine grained structure with layers consisting of Nb + Nb 2 C or Nb 2 C + NbC depending on their deposition temperature. In contrast, the coatings deposited at a higher temperature were found to have decreased smoothness of the surface and a coarse grained structure with a single layer consisting of NbC. Annealing experiments on the coatings with Nb and Nb 2 C layers showed that these layers readily transformed to a single layer of NbC at 900 °C within 10 h. It was found that the layer growth rates of carbides in the deposits are much faster than the layer growth rates estimated from ordinary diffusion data.

Study on the hydrogen solubility in zirconium alloys

Pure zirconium and zirconium alloys of Zircaloy-2 (1.47 wt% Sn, 0.14 wt% Fe, 0.11 wt% Cr, 0.05 wt% NO, Zircaloy-4 (1.52 wt% Sn, 0.21 wt% Fe, 0.11 wt% Cr) and Zr-1 wt% Nb alloy and Zr(O) alloys were selected as specimens. The hydrogen solubility measurement was performed in the temperature range 500 – 050°C at a hydrogen pressure below 104 Pa. The hydrogen solubilities were different among alloys and the phase relationships were affected by the alloying element. The partial thermodynamic quantities of hydrogen in the alloys were derived from the experimental data and the effect of the alloying element was discussed.

Simulation of tokamak runaway-electron events

High energy runaway-electron events, which can occur in tokamaks when the plasma hits the first wall, are a critical issue for the materials selection of future devices. Runaway-electron events are simulated with an electron linear accelerator to better understand the observed runaway-electron damage to tokamak first wall materials and to consider the runaway-electron issue in further materials development and selection. The electron linear accelerator produces beam energies of 20–30 MeV at an integrated power input of up to 1.3 kW. Graphite, SiC + 2% AlN, stainless steel, molybdenum and tungsten have been tested as bulk materials. To test the reliability of actively cooled systems under runaway-electron impact, layer systems of graphite fixed to metal substrates have been tested. The irradiation resulted in damage to the metal compounds but left graphite and SiC + 2% AlN without damage. Metal substrates of graphite-metal systems for actively cooled structures suffer severe damage unless thick graphite shielding is provided.

Self-Diffusion of Carbon in Uranium Monocarbide

The diffusion of C in UC has been measured using radioactive tracer and sectioning technique. Self-diffusion coefficients of C in UC are described well by the equation over the temperature range of 1,400°–2,000°C.

Thermal desorption of helium from graphite irradiated by He+ ions

Thermal desorption of helium from graphite irradiated with 20 keV He + ions and surface erosion caused by ion irradiation have been studied. After He + ion irradiation, localized protrusions and uniform domed surface uplifting were observed on isotropic graphite at irradiation doses of 5.0 × 10 17 ions/cm 2 and 5.0 × 10 18 ions/cm 2 , respectively. The peak temperatures in thermal desorption curves have a tendency to rise with irradiation dose, and become constant at about 330°C for irradiation doses above 5.0 × 10 17 ions/cm 2 . Thermal desorption of helium considerably depends on the structure of graphite samples studied here. Glassy carbon and graphitized paper (PAPYEX) retained only 22% and 12% of the helium retention of isotropic graphite, respectively.

Self-diffusion of carbon in hyperstoichiometric uranium monocarbide

Abstract The self-diffusion of C in hyperstoichiometric UC has been measured in the single-phase region of UC, using a radioactive tracer and sectioning technique. The diffusion equations as a function of temperature are: 1. (i) for UC1.38, D = 0.10 exp (−(53.5 ± 10.6)/RTkcal/mole) cm2/sec for the temperature range 2100–2230°C; 2. (ii) for UC1.52, D = 0.11 exp (−(54.6 ± 4.6)/RTkcal/mole) cm2/sec for the temperature range 1950–2150°C. The dependence of the diffusion coefficients and the activation energy is described as follows, 1. (i) when x (C/U molar ratio) ≲ 1.11, the diffusion coefficients increase with x, while the activation energy decreases, 2. (ii) when ≳, the diffusion coefficients no longer increase with x but remain constant, as does the activation energy, r.e., E is about 54 kcal/mole. The mechanism for C diffusion in hyperstoichiometric UC is considered to be an interstitial or interstitialcy.

Reaction of several iron and nickel based alloys with sintered Li2O Pellets

Abstract The reaction of type 316 stainless steel, Incoloy 800, Hastelloy X-R, Inconel 600 and pure Ni with sintered Li 2 O pellets has been studied between 800 and 1100°C under dynamic vacuum. The reaction products were analyzed by means of metallographic, microprobe and X-ray diffraction methods. The reactions proceeded measurably between 800 and 950°C and appreciably at 1000°C, being greatest with Incoloy 800 and least with Hastelloy X-R. Among the primary alloy constituents, chromium was exclusively attacked by lithium and oxygen diffusing from the Li 2 O into the alloys to form LiCrO 2 . This phase grew into a reaction zone (subscale) of uniform thickness beneath the surface of each alloy. Preferential growth of LiCrO 2 along the grain boundaries was observed only in the case of Inconel 600 below 950°C. On the other hand, iron diffused toward the Li 2 O pellets to form volatile Li 5 FeO 4 . However, any reaction product associated with Ni was not detected and Ni metal was little attacked by the Li 2 O pellet over the whole range of reaction temperature.