Toshiaki Yoneoka

Diffusion coefficient of tritium in molten lithium-lead alloy (Li17Pb83) under neutron irradiation at elevated temperatures

Abstract The diffusion coefficient of tritium in molten Li 17 Pb 83 alloy was determined under neutron irradiation at 300–700°C. Tritium residence time in the experimental system decreased with increasing H 2 pressure in He sweep gas up to 1000 Pa, and above this limiting concentration it became constant. This result suggests that the tritium release rate was controlled only by the tritium diffusion processes in the molten Li 17 Pb 83 alloy sample and in the Fe sample holder above P H 2 = 1000 Pa. From the data on tritium residence time obtained in P H 2 = 3000 Pa, the diffusion coefficient of tritium was determined as follows: D / m 2 s −1 = 2.50 × 10 −7 exp (−27.0 kJ mol −1 / RT ), which was slightly larger than the other literature values.

Compatibility of yttria (Y2O3) with liquid lithium

Abstract Compatibility of sintered specimens and plasma sprayed coating specimens, of Y 2 O 3 with liquid lithium was tested at 773 K. No configuration change was observed for the sintered specimens with a slight increase of thickness for 1419 h. Lithium—yttrium complex oxide (LiYO 2 ) was formed on the surface, and the inner part changed to gray or black nonstoichiometry Y 2 O 3− x with lower electrical resistivity. The plasma sprayed coating specimens were severely attacked by liquid lithium with or without applied electric field. Lithium penetrated into the coating layer through small cracks and reacted on Y 2 O 3 to form LiYO 2 , which has a different density from Y 2 O 3 and is more brittle than Y 2 O 3 . It is concluded that Y 2 O 3 has a possibility as a ceramic coating material for liquid blankets if it can be made into a dense coating on the surface of piping materials.

Development of tritium processing material — A U-Zr alloy as a promising tritium storage material

Hydrogen absorption-desorption properties of UZr 2.3 , 6-phase intermetallic compound, were studied at temperatures ranging from room temperature to 973 K at a hydrogen pressure of 10 to 10 5 Pa. As a result, separation into ZrH 2 and UH 3 phases occurred on hydrogenation of UZr 2.3 . However, the decomposition isotherms of these phases were considerably different from those of the respective pure metals, and this result was attributed to the dissolution of one metal element into the hydride phase of the other metal element. Despite the formation of the UH 3 phase, powdering of the alloy specimen on its hydrogenation was remarkably reduced, possibly due to zirconium dissolving into it. Hence, the UZr 2.3 is evaluated to be a promising storage material for tritium.

Hydrogen permeation of vanadium and an in situ surface analysis

Abstract The effect of surface-segregated sulfur on the hydrogen permeation rate of vanadium has been studied in a new apparatus installed with an in situ AES analyzer. It was observed that during a high temperature anneal a clean vanadium surface was rapidly covered with sulfur. Besides, the surface coverage of sulfur was found to be controllable by means of a combination of in situ sputter cleaning and subsequent annealing, so that a “well characterized dirty” surface for S on V can be prepared. After a stepwise introduction of hydrogen into the upper part of the chamber, the hydrogen permeation rate of a clean vanadium quickly increased, reached a peak, then gradually diminished. The permeation rate of a sulfur-covered vanadium, however, increased much more slowly and gradually reached a lower steady-state level. The maximum permeation rate definitely decreased with increasing the surface coverage of sulfur, revealing that a surface sulfur layer has a strong barrier effect upon the hydrogen permeation of vanadium.

Corrosion behaviour of AlN for self-cooled Li/V blanket application

Abstract Corrosion behaviour of Aluminum nitride (AlN) as a candidate material for insulating coating for V/Li blanket was investigated by corrosion experiments in liquid lithium (Li) up to 1073 K for 1000 h. High purity AlN samples decreased their weights after the sintering test in Li in contact with vanadium alloy over 973 K, while those in Li not in contact with the vanadium alloy survived up to 1073 K. Nitrogen dissolution from AlN into the liquid Li and absorption of dissolved nitrogen by vanadium alloy are considered to be a corrosion mechanism. Small decreases of electrical resistance were observed after the sintering tests over 873 K because of the conductive corrosion layer on the surface caused by the nitrogen dissolution. In the cases of low purity AlN samples sintered over 973 K, large weight decreases were explained by fragile grain boundary caused by oxygen dissolution. The oxygen dissolution may also results in the resistivity decrease even at 723 K. Therefore, decrease of oxygen impurity in AlN and addition of nitrogen in liquid Li are considered to give a possible solution to the Li/AlN corrosion problem in the Li/AlN/V alloy blanket system.

Compatibility of insulating ceramic materials with liquid breeders

Abstract The development of ceramic coating is one of the most important subjects in liquid blanket R&D. The compatibility of candidate ceramic materials (Y 2 O 3 , Al 2 O 3 , MgO, 3Al 2 O 3 –MgO, AlN–BN and BN) with liquid metal breeders such as metallic lithium and lithium-lead alloy (Lil7–Pb83) was investigated at 773 K up to 5 Ms with the change in insulating property. Al 2 O 3 and 3Al 2 O 3 –MgO were severely corroded and dissolved or broken by lithium, while MgO was corroded uniformly with a moderate rate (e.g. 27 μm for 4.8 Ms). The most thermodynamically stable Y 2 O 3 was a little corroded and showed a slight increase in electrical conductivity. On the other hand, all the ceramic materials were not corroded at all by Lil7–Pb83, as predicted from a thermodynamical analysis. AlN–BN and BN corroded by lithium became more fragile because impurities included in the specimens were dissolved in lithium.

High temperature liquid metal corrosion and high temperature electrical conductivity of Y2O3

Abstract Yttrium sesquioxide has been proposed as a promising candidate material for collector electrodes used in the laser enrichment system of uranium-235. For this purpose, yttria is expected to be compatible with molten uranium and electrically conductive. A corrosion test of yttria with molten lanthanum as a simulating metal for uranium and a measurement of its electrical conductivity under extremely low oxygen pressure were performed. It was shown from the corrosion test that a yttria sample was considerably corroded by the molten lanthanum at 1513 K and the maximum corrosion depth for 5 Ms was 0.162 mm. The electrical conductivity of hypo-stoichiometric yttria reduced by titanium was higher than that of pure germanium at room temperature (2.1 S/m). The oxygen pressures equilibrated with the yttria specimens were estimated to discuss the relation to measured conductivities.

Tritium permeation through austenitic stainless steel with chemically densified coating as a tritium permeation barrier

Chemically densified coating formed on the surface of austenitic stainless steel (SUS 316) was examined for compatibility with molten lithium-lead eutectic alloy (Li17Pb83) and tritium permeability. The chemically densified coating (CDC) consisting of SiO2 particles and a Cr2O3 matrix with a thickness of 60 μm was unstable in contact with the molten alloy as predicted from a thermodynamic calculation at 600°C, and it was degraded in several days. In an in-pile experiment, specimens with the coating on the front surface or the rear surface were immersed in Li17Pb83 molten alloy, and their tritium permeabilities were measured. The permeability of the former was reduced to 110 of the ideal value in the diffusion-limited case, while that of the latter was less than 1100 of the diffusion-limited value even in a pure H2 atmosphere. It is concluded that CDC is quite effective to reduce tritium permeability in the condition of not contacting molten Li17Pb83 alloy.

H2O trapping on various materials studied by AFM and XPS

Abstract Adsorption and desorption of water on thin films of iron oxide formed on the iron surface were studied by AFM and X-ray photoemission spectroscopy (XPS). The morphological change of iron oxide or water droplets on the surface of iron oxide were observed by AFM. Electronic state of O 1s was observed by XPS after a sample was exposed to distilled water. Several peaks appeared and assignment to oxygen-containing surface species was discussed.

Compatibility of structural candidate materials with LiF–BeF2 molten salt mixture

Abstract Compatibility of structural materials such as JLF-1 (Fe–9Cr–2W), vanadium alloys (e.g., V–5Cr–5Ti) and SiC with Flibe (LiF–BeF2) is a key issue for the force-free helical reactor (FFHR) blanket concept. In the present study, the corrosion behavior of SUS430 (Fe–18Cr) and SiC in static Flibe was investigated as a first step. After being dipped in Flibe containing a trace of HF at 550°C for 1 day, 3 days and 10 days, specimens were analyzed by X-ray diffractometry (XRD) and Rutherford backscattering spectroscopy (RBS). It was found that SUS430 specimens formed an oxide layer on the surface. However, it was not clear whether the SiC specimens were corroded or not due to a thick deposit.