Takanori Nagasaki

Reversible disproportionation of ZrCo under high temperature and hydrogen pressure

Abstract Chemical behavior of an intermetallic compound ZrCo with hydrogen at high temperature region was studied with respect to application utilizing the reversible hydrogenation. While ZrCo compound reacts with hydrogen to form ZrCoH (0–3) below 400°C, the disproportionation reaction, 2ZrCo + H 2 → ZrH 2 + ZrCo 2 occurs above 400°C under hydrogen pressure higher than the equilibrium decomposition pressure of ZrCoH (0–3) . X-ray diffraction analysis showed that the reaction is completed in 5 h at 500°C and it takes more than 40 h at 400°C. No sign of generation of ZrH 2 + ZrCo 2 phase was observed by the X-ray diffraction in 10 h at 400°C, however. The disproportionated product completely returned to single-phase ZrCo when evacuated at or above 500°C for several hours, thus this reaction is reversible. Kinetics of the disproportionation reaction was expressed by an Avrami equation R = 1 − exp{−( t / π ) n }. The reaction speed was expressed by an Arrhenius form. For practical application of ZrCo, the results suggest some operational precaution and limitation, and recovery treatment when the material is suspected to have lost some capacity by disproportionation reaction.

Development of zirconium-cobalt beds for recovery, storage and supply of tritium

Abstract A ZrCo intermetallic compound is proposed as the material for recovery, storage and supply of gaseous tritium. It is a substitute for the uranium that has been extensively used for this purpose in spite of its restricted handling as a nuclear fuel material. Pressure-composition isotherms of a ZrCoH2 system were investigated and this material proved to be applicable and advantageous. Some ZrCo beds were designed, fabricated and tested for actual use. A bed for the handling of large amounts of hydrogen isotopes was developed. The test establishes ZrCo beds as practical components for tritium service.

A zirconium-cobalt compound as the material for a reversible tritium getter

An intermetallic compound of zirconium-cobalt was prepared, and the pressure-composition isotherms for the ZrCo-H system were measured in the pressure range from 10 Pa (0.1 Torr) to approx. =130 kPa (1000 Torr) and in the temperature range from 130 to 400/sup 0/C. The equilibrium hydrogen pressures of plateaus under the the experimental conditions were one or two orders of magnitude higher than that of a uranium-hydrogen system. A pronounced hysteresis between hydrogenating isotherms and dehydrogenating ones was observed. Based on the results, ZrCo is proposed as a substitute for uranium for the purpose of recovering, storing, and supplying gaseous tritium.

Correlation behavior of lithium and tritium in some solid breeder materials

Abstract Tritium behavior in candidate solid breeder materials such as Li 2 O, Li 2 SiO 3 or γ-LiAlO 2 has been clarified to have a close relation with the diffusion of lithium by the analysis of spin-lattice relaxation with nuclear magnetic resonance and ionic conductivity. The conductivity measurement is a simple and brief technique and can be performed even in a in situ condition under irradiation.

Electrical conductivity of a sintered pellet of octalithium zirconate

Abstract The conductivity of a sintered pellet of Li 8 ZrO 6 was measured in dry or wet argon atmosphere by the two-terminal technique with an AC Wheatstone bridge in the temperature range 350–1050 K. The temperature dependence of conductivity multiplied by temperature (σT) of Li 8 ZrO 6 in a dry environment had two transition points at 426 and 833 K. The activation energies of conductivity in high, middle and low temperature regions were 103.2, 52.1 and 37.4 kJ/mol, respectively. The activation energies of conductivity agreed well with those obtained by temperature dependence of spin-lattice relaxation time T 1 of 7 Li with pulsed nuclear magnetic resonance.

Thermal expansion and solubility limits of plutonium-doped lanthanum zirconates

Abstract Plutonium-doped and undoped lanthanum zirconates were prepared by solid state reaction. The solubility limit of Pu in La 2 Zr 2 O 7 was studied by X-ray diffraction, and the solubility limits of Pu in pyrochlore-type phase prepared in air, vacuum and Ar–8%H 2 atmospheres were determined to be 10, 35 and 100 mol%, respectively. The compound with 80 mol% Pu prepared in air and the compounds with 80 and 100 mol% Pu prepared in vacuum showed single fluorite-type phases. Thermal expansion coefficients of (La 1− x Pu x ) 2 Zr 2 O 7+ y ( x =0, 0.05, 0.10 and 0.80) were also measured by high-temperature X-ray diffraction at temperatures between room temperature and 1273 K in air and He–8%H 2 atmospheres and were found to increase with increasing Pu content. The dependence of the linear thermal expansion coefficients on Pu content is discussed from the point of view of the radii of both tetravalent Pu in air and trivalent Pu in He–8%H 2 .

Simultaneous ion and gas driven permeation of deuterium through nickel

Abstract The permeation rate of deuterium through nickel exposed to a deuterium ion beam has been measured as a function of temperature (100–1000°C), incident ion flux ( 0.2−1.1 × 10 15 D-atom cm −2 s −1 ), incident ion energy (1–2.5 keV/D-atom) , and deuterium partial pressure. The results are analyzed with a model which is developed to describe ion driven permeation when gas driven permeation coexists; the rate-limiting process of recycling during ion implantation is identified as surface recombination above 400°C and as bulk diffusion below 300°C, and the recombination coefficient is estimated. The model predicts the whole parameter dependence observed. The permeation spike is also investigated.

High temperature phase transitions of CaTiO3 and (Ca0.85Nd0.15)TiO3 by X-ray diffractometry and differential thermal analysis

Abstract The two phase transitions of CaTiO3 and (Ca0.85Nd0.15)TiO3 were studied by high temperature X-ray diffraction and differential thermal analysis in the temperature range 300–1723 K. The presence of the two kinds of phase transitions in CaTiO3 and (Ca0.85Nd0.15)TiO3 at about 1390 K and about 1530 K recently found by the heat capacity measurement in our laboratory was confirmed. From the X-ray diffraction patterns of CaTiO3 and (Ca 0.85Nd0.15)TiO3, the first phase transition at lower temperature was due to the change from the orthorhombic Pbnm to the orthorhombic Cmcm structure, and the second one at higher temperature was due to change from Cmcm to the cubic Pm3m structure, originating from the sequential rotations of the TiO6 octahedra about one of the axes. Since no clear peak was seen at the phase transition around 1390 K in the DTA curve in contrast with the presence of the endothermic peak at the transition around 1530 K, it is thermodynamically considered that the former and the latter transition were second- and first-order phase transition, respectively.

Thermoelectric properties of Rh-doped Ru2Si3 prepared by floating zone melting method

Precipitation-free samples of Rh-doped Ru 2 Si 3 were prepared by the floating zone (FZ) method, The temperature dependences of the electrical resistivities and the Seebeck coefficients of Rh-doped Ru 2 Si 3 (Rh content = 0, 4, 6 mol%) were measured. The electrical resistivities of both 4% and 6% Rh-doped Ru 2 Si 3 were smaller than those of undoped Ru 2 Si 3 prepared by the FZ method and 4% Rh-doped one prepared by other methods. The maximum value of the Seebeck coefficients for all samples was -175 μV/K at 673 K for 4% Rh-doped Ru 2 Si 3 . The dimensionless thermoelectric figure of merit reached 0.8 for 4% Rh-doped Ru 2 Si 3 at 1073 K, which was about 50% larger than that of optimized n-type SiGe.

EXAFS analyses of CaTiO3 doped with Ce, Nd and U

Abstract The EXAFS (extended X-ray absorption fine structure) analyses were carried out for Ti–K, Ce–L3, Nd–L3 and U–L3 edges to make clear the local structures in (Ca 1− x Ce x )TiO 3 ( x =0.10, 0.20), (Ca 1− x Nd x )TiO 3 ( x =0.05, 0.10, 0.15, 0.20) and (Ca 1− x U x )(Ti 1−2 x Al 2 x )O 3 ( x =0, 0.05). With increasing neodymium content in (Ca 1− x Nd x )TiO 3 the first nearest titanium–oxygen distances decreased. On the other hand, with increasing cerium content in (Ca 1− x Ce x )TiO 3 , the first nearest titanium–oxygen distances increased. The first nearest titanium–oxygen distance in (Ca 1− x U x )(Ti 1−2 x Al 2 x )O 3 was larger than that in undoped CaTiO 3 . With increasing dopant content, the first nearest dopant (cerium or neodymium)–oxygen distances increased. From the result of the compositional dependencies of bonding length (cation–oxygen distance), which is inversely proportional to the bonding energy, it is estimated that magnitude of the leaching rate of Ce-doped CaTiO 3 is larger than those of Nd-doped and undoped CaTiO 3 . The experimental results on the leaching rates of doped and undoped CaTiO 3 were consistent with this estimation. The structural change of (Ca 0.80 Ce 0.20 )TiO 3 after a leaching test in HCl at 363 K for 140 days was also measured by X-ray diffraction and XANES analysis.