Kazutaka Kawamura

X-ray and neutron diffraction and molecular dynamics simulation of molten lithium and rubidium nitrates

Molecular dynamics simulations have been performed for lithium and rubidium nitrate melts at 550 and 600 K, respectively, together with X-ray and neutron diffraction experiments. Simple Coulomb pair potentials with Born-type repulsions have been adopted in the simulations with a rigid body model for the nitrate ion. Structure functions derived from the X-ray and neutron experiments are well reproduced by the simulations, from which the three-dimensional cation distribution around the nitrate ion has been revealed. The self-diffusion coefficients, the velocity autocorrelation functions and the self-exchange velocities of lithium, rubidium and nitrate ions have been calculated. Anisotropic motion of nitrate ions has been found and is discussed on the basis of the structure of the melts.

Structural changes of amorphous Pd80Si20 with neutron irradiation, with an interpretation in terms of the imperfections in the structure of amorphous solids

Abstract Wide and small-angle X-ray scattering measurements were made, as well as differential scanning calorimetry, for amorphous Pd80Si20 before and after irradiation by neutrons (5 × 1020 neutrons/cm2, > 1 Mev), and the structural changes thereby produced are described. Before irradiation the structure is well assimilated with a dense random packing (DRP) structure except that the coherence between neighboring Pd atoms extends farther than expected from the DRP structure, the coherent length estimated from the X-ray scattering being about 8 A. With irradiation the coherent regions of 8 A diameter get separated from each other by a mean distance of 20 A, thus producing structural inhomogeneities which do not exist at all before irradiation. The glass transition temperature is found to be increased by about 10 K after irradiation. The effects of irradiation are interpreted as being due to the structural imperfections produced by particle bombardment. The DPR structure is adopted as a model structure for the perfect amorphous solids.

Structural determination of molten NaNO3, NaNO2 and their eutectic mixture by molecular dynamics simulation and X-ray diffraction

Molecular dynamics (MD) simulation was carried out for the molten NaNO3, NaNO2 and their eutectic mixture systems to compare their simulated structures with those determined experimentally by X-ray diffraction. As for the pure NaNO3 and NaNO2, parameters for repulsion potential were adjusted so that the structure factors of their simulated systems coincide reasonably with the corresponding experimental ones. The MD simulation of the mixture was carried out with the parameters determined for the component salts. The structure factor of the simulated mixture system successfully reproduced the experimental one, which shows validity of the proposed pair potential. The assignment of most of the peaks in the experimental correlation function was performed with the aid of simulated results. The simulated data has been used to determine the diffusion coefficients of the component ions.

The effect of 14 MeV neutron irradiation on tritium recovery from lithium oxide

Post-irradiation experiments were performed to study the effect of 14 MeV neutron irradiation on the recovery of tritium from sintered lithium oxide. The chemical state distribution of tritium produced in the lithium oxide was found to depend on the neutron fluence because of the influence of radiation defects. When the lithium oxide was heated with a helium sweep gas, tritium was recovered mainly as HTO and its recovery was controlled by bulk diffusion of tritium in the T+ state except in an early period of recovery. The diffusion coefficient was expressed as D = 4.4 exp [−143 (kJ mol−1)/RT] cm2 s−1. When hydrogen was added to the sweep gas, on the other hand, a considerable amount of tritium was recovered as HT. The HT recovery was analyzed with first-order kinetics using the activation energy of 137 kJ mol−1. The reactions of tritium with F+ centers may be responsible for the observed behaviour.

Internal cation mobilities in the molten binary system Li2CO3K2CO3

Abstract Internal mobility ratios of two cations in the molten binary system (Li, K)(CO 3 ) 0.5 have been measured with the Klemm method. From these and available data on the densities and conductivities, the internal mobilities of Li + and K + have been calculated. Except at a region of relatively high concentration of Li 2 CO 3 , the Chemla effect occurs, that is, the mobility of the large cation K + is greater that of the small one Li + . The internal mobilities of Li + ions are well expressed by b Li = [ A /( V − V 0 )] exp (− E/RT ), where A , V 0 and E are constants characteristic of the cation and independent of temperature T , V the molar volume of mixtures, and R the gas constant. Negative deviations from this equation at a small V region may be attributed to the free space effect. For b K such a simple equation does not hold over an extended region of concentration, which suggests that the agitation effect and the free space effect would also play a role for the mobility.

Thermal behaviour and crystallization kinetics of amorphous CuTi alloys

Etude de la structure partiellement et pleinement cristallisee des alliages amorphes Cu 1−x Ti x avec 0,35≤x≤0,65, par chauffage continu jusqua une temperature determinee de la courbe de calorimetrie differentielle a balayage. Determination des energies dactivation

The effect of fusion neutron irradiation on tritium recovery from lithium salts

Abstract The effect of 14 MeV neutron irradiation on tritium recovery was studied from the lithium salts LiF, LiCl, LiBr, Lil, LiF-BeF 2 and LiF-PbF 2 . The chemical state distribution of tritium formed in the irradiated lithium salts was found to depend on the neutron fluence. The dependence was explained by taking into account the interactions of tritium with radiation defects. In the tritium recovery experiments, the effects of irradiation were apparently observed with the highly irradiated samples; the TX recovery with a helium sweep gas and the HT recovery with a hydrogen-containing sweep gas were well described by the reaction kinetics in which F centers play an important role. In the case of the less-irradiated samples, on the other hand, the tritium recoveries were found to follow a diffusion-controlled kinetics and were explained on the basis of atomic theory of diffusion.

Diffusion behaviors of tritium formed in lithium-lead alloys by 14 MeV neutron irradiation

Abstract Tritium recovery experiments were performed with Li-Pb alloys irradiated with 14 MeV neutrons. The effects of temperature, alloy composition, and neutron fluence were observed on tritium recovery. Tritium was released mainly as HT from the alloy and its release rate was controlled by bulk diffusion in the alloy. From the chemical analysis of tritium species in the alloy, it was found that free tritium atoms in the T 0 state diffuse more rapidly than the others such as tritide ions in the T − state. The diffusion mechanism for each species was discussed based on the obtained diffusion coefficients. It was also found that free tritium atoms might be trapped by radiation defects produced in the alloy.

Diffusion of hydrogen in amorphous Cu45Ti55 ribbon

Abstract The diffusion of hydrogen in amorphous Cu 45 Ti 55 ribbon and in a crystalline TiH 2 ue5f8Cu phase produced by the decomposition of Cu 45 Ti 55 H x was investigated in the temperature range 360–583 K using the evolution method. The rate of evolution of hydrogen was related to the amount of hydrogen absorbed in the amorphous specimens. The activation energy for the rate of evolution of hydrogen was much larger in the crystalline TiH 2 ue5f8Cu phase than in the amorphous phase.

Radiation damage of amorphous alloy by heavy ion

Abstract According to a molecular dynamics method with some sophistications, radiation effects of amorphous Pd 80 Si 20 alloy were simulated. Structural changes detected experimentally were reproduced and explained in the simulation. Then cascade process was observed as a result of analysis in detail.