Yasufumi Suzuki

Stability and structure of the δ phase of the U-Zr alloys

Abstract Homogeneity range and crystal structure of the intermediate δ phase in the UZr alloy were examined by electron probe microanalysis, X-ray diffraction and differential thermal analysis, using the alloys prepared by arc-melting. The homogeneity range of the δ phase was found to be 64.2–78.2 at% Zr at 600°C and 66.5–80.2 at% Zr at 550°C. The powder diffraction patterns of the δ phase agreed with an unusual structure of modified C32, in which the (0,0,0) sites are preferentially occupied by Zr atoms, and the ( 2 3 , 1 3 , 1 2 ) and ( 1 3 , 2 3 , 1 2 ) sites randomly shared by U and Zr atoms.

Distribution behavior of plutonium and americium in LiCl–KCl eutectic/liquid cadmium systems

Abstract The thermodynamics of plutonium and americium in LiCl–KCl eutectic/liquid cadmium systems was studied with interest in the oxidation state of americium in the salt phase. The standard potential of plutonium vs. the Ag/AgCl (1 wt% AgCl) electrode, E 0 Pu/Pu(III) , in the LiCl–KCl eutectic was measured in the temperature range of 400–500°C and given by the equation with a standard deviation, σ =0.0009 V: E 0 Pu/Pu(III) (V)=−2.204+0.000845 T (K). The Ag/AgCl electrode had been carefully calibrated using the Li–Al electrode. The potential of the cadmium containing plutonium and americium, E Cd , was measured at 500°C as a function of the distribution coefficient ( D : mole fraction in salt divided by mole fraction in cadmium), and represented by the following equations. over the range of E Cd >−1.45 V: E Cd =−1.360 (±0.004)+0.0511 log D Am =−1.348 (±0.002)+0.0511 log D Pu . It is indicated that americium as well as plutonium is present in the trivalent oxidation state in the salt under this condition. Based on the potential data, the activity coefficient of plutonium in liquid cadmium and the separation factor of americium relative to plutonium were determined to be (1.74±0.28)×10 −4 and 1.77±0.46, respectively. Under the reducing conditions (i.e. E Cd E Cd and log D Am , indicates that divalent americium is possibly present in the salt phase.

Electrode reaction of plutonium at liquid cadmium in LiCl–KCl eutectic melts

The electrode reaction of the Pu3+/Pu couple at the surface of a liquid Cd electrode was investigated by cyclic voltammetry at 723, 773 and 823 K in LiClKCl eutectic melt. It was found that the diffusion of Pu3+ in the salt phase was a rate-determining step in the cathodic reaction, but the dissolution of Pu from the liquid Cd phase was so slow that the anodic wave broadened slightly. The redox potentials of the Pu3+/Pu couple at the liquid Cd electrode at 723, 773 and 823 K were observed at more positive potentials, 0.299, 0.269 and 0.239 V, respectively, than those at a solid Mo electrode. This potential shift was thermodynamically analyzed by a lowering of activity of Pu in the Cd phase due to the formation of the alloy PuCd6 at the interface. As well as the Gibbs energy of formation of PuCd6, the Gibbs energies of formation of the other intermetallic compounds PuCd2 and PuCd4 could also be estimated by the analysis of the anodic peaks in cyclic voltammograms of the Pu3+/Pu redox couple at a Cd-coated Mo electrode.

Dependence of the thermal conductivity of (U,Pu)N on porosity and plutonium content

Abstract The dependence of the thermal conductivity of (U, Pu)N on porosity and plutonium content was investigated from 680 to 1600 K. The modified Maxwell-Eucken equation was applied to the experimental results for (U0.8Pu0.2)N pellets with 82–95% of theoretical density. The coefficients β in the equation were separately estimated for the pellets sintered at different temperatures and for those sintered with pore former particles. The plutonium content dependence was examined by use of (U1−xPux)N pellets with x = 0, 0.2, 0.35, 0.6, 0.8 and 1.0. Prominent decrease in thermal conductivity with plutonium content was found in the UN-rich region and the temperature dependence diminished with the increase of plutonium content.

Thermophysical and thermodynamic properties of actinide mononitrides and their solid solutions

Abstract The recent investigations on the thermophysical and thermodynamic properties of actinide mononitrides, which are essential for the development of nitride fuel, are described. The thermal conductivities of UN, NpN and PuN are found to have a tendency to increase gradually with temperature, while to decrease with the atomic number of the actinide element component because of the decrease of electronic contribution. The solid solutions of the mononitrides show a temperature dependence similar to those of two component nitrides each and possess intermediate thermal conductivity values. Knudsen–effusion mass spectrometric measurements of UN, NpN, and PuN have been performed. Predominant vapor species observed were only elemental metals except nitrogen gas. It is considered that UN and NpN form liquid phases and show the partial pressures of metal gas species close to those over pure respective metals, while PuN evaporates congruently.

Vaporization behaviour of (Pu,Am)N

Abstract An unusual vaporization behaviour of plutonium and americium from a reactor grade PuN sample was found in a Knudsen cell effusion mass spectrometric analysis, where initially the mass-239 signal was much lower than the thermodynamic expectation while the mass-241 signal was significantly large. The observation was adequately explained with a thermodynamic model of (Pu,Am)Ni 1− x . The analysis was based on a working hypothesis that the Gibbs free energy of formation of AmN is not so different from those of UN, PuN and LaN. The agreement of the calculation and observation supported this hypothesis. The second-law enthalpy of formation of AmN was estimated to be −294 kJ mol −1 at 1600 K from the measured vapour pressure of Am over the PuN sample.

Vaporization behavior of uranium-plutonium mixed nitride

The vaporization behavior of UN, PuN and uranium-plutonium mixed nitride, (U, Pu)N, was observed by a Knudsen-effusion mass spectrometry. The mixed nitride samples with the wide PuN composition range were prepared by the homogenization of the mixtures of UN and PuN synthesized by the carbothermic reduction. The results of evaporation for UN and PuN agree reasonably with those reported previously. The partial pressures of U and Pu over mixed nitride were depressed by the formation of the solid solution of UN and PuN. The estimation of the activities and activity coefficients of UN and PuN in mixed nitride revealed that mixed nitride does not behave as an ideal solution. It is also suggested that mixed nitride evaporates congruently in the temperature range of the present study without the formation of any molten phase in a condition of Knudsen effusion.

Vaporization behavior of neptunium mononitride

Vaporization behavior of NpN(s) was investigated by mass spectrometry with a Knudsen-cell in the temperature range of 1690–2030 K. It is suggested from the vapor pressure measurements that NpN(s) decomposed into Np(1) and N2(g). The free energy of formation of NpN(s) was evaluated by using the partial pressures of Np(g), the decomposition pressures of N2(g) reported previously and the free energy of formation of Np(g). The free energy of formation of NpN(s) obtained lay between those of UN and PuN reported in the literature.

Electrolysis of plutonium nitride in LiCl-KCl eutectic melts

Abstract The electrolysis of plutonium nitride, PuN, was investigated in the LiCl–KCl eutectic salt with 0.54 wt% PuCl3 at 773 K in order to understand the dissolution of PuN at the anode and the deposition of metal at the cathode from the viewpoint of the application of a pyrochemical process to nitride fuel cycle. It was found from cyclic voltammetry that the electrochemical dissolution of PuN began nearly at the theoretically evaluated potential and this reaction was irreversible. Several grams of plutonium metal were successfully recovered at the molybdenum electrode as a deposit with a current efficiency of about 90%, although some fractions of the deposited plutonium often fell from the molybdenum electrode.

X-ray Debye temperature and Grüneisen constant of NpO2

Abstract The Debye temperature, Θ D , and the Gruneisen constant, γ , are useful parameters to estimate thermal properties of ceramics. In the present study, those for NpO 2 were determined through a high-temperature X-ray diffraction analysis. The measurements were carried out in the temperature range from room temperature up to 1174 K. The temperature factor of Np at each measured temperature was refined by Rietveld analysis to obtain Θ D , assuming that the factor of O is the same as in UO 2 . The calculated Θ D values tended to decrease with increasing temperature, which was attributed to the anharmonicity of lattice vibration. From the decrease, we determined γ using Paskins temperature calibration which corrects the effect of anharmonicity. The values of Θ D and γ obtained in this study were 435±2 K and 1.93±0.03, respectively.