[en] To reveal the nature of the oxygen defects in actinide, lanthanide oxides, and their solid solutions, the relations between the molar ratio of oxygen to metal, O/M, and the oxygen potentials of these oxides in our previous studies were reviewed. These relations were then compared with those between O/M and the oxygen potentials in the literature, with reference to the phase relations and the constituent cationic valences and radii of these oxides. Consequently, we found consistent properties in the relations between O/M and the oxygen potentials of actinide, lanthanide oxides, and their solid solutions, in addition to the dependence of the relations on the crystal structures (e.g., fluorite-type, pyrochlore-type, and cubic sesquioxide-type)

[en] Correlation of crystal structure with electric field gradient (EFG) in the fluorite- and pyrochlore-type compounds in the Gd₂O₃-ZrO₂ system Gd_xZr_1-xO_2-x/2 with 0.18≤x≤0.62 were investigated by ¹⁵⁵Gd Moessbauer spectroscopy, powder X-ray diffraction and point-charge model (PCM) calculation. An intermediate ordered pyrochlore phase forms for 0.45≤x≤0.55, sandwiched with a disordered fluorite phase for 0.18≤x<0.45 and 0.55< x≤0.62. Some ¹⁵⁵Gd Moessbauer parameters, especially the quadrupole coupling constant (e²qQ), were found to exhibit a characteristic maximum around the ideal-pyrochlore Gd₂Zr₂O₇ (x=0.50) composition. The validity of the proposed pyrochlore-based structural model was examined by comparing the experimental values of EFG at the Gd sites with those calculated by the PCM calculations

[en] We have studied chemical properties of fluorite-type structure compounds by using powder XRD and Moessbauer spectroscopy. ¹⁵¹Eu Moessbauer spectroscopic study on mixed oxide, U_1-yEu_yO_2-x revealed that oxygen coordination number around Eu³⁺ was 8 for 0< y<0.5 and decreased smoothly with increase in y for y>0.5. ²³⁸U Moessbauer spectroscopy was applied to this system. The γ-ray source used for ²³⁸U Moessbauer spectroscopy was ²⁴²PuO₂ (²⁴²Pu: 99.99%). 44.9 keV ²³⁸U Moessbauer γ-ray is distinguished from the 59.5 keV γ-ray from the decay of ²⁴¹Am isotope which is the daughter of isotopic impurity ²⁴¹Pu by using a Ge detector. The magnetic splitting of ²³⁸U Moessbauer spectra for UO₂ was observed as obvious broadening of the line shapes below the Neel temperature, 30K. (author)

[en] The debris composed of U, Pu and the structural materials of the nuclear reactors is being cooled and corroded in several types of aqueous solution in the Unit 1 through 3 of the Fukushima Daiichi Nuclear Power Plants. The chemical and transport behaviors of Pu and U in the corroded debris must be understood for the criticality safety control of Pu and U in the debris, especially for the removal operations and storage. Therefore, the chemical changes of UO_2 and PuO_2 powders, disks and U and Pu metal disks in 10 wt% H_2O_2 aqueous solution have been checked, where H_2O_2 is formed by the radiolysis of H_2O. Each sample was kept in a vial bottles filled with 10 wt% H_2O_2 aqueous solution for a month at 50° Celsius. As a result, the UO_2 powder and disk changed to the yellow slurry. The slurry was dried and identified to be hydrated uranium peroxide by the powder x-ray diffraction (XRD) method, whereas the PuO_2 powder and disk remained unchanged. U metal was more reactive with H_2O_2 aqueous solution than Pu metal. The chemical changes of the mixed UO_2/PuO_2 powders in H_2O_2 aqueous solution have been investigated. The equimolar amounts of UO_2 and PuO_2 powders were mixed and ground in an agate mortar. The mixture was kept in a vial bottle filled with 10 wt% H_2O_2 aqueous solution for a month at 50° Celsius. The slurries sampled from the middle and bottom zones of the vial bottle were dried and identified by the powder XRD method. The dried slurry of the middle zone was mainly composed of hydrated uranium peroxide, whereas the dried powder of the bottom zone was mainly composed of PuO_2. It was found that UO_2 in the H_2O_2 aqueous solution is more reactive and mobile than PuO_2. (author)

[en] For the remote analysis of a next generation nuclear fuel material containing minor actinide (MA), we applied laser-induced breakdown spectroscopy (LIBS) to uranium oxide (U_3O_8), which included a small amount of neodymium oxide (Nd_2O_3), as a simulated sample of MA. By using a deconvolution technique for the Nd spectra in U, we separated the complex, overlapped, and confused spectra and determined their actual intensities. As a result, a calibration curve with good linearity and a detection limit of less than 700 ppm were demonstrated. (author)

[en] Research and development of laser based quick analysis without chemical analysis and neutron measurement for next-generation Minor Actinide containing MOX fuel has been carried out, and the basic performances by using un-irradiated MOX fuel were demonstrated. The glove box had been re-constructed and specialized for laser spectroscopy, and the remote spectroscopy of MOX sample contained several concentrations of Pu was performed. In elemental analysis by Laser Induced Breakdown Spectroscopy (LIBS) with high resolution spectrometer, relative error of 2.9% at 30% Pu and the detection lower limit of 2500ppm in natural U oxide were demonstrated with the operation time of 5 min. In isotope ratio analysis by Ablation Resonance Absorption Spectroscopy, tunable semiconductor laser system was constructed, and the performances such as relative deviation less than 1% in the ratio of "2"4"0Pu/"2"3"9Pu and the sensitivity of 30-100ppm in natural U were also accomplished with laser operation time of 3 to 5min. As for an elemental analysis of the simulated liquid sample, ultra-thin laminate flow was experimented as LIBS target, and the sensitivity comparable to conventional ICP-AES was confirmed. Present study includes the result of the entrusted project by the Ministry of Education, Culture, Sports, Science and Technology of Japan (MEXT). (author)

[en] Electrode reactions of U(VI) in weak acid solution are different from those in acid solution. Electrolysis of U(VI) can produce UO₂ fine crystal, not hydrolyzed U(IV). The formed UO₂ is easily deposited on an electrode and is electrode active. (author)

[en] Pyrochlore-type (P-type) zirconia has been widely studied as a disposable form of the high-level radioactive nuclear waste. We have developed the EMF (electromotive force) measurement system using the cubic zirconia oxygen sensor, and investigated the relation between oxygen potential (g(O₂)), oxygen composition (x) and temperature (T) for oxygen-excess pyrochlore Zr₂Pu₂O_7+x between 798K and 1078K over 0.422Ce₂O_7+x. It was found that g(O₂) of P-type Zr₂Pu₂O_7+x are significantly higher than those of oxygen deficient fluorite PuO_2-x at the same values of the oxygen to metal molar ratio (O/M) or the molar ratio of Pu(Ce)³⁺ ions to the total Pu(Ce)³⁺Pu(Ce)⁴⁺ ions(y). g(O₂) of P-type Zr₂Pu₂O_7+x are about -150 kJ/mol lower than those of P-type Zr₂Ce₂O_7+x at the same values of O/M or y, though the shape of g(O₂) versus x curves resembles each other over the range of x investigated in this study. These results are consistent with the fact that g(O₂) of PuO_2-x are about -150 kJ/mol lower than those of CeO_2-x at the same values of O/M or y, and their g(O₂) curves resemble each other. (author)

[en] For applying Laser Induced Breakdown Spectroscopy (LIBS) to the analysis of nuclear fuel materials, it is very important to identify the emission spectrum and its intensity on impurities intermingled within complex emission spectra of matrix elements such as uranium (U) and plutonium (Pu). Then, the spectra of natural uranium are measured using LIBS, 165 atomic spectra and 381 single ion spectra were identified. (author)

[en] The formation of MPd_3+x (M = Gd, Np) by the reaction of MN with Pd at 1323 K in Ar gas flow was observed. Cubic AuCu₃-type GdPd_3.3 (a = 0.4081 ± 0.0001 nm) and NpPd₃ (a = 0.4081 ± 0.0001 nm) were identified, respectively. The product obtained from the reaction of NpN with Pd contained additional phases, including the hexagonal TiNi₃-type NpPd₃. Chlorination of the MPd_3+x (M = Gd, Np) samples was accomplished by the solid-state reaction using cadmium chloride at 673 K in a dynamic vacuum. (author)