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Funaishikawa, Japan

Iijima K.,Japan Atomic Energy Agency | Tomura T.,Inspection Development Co. | Shoji Y.,Japan Atomic Energy Agency | Shoji Y.,JDC Corporation
Applied Clay Science | Year: 2010

In the geological disposal system with stable colloidal particles, the influence of the colloidal particles on the migration of radionuclides has to be evaluated. Not only the distribution coefficient, which is a measure for adsorption of radionuclides on colloidal particles, but also the reversibility of adsorption should be determined. In this study, adsorption and desorption of Cs+ ions on colloidal montmorillonite particles were investigated in the wide range of the initial Cs+ concentrations. A rapid, almost linear and reversible adsorption of Cs+ on montmorillonite was observed at basic condition, indicating that the adsorption of Cs+ on montmorillonite was mostly dominated by ion exchange. The size of the montmorillonite particles slightly affected the distribution coefficients at lower Cs+ concentrations, suggesting the contribution of surface complexation. The ion exchange and surface complexation model reproduced the adsorption and desorption behavior of Cs+. To explain the fixation of Cs+, montmorillonite was conditioned with the solutions of various Cs+ concentrations and the basal spacing was measured by X-ray diffraction (XRD). Conditioning montmorillonite at higher Cs+ concentrations >5×10-3M decreased the basal spacing which may result in the fixation of Cs+ in the interlayer space. Since the adsorption experiments were carried out at lower Cs+ concentration <10-4M, Cs+ adsorption was reversible. © 2010 Elsevier B.V.

Kato M.,Japan Atomic Energy Agency | Takeuchi K.,Japan Atomic Energy Agency | Uchida T.,Japan Atomic Energy Agency | Sunaoshi T.,Inspection Development Co. | Konashi K.,Tohoku University
Journal of Nuclear Materials | Year: 2011

The oxygen potential of (U0.88Pu0.12)O 2±x (-0.0119 < x < 0.0408) and (U 0.7Pu0.3)O2±x (-0.0363 < x < 0.0288) was measured at high temperatures of 1673-1873 K using gas equilibrium method with thermo gravimeter. The measured data were analyzed by a defect chemistry model. Expressions were derived to represent the oxygen potential based on defect chemistry as functions of temperature and oxygen-to-metal ratio. The thermodynamic data, ΔGO2, ΔHO2 and ΔSO2, at stoichiometric composition were obtained. The expressions can be used for in situ determination of the oxygen-to-metal ratio by the gas-equilibration method. The calculation results were consistent with measured data. It was estimated that addition of 1 wt.% Pu content increased oxygen potential of uranium and plutonium mixed oxide by 2-5 kJ/mol. © 2010 Elsevier B.V. All rights reserved.

Nakamichi S.,Japan Atomic Energy Agency | Kato M.,Japan Atomic Energy Agency | Tamura T.,Inspection Development Co.
Calphad: Computer Coupling of Phase Diagrams and Thermochemistry | Year: 2011

Oxygen potentials of (Am 0.024Pu 0.311U 0.665)O 2-x and (Am 0.01Np 0.12Pu 0.29U 0.58)O 2-x in the near-stoichiometric region were measured at 1473, 1573 and 1623 K by thermogravimetry and the influences of Am and Np on the MOX fuels were evaluated. The measurements were carried out by a gas equilibrium method using an (Ar, H 2,H 2O) gas mixture. Oxygen potentials of (Am 0.024Pu 0.311U 0.665)O 2-x were slightly higher than those of (Pu 0.3U 0.7)O 2-x while those of (Am 0.01Np 0.12Pu 0.29U 0.58)O 2-x and (Pu 0.3U 0.7)O 2-x were almost the same. It was concluded that oxygen potentials of MOX fuels were slightly increased by a small amount of Am and not affected by Np. © 2011 Elsevier Ltd. All rights reserved.

Doi R.,Japan Atomic Energy Agency | Uchikoshi K.,Inspection Development Co. | Beppu H.,Inspection Development Co.
Journal of Nuclear Science and Technology | Year: 2016

To determine the equilibrium constant for ferroselite (FeSe2(cr)) dissolution reaction, FeSe2(cr) solubility experiments were performed at 298 ± 1 K from both the over- and under-saturation directions with Fe–Se precipitates that were aged at 348 K. X-ray diffraction (XRD) analysis detected only FeSe2(cr) as the Se solid phase in the equilibrated precipitates. The Eh values of the equilibrated suspensions ranged from −188.6 to −4.9 mV vs. standard hydrogen electrode (SHE) and the pH values ranged from 6.00 to 8.76. Based on the available thermodynamic data, Se4 2− and Fe2+ are thermodynamically stable within this Eh–pH range. Agreement between the solubility data obtained from the over- and under-saturation directions lends credence to the attainment of equilibrium at 298 ± 1 K. The thermodynamic interpretations using the specific ion interaction theory (SIT) model showed that Eh values and the concentrations of Se and Fe are well represented by the 2FeSe2(cr) solubility reaction (2FeSe2(cr) ⇌ 2Fe2+ + Se4 2− + 2e−) with log10K○ = −17.09 ± 0.28. The obtained log10K○ value falls within the uncertainty limits of the log10K○ value calculated from the available thermodynamic data. © 2016 Atomic Energy Society of Japan. All rights reserved.

Takeuchi K.,Japan Atomic Energy Agency | Kato M.,Japan Atomic Energy Agency | Sunaoshi T.,Inspection Development Co.
Journal of Nuclear Materials | Year: 2011

The shrinkage of (U 0.8, Pu 0.2)O 2±x pellets was investigated with the help of a thermal dilatometer in isothermal and isochronal heating tests. During shrinkage measurements in isothermal heating, the oxygen-to-metal ratio of the pellets was maintained at a constant value by controlling the oxygen potential in the sintering atmosphere. The influence of the oxygen-to-metal ratio on the sintering behavior was evaluated from the measurement results. Mainly two mechanisms dominated the sintering of mixed oxide pellets. When the oxygen-to-metal ratio was close to the stoichiometric composition, pellet shrinkage progressed at low temperatures of 1200-1600 K, and the shrinkage rate of the pellets drastically changed with a small deviation from stoichiometric composition. The result showed that a diffusion process was dominated during the sintering of near-stoichiometric compositions. On the other hand, the sintering of reduced mixed oxide pellet proceeded at high temperatures of 1600-1900 K, and the shrinkage rate was very low as compared with stoichiometric mixed oxide. © 2010 Elsevier B.V. All rights reserved.

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