Tetsuji Yamaguchi

Diffusivity of U, Pu and Am carbonate complexes in a granite from Inada, Ibaraki, Japan studied by through diffusion

Abstract The diffusivities of uranium, plutonium and americium in Inada granite have been determined using through-diffusion method. Experiments were performed at (25±1)°C in a 0.1 mol l −1 NaHCO 3 solution where the actinides are present as carbonate or hydroxy–carbonate complexes. Effective diffusivity ( D e ) values of (1.42±0.24)×10 −13 m 2 s −1 and (5.1±2.0)×10 −14 m 2 s −1 were obtained for uranium and plutonium, respectively. Diffusion through 5 mm thick granite was not observed for americium within the experimental period of 366 days. The D e value for uranyl carbonate species obtained in these experiments was found to be four times higher than the previously obtained D e value for the uncomplexed uranyl ion.

Effective diffusivity of the uranyl ion in a granite from Inada, Ibaraki, Japan

The effective diffusivity of uranium(VI) in Inada granite has been determined by through-diffusion. Experiments were performed at room temperature (20–25°C) in a 0.1 mol 1−1 KCl solution where uranium is present predominantly as the poorly sorbing UO22+. An effective diffusivity (De) of (3.6 ± 1.6) × 10−14 m2 s−1 was obtained, close to that for uranine (nonsorbing organic tracer), but one order of magnitude lower than those obtained for Sr2+ and NpO2+, and two orders of magnitude lower than that obtained for I−. According to well established theory, a proportional relationship exists between De and the diffusivity in the bulk of the solution (Dv). The effective diffusivity obtained in granite was not proportional to Dv. This agrees with results obtained for effective diffusivity in a Swedish granite. The ratio DeDv was found to be not constant but increased with De or Dv. This result suggests a limit to the application of the theory.

XPS Study of Pb(II) Adsorption on γ-Al2O3 Surface at High pH Conditions

Lead(II) adsorption on γ-Al2O3 at high pH condition (11 <pH<13) was studied in NaNO3 solutions. With increasing pH from 11, Pb(II) uptake by γ-Al2O3 was decreased regardless to sodium concentration (0.1 and 1.0 M). XPS spectra showed no chemical shifts occurred in substrate Al (2s, 2p) and O (1s) binding energy by Pb(II) adsorption. Binding energy of adsorbed Pb (4f) had no dependence on pH and adsorption density and was higher than that of orthorhombic yellow PbO and Pb(NO3)2. Molecular orbital (MO) calculation revealed lowest unoccupied molecular orbital (LUMO) energy of Pb(OH)- 3 was about 6 eV higher than that of Pb(OH)2, and suggested Pb(OH)- 3 was less reactive than Pb(OH)2 with deprotonated surface hydroxyl ligand. These results suggest that Pb(II) is adsorbed on γ-Al2O3 as inner-sphere surface complex form at high pH condition. Although Pb(II) is adsorbed on γ-Al2O3 as inner-sphere surface complex, bonding between Pb(II) and γ-Al2O3 shows less covalent nature.

Diffusive transport of neptunium and plutonium through compacted sand-bentonite mixtures under anaerobic conditions

Diffusive transport of neptunium, plutonium, tritiated water (HTO), Cs+ and I- in compacted sand-bentonite mixtures was studied by a through-diffusion method. Experiments for Np were performed in the presence of carbonate where Np is stable as NpIV(CO3)2(OH)22- and those for Pu in the presence of fulvic acid where the Pu is stable as fulvic complexes. These experiments were performed under Ar (pO2 < 10-6 atm). Effective diffusivity (De) values of (1.81 ± 0.03) × 10-10, (1.8 ± 0.8) × 10-10, (5.1 ± 0.8) × 10-11 and (9.0 ± 4.1) × 10-11 m2 s-1 were obtained for HTO, Cs+, I- and Np(CO3)2(OH)22-, respectively. The ratio of the De to the diffusivity in bulk of the water was around 0.1 for Np(CO3)2(OH)22-, HTO and Cs+, which is consistent with the pore diffusion model. Observed diffusive transport of Pu was much smaller than those of HTO, Cs+, I- and Np(CO3)2(OH)22- probably because Pu was present as colloidal forms and that confined pore space in the compacted sand-bentonite mixtures does not allow diffusive transport of colloidal plutonium.

Interactions between anionic complex species of actinides and negatively charged mineral surfaces

Summary Sorption of actinides onto negatively charged mineral surfaces was investigated under conditions that actinides were predominantly present as anionic complex species. U(VI), Np(V) and Sn(IV) were prepared as 1 mol kg-1 Na-ClO4/OH solutions and equilibrated with γ-Al2O3 at pcH 11-13.6 where these elements form UO2(OH)42-, NpO2(OH)2- and Sn(OH)5- species, respectively. Sorption of Np(IV), Th(IV) and Am(III) on amorphous SiO2 and on γ-Al2O3 was investigated in 0.03-0.3 mol kg-1 Na-HCO3/CO3 solutions where these actinides are expected to be stable as anionic carbonate complexes. Distribution coefficients of U(VI), Np(V), Sn(IV), Th(IV) and Am(III) decreased with the increasing pH or with the increasing carbonate concentrations. The monotonous decrease in the distribution coefficients in the investigated pcH range suggests that anionic complex species of actinides were not sorbed on negatively charged mineral surfaces. A proportional relation was found between the experimentally obtained Kd values and calculated mole fraction of electrically neutral species in equilibrated solutions. Sorption occurs under the condition that both actinides and mineral surfaces are negatively charged because minor neutral species are sorbed on mineral surfaces.

The Solubility of Metallic Selenium under Anoxic Conditions

The solubility of metallic selenium was measured in a mixture of 0.1M-NaCl and 0.05M-N 2 H 4 under anoxic conditions (O 2 4 2 at pH between 10 and 13, by UV-Vis absorption spectrometry. The solid phase was identified as Se (cr) by X-ray diffraction. The equilibrium constants of Se(cr) + H + + 2e - = HSe - logK 0 = -6.5±0.5 and 4Se(cr) + 2e - = Se 4 2- logK 0 = -16.8±0.5 were determined. The standard molar free energy of formation of HSe - and Se 4 2- was determined to be (37.1±2.9) and (95.9±2.9) kJ/mol, respectively.

An attempt to select thermodynamic data and to evaluate the solubility of radioelements with uncertainty under HLW disposalconditions

Abstract An attempt was made to select thermodynamic data with uncertainties and to evaluate the solubility of radioelements with uncertainties considering the uncertainties of the thermodynamic data and variation in groundwater chemistry. The thermodynamic data were selected by reviewing the JAEA-TDB released in 2012 from the viewpoint of consistency of the data selection process, the conservativeness of the solubility evaluation and testing up-to-date data. A Monte Carlo-based probabilistic solubility calculation code, PA-SOL, was used for the uncertainty analysis of the solubility. The uncertainty analysis for the solubility of 13 elements showed large uncertainties for Nb, Tc and U. The variations in pH, pe and total carbonate concentration assumed in this study induce significant uncertainties to the evaluated solubility. The reactions whose log K0 with uncertainties induce significant uncertainties to the evaluated solubilities were identified.

SrI 2 Diffusion Experiment in Granite From Underground Research Laboratory in Manitoba, Canada

It is important to quantify the diffusion of radionuclides into pores in rock and to understand the diffusion mechanism to evaluate the migration of radionuclides in deep geological formations. The diffusivity of ions into the rock matrix is usually estimated using a theory for the diffusion in porous material, in which a proportional relationship exists between the effective diffusivity in rock and the diffusivity in the bulk of the solution. In this study, diffision of nonsorbing Γ and sorbing Sr 2+ was observed simultaneously and the results were compared.