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Neptunium(V) / Diffusion / Sorption / Montmorillonite / ModelingSummary. Diffusion and sorption of radionuclides in compacted bentonite/montmorillonite are key processes in the safe geological disposal of radioactive waste. In this study, the effects of carbonate and salinity on neptunium(V) diffusion and sorption in compacted sodium montmorillonite were investigated by experimental and modeling approaches. Effective diffusion coefficients (D e ) and distribution coefficients (K d ) of 237 Np(V) in sodium montmorillonite compacted to a dry density of 800 kg m −3 were measured under four chemical conditions with different salinities (0.05/0.5 M NaCl) and carbonate concentrations (0/0.01 M NaHCO 3 ). D e values for carbonate-free conditions were of the order of 10 −10 -10 −11 m 2 s −1 and decreased as salinity increased, and those for carbonate conditions were of the order of 10 −11 -10 −12 m 2 s −1 and showed the opposite dependence. Diffusion-derived K d values for carbonate-free conditions were higher by one order of magnitude than those for carbonate conditions. Diffusion and sorption behaviors were interpreted based on mechanistic models by coupling thermodynamic aqueous speciation, thermodynamic sorption model (TSM) based on ion exchange, and surface complexation reactions, and a diffusion model based on electrical double layer (EDL) theory in homogeneous narrow pores. The model predicted the experimentally observed tendency of D e and K d qualitatively, as a result of the following mechanisms; 1) the dominant aqueous species are NpO 2 + and NpO 2 CO 3 − for carbonate-free and carbonate conditions, respectively, 2) the effects of cation excess and anion exclusion result in opposite tendencies of D e for salinity, 3) higher carbonate in solution inhibits sorption due to the formation of carbonate complexes.

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Leachabilities and solubilities of the synthesized iodide sodalite and natural (chloride) sodalite were measured by leach test. The iodide sodalite was synthesized in nitrogen gas flow at 800°C for 2 hours. The crystalline structure of the product was certified by XRD analysis. The natural sodalite containing chlorine was obtained at Bancroft, Canada. The several types of solution were used to evaluate the influence of the solubility of sodalite that included the varied pH and the chemical compositions.The solubilities of chloride sodalite were calculated by the thermodynamics data. The solubility measured for the synthesized iodide sodalite was compared with that calculated. The solubility of the synthesized iodide sodalite was approximately 2 x 10 -4 mol/L, which shows a good agreement with the calculated one of the natural sodalite.

To perform a safety assessment for the geological disposal of radioactive waste, it is important to understand the response characteristics of the disposal system. In this study, approximate analytical solutions for steady-state nuclide releases from the engineered barrier system (EBS) of a repository were derived for an orthogonal one-dimensional diffusion model. In these approximate analytical solutions, inventory depletion, decay during migration and the influence of groundwater flow in the excavation damaged zone (EDZ) were considered. These solutions were simplified by the Taylor theorem in order to clearly represent the response characteristics of the EBS. The validity of these solutions was shown by comparison with numerical solutions. The response characteristics of the EBS are useful for identifying target values for important parameters that would have the effect of improving the robustness of system safety. The robustness of the geological disposal system and the reliability of the safety assessment can thus potentially be improved using the approximate analytical solutions.

Effective diffusivities of iodine, chlorine, and carbon in mixtures of bentonite and sand were determined by measuring the effective diffusivities of common chemical compounds labeled with radioactive isotopes of these elements. For carbon, both inorganic and organic carbon compounds were used in order to consider the variety of chemical forms of carbon possible in a radioactive waste repository. The bentonite content and dry density of the bentonite–sand mixture were varied. Two chemically different aqueous solutions, representing concrete pore water and bentonite pore water, were used to represent different conditions that could affect diffusivity in bentonite buffer material in a hypothetical radioactive waste disposal situation.The effective diffusivities of iodine, chlorine, and carbon tended to decrease with increasing bentonite content and dry density of the mixture. In the presence of simulated concrete pore water, the effective diffusivities for iodine, chlorine, and carbon in the bentonite mixtures were not higher than those obtained when simulated bentonite pore water was used. Except for some organic compounds, the measured effective diffusivities were lower than that of tritiated water under the same experimental conditions. This was attributed primarily to exclusion of anions from the bentonite pores. The effective diffusivity of carbon depended on its chemical form. The effective diffusivity of the anionic forms of organic carbon tested (carboxylic acids ) was as low as that of inorganic anionic carbon.Measured effective diffusivities were compared with those calculated using a model based on electrical double layer theory. The theory was applied to calculate distributions of electrolyte ions and diffusing ions in the bentonite pores. The calculated effective diffusivities showed good agreement with the measured values.

Part of TRU waste includes a large amount of nitrate salt, the effects of which have to be evaluated in a safety assessment of co-location disposal with high level radioactive waste (HLW). High concentrations of nitrate ions from TRU waste might affect the solubility of different radionuclides in the HLW. In the current study, the effects of nitrate salt on radionuclide solubility were investigated experimentally. Solubility experiments of important and redox sensitive radionuclides, Tc(IV), Np(IV) and Se(0), were performed using various concentrations of sodium nitrate (NaNO3) and of Np(V) in NaNO3 solutions to investigate complex formation with NO3− ions. Solubility experiments of Pd(II), Sn(IV) and Nb(V) using ammonium chloride (NH4Cl) solution were also undertaken to investigate complex formation with NH3/NH4+ ions. No significant solubility enhancement was observed for Np and Se. Tc solubility in ≥0.1 mol/dm3 NaNO3 solution increased due to oxidation by nitrate ions. An increase of Np(V) solubility was expected by the chemical equilibrium model calculation with JNC-TDB, however, solubility enhancement by complex formation of Np(V) with nitrate ions was not observed. Solubility enhancement by complex formation of Sn and Nb were also not observed, only Pd solubility was increased by complex formation with NH3/NH4+ ions.

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