Radiocesium interaction with clay minerals; Theory and simulation advances Post-Fukushima

Okumura, Masahiko; Kerisit, S.*; Bourg, I. C.*; Lammers, L. N.*; Ikeda, Takashi*; Sassi, M.*; Rosso, K. M.*; Machida, Masahiko

Journal of Environmental Radioactivity, 189, p.135 - 145, 2018/09

https://doi.org/10.1016/j.jenvrad.2018.03.011

no abstracts in English

Reply to "Comments on radiation-damage resistance in phyllosilicate minerals from first principles and implications for radiocesium and strontium retention in soils"

Sassi, M.*; Rosso, K. M.*; Okumura, Masahiko; Machida, Masahiko

Clays and Clay Minerals, 65(5), p.371 - 375, 2017/11

https://doi.org/10.1346/CCMN.2017.064078

no abstracts in English

Origin of 6-fold coordinated aluminum at (010)-type pyrophyllite edges

Okumura, Masahiko; Sassi, M.*; Rosso, K. M.*; Machida, Masahiko

AIP Advances (Internet), 7(5), p.055211_1 - 055211_9, 2017/05

https://doi.org/10.1063/1.4983213

no abstracts in English

Issues concerning the determination of solubility products of sparingly soluble crystalline solids; Solubility of HfO(cr)

Rai, D.*; Kitamura, Akira; Rosso, K. M.*; Sasaki, Takayuki*; Kobayashi, Taishi*

Radiochimica Acta, 104(8), p.583 - 592, 2016/08

https://doi.org/10.1515/ract-2015-2540

Solubility studies were conducted with HfO(cr) solid as a function of acid concentrations. These studies involved (1) using two different amounts of the solid phase, (2) acid washing the bulk solid phase, (3) preheating the solid phase to 1400 C, and (4) heating amorphous HfO(am) suspensions to 90 C to ascertain whether the HfO(am) converts to HfO(cr) and to determine the solubility from the oversaturation direction. Based on the results of these treatments it is concluded that the HfO(cr) contains a small fraction of less crystalline, but not amorphous, material [HfO(lcr)] and this, rather than the HfO(cr), is the solubility-controlling phase in the range of experimental variables investigated in this study. The solubility data are interpreted using both the Pitzer and SIT models. The log of the solubility product of HfO(cr) is estimated. The observation of a small fraction of less crystalline higher solubility material is consistent with the general picture that mineral surfaces are often structurally and/or imperfect leading to a higher solubility than the bulk crystalline solid. This study stresses the urgent need, during interpretation of solubility data, of taking precautions to make certain that the observed solubility behavior for sparingly-soluble solids is assigned to the proper solid phase.

Shell evolution beyond =40; Cu

Sahin, E.*; Doncel, M.*; Sieja, K.*; De Angelis, G.*; Gadea, A.*; Quintana, B.*; Grgen, A.*; Modamio, V.*; Mengoni, D.*; Valiente-Dobn, J. J.*; et al.

Physical Review C, 91(3), p.034302_1 - 034302_9, 2015/03

https://doi.org/10.1103/PhysRevC.91.034302

Large-scale molecular dynamics simulations of cesium diffusion in systems with clay particles and water molecules

Hiraguchi, Atsuki; Zheng, X.*; Underwood, T. R.*; Kobayashi, Keita; Yamaguchi, Akiko; Itakura, Mitsuhiro; Machida, Masahiko; Rosso, K. M.*; Bourg, I. C.*; Okumura, Masahiko

no journal, , 

Understanding the radionuclide diffusion phenomena is crucial for the safe geological disposal of high-level radioactive waste. The diffusion in clay-water systems is particularly important for the performance of the artificial barrier made of bentonite. Numerical simulation is one of the best research methods for understanding the phenomenon at the microscopic level. Recently, large-scale molecular dynamics (MD) simulations of the systems with clay particles and water molecules were realized. In this presentation, we will show numerical simulation results of diffusion of cesium in the large system with MD. Our recent results suggest that cesium is less diffuse than sodium.

Free energy landscapes of cesium adsorption from mesopore to the interlayer of montmorillonite; Classical molecular dynamics simulation studies

Hiraguchi, Atsuki; Zheng, X.*; Underwood, T. R.*; Kobayashi, Keita; Yamaguchi, Akiko; Itakura, Mitsuhiro; Machida, Masahiko; Rosso, K. M.*; Bourg, I. C.*; Okumura, Masahiko

no journal, , 

The migration behavior of radionuclides in clay minerals used as buffer material must be clarified for the long-term safety of geological disposal of high-level radioactive waste. The recent development of supercomputers enables us to perform large-scale classical molecular dynamics (MD) simulations to evaluate the adsorption properties of cation to clay minerals. We evaluated the free-energy profiles of cesium adsorption from mesopore to the interlayer of Na-montmorillonite by classical MD on a supercomputer. We found that the free-energy profiles of cesium adsorption depend on the interlayer distance of the Na-montmorillonite and mesopore salinity in the water system. In the presentation, we also discuss the distribution and diffusion coefficients of cesium ions.

Molecular dynamics investigation of the temperature dependence of cesium adsorption on montmorillonite

Hiraguchi, Atsuki; Zheng, X.*; Underwood, T. R.*; Kobayashi, Keita; Yamaguchi, Akiko; Itakura, Mitsuhiro; Machida, Masahiko; Rosso, K. M.*; Bourg, I. C.*; Okumura, Masahiko

no journal, , 

Bentonite is the primary buffer material to inhibit radionuclide diffusion in geological disposal of high-level radioactive waste. Cesium ions are strongly adsorbed by montmorillonite, the main component of bentonite. However, the detailed adsorption states and mechanisms remain unclear. We conducted classical molecular dynamics simulations of the cesium adsorption on a Na-montmorillonite at 300 K, 330K, 350 K, and 370 K to investigate the temperature dependence of cesium adsorption by evaluating free energy profiles. Our simulation showed that cesium ions consistently adsorb within the interlayer and form inner-sphere complexes at all temperatures. This temperature-independent behavior supports bentonite's reliability as a buffer material for high-level radioactive waste even at high temperatures.

Molecular dynamics simulations of free energy profiles of cesium ions on adsorption to interlayers of Na-montmorillonite with different composition

Hiraguchi, Atsuki; Zheng, X.*; Underwood, T. R.*; Kobayashi, Keita; Yamaguchi, Akiko; Itakura, Mitsuhiro; Machida, Masahiko; Rosso, K. M.*; Bourg, I. C.*; Okumura, Masahiko

no journal, , 

The migration behavior of radionuclides in clay minerals used as buffer material must be clarified for the long-term safety of geological disposal of high-level radioactive waste. We can investigate diffusion/adsorption behavior of cesium ion and sodium ion in clay minerals and water systems at molecular scale using molecular dynamics simulation on a supercomputing system. In this study, we evaluated the free energy profiles of cesium and sodium ions adsorption to the interlayer of Na-montmorillonite from bulk water in mesopore. As a result, we found that the minimum of free energy profiles of these cations locates in the interlayer of Na-montmorillonite, and the minimum of free energy profile of cesium is lower than that of sodium. This result shows that the affinity of cesium ion to Na-montmorillonite is stronger than that of sodium ion. Moreover, we confirmed that the affinities of both cations to Na-montmorillonite decrease with salinity increase, and these cations maintain their adsorption mechanism. In the presentation, we discuss layer charge and layer size dependence on the free energy profiles of these cations.

Free energy calculations of cesium ion adsorption to the interlayer of montmorillonites using classical molecular dynamics

Hiraguchi, Atsuki; Zheng, X.*; Underwood, T. R.*; Kobayashi, Keita; Yamaguchi, Akiko; Itakura, Mitsuhiro; Machida, Masahiko; Rosso, K. M.*; Bourg, I. C.*; Okumura, Masahiko

no journal, , 

The migration behavior of radionuclides in clay minerals used as buffer material must be clarified for the long-term safety of geological disposal of high-level radioactive waste. We can investigate diffusion/adsorption behavior of cesium ion and sodium ion in clay minerals and water systems at molecular scale using molecular dynamics simulation on a supercomputing system. In this study, we evaluate the free energy profiles of the cesium and sodium ions adsorption to the montmorillonite interlayer from bulk water in mesopore. We found that the free energies in the interlayer of Na-montmorillonite are smaller than those in the bulk water for the cesium and sodium ions, and the averaged free energy value for cesium in the interlayer is lower than that for sodium. This result shows that cesium ion is more strongly adsorbed to the interlayer of Na-montmorillonite than sodium ion. Moreover, we found differences in the shape of these free energy profiles in the interlayer. The free energy profile for the cesium ion has a spiky structure with many local minimums, although that for the sodium ion is almost uniform. These results suggest that the cesium ion forms the inner-sphere complex at these local minimums and the sodium ion forms the outer-sphere complex. Furthermore, we investigated salinity dependence on the free energy profiles. We confirmed that the affinities of both cations to Na-montmorillonite decrease with increasing salinity, and the adsorption mechanism of these cations does not depend on salinity. In the presentation, we discuss the free energy profiles with different layer charges, interlayer distances, etc.