Using natural systems evidence to test models of transformation of montmorillonite

Savage, D.*; Wilson, J.*; Benbow, S.*; Sasamoto, Hiroshi; Oda, Chie; Walker, C.*; Kawama, Daisuke*; Tachi, Yukio

Applied Clay Science, 195, p.105741_1 - 105741_11, 2020/09

https://doi.org/10.1016/j.clay.2020.105741

Safety functions for the clay buffer in a repository for high-level radioactive waste (HLW) are fulfilled if the presence of montmorillonite with high swelling capacity and low permeability is maintained in the long-term. The transformation of montmorillonite to the non-swelling mineral likely illite is addressed in most safety assessments by using simple semi-empirical kinetic models, but this approach contrasts with more complex reactive-transport simulations. In the present study, reactive-transport simulations are compared with simple semi-empirical kinetic models. Results suggest that reactive-transport simulations err on the side of conservatism, but may produce unrealistic estimates of illitization. This comparison demonstrates that reactive-transport models may be carefully applied to simulate the long-term evolution of near field environment for HLW disposal.

A Comparative study of the modelling of cement hydration and cement-rock laboratory experiments

Savage, D.*; Soler, J. M.*; Yamaguchi, Kohei; Walker, C.; Honda, Akira; Inagaki, Manabu; Watson, C.*; Wilson, J.*; Benbow, S.*; Gaus, I.*; et al.

Applied Geochemistry, 26(7), p.1138 - 1152, 2011/07

https://doi.org/10.1016/j.apgeochem.2011.04.004

The use of cement and concrete as fracture grouting or as tunnel seals in a geological disposal facility for rad wastes creates potential issues concerning chemical reactivity. From a long term safety perspective, it is desirable to be able model these interactions and changes quantitatively. As part of the LCS (Long-term Cement Studies) project programme, a modelling inter-comparison has been conducted, involving the modelling of two experiments describing cement hadration and cement-rock reaction, with teams representing the NDA (UK), Posiva (Finland), and JAEA. This modelling exercise showed that the dominant reaction pathways in the two experiments are fairly well understood and are consistent between the different modelling teams, although significant differences existed amongst the precise parameterisation. Future modelling exercises of this type should focus on a suitable natural or industrial analogue that might aid assessing mineral-fluid reactions at these longer timescales.

Multiple Scenarios of Bentonite Alteration

Oda, Chie; Sasaki, Ryoichi; Honda, Akira; Savage, D.*; Arthur, R. C,*

JNC TN8400 2005-020, 39 Pages, 2005/09

JNC-TN8400-2005-020.pdf:3.16MB

The multiple scenarios of bentonite alteration were ideveloped based on the possible mineralogical changes derived from knowledge of both experiments and observation of natural systems. It was focused that the mineral reaction involving hyperalkaline fluid thermodynamically depends on the variable chemical condition in bentonite buffer and that kinetics is important as well as thermodynamic stability in controlling their occurrence, i.e., the kinetic controls operate to remain metastable minerals over the long term. The mineralogical consequences of the interaction between clays and alkaline fluid are summarized as follows. / -Clay and gel illite / -Clay and gel metastable zeolite stable zeolite and feldspar / -Clay and gel stable zeolite and feldspar The typical minerals for each category (illite group, metastable zeolite group and stable zeolite and feldspar group) were selected. The scenarios give the range of potential secondary minerals occurring in chemical schemes of the bentonite alteration by cement pore fluids, and thus reasonable assumptions on the simulation of the chemical and mineralogical evolutions of geological disposal system for TRU waste.

An Analysis of Bentonite-Cement Interaction and Evolution of Pore Water Chemistry

Oda, Chie; Honda, Akira; Sasaki, Ryoichi; Savage, D.*

Extended Abstractof International Workshop on Bentonite-Cement Interaction in Repository Environments, 7 Pages, 2004/04

The possible multiple scenarios of chemical-mineralogical evolution in Bentonite buffer were considered on the basis of the information of both experimental results and observations of natural systems. The range of variable chemical conditions according to each scenario was examined in terms of the potential for locally-attained chemical equilibrium and kinetics. These analyses have been intended to provide reasonable assumptions on simulations using chemistry-transport-hydrology coupling models that aims at estimating the possible evolution of hydraulic condition of bentonite in a TRU waste repository.

Compilation of Kinetic Data for Geochemical Calculations

Arthur, R. C,*; Savage, D.*; Sasamoto, Hiroshi; Shibata, Masahiro; Yui, Mikazu

JNC TN8400 2000-005, 61 Pages, 2000/01

JNC-TN8400-2000-005.pdf:2.83MB

Kinetic data, including rate constants, reaction orders and activation energies, are compiled for 34 hydrolysis reactions involving feldspars, sheet silicates, zeolites, oxides, pyroxenes and amphiboles, and for similar reactions involving calcite and pyrite. The data are compatible with a rate law consistent with surface reaction control and transition-state theoly, which is incorporated in the geochemieal software package EQ3/6 and GWB. Kinetic data for the reactions noted above are strictly compatible with the transition-state rate law only under far-from-equilibrium conditions. It is possiblethat the data are conceptually consistent with this rate law under both far-from-equilibrium and near-to-equilibrium conditions, but this should be confirmed whenever possible through analysis of original experimental results, Due to limitations in the availability of kinetic data for mineral-water reactions, and in order to simplify evaluations of geochemical models of groundwater evolution, it is convenient to assume local-equilibrium in such models whenever possible. To assess whether this assumption is reasonable, a modeling approach accounting for coupled fluid flow and water-rock interaction is described that can be used to estimate spatial and temporal scale of local equiliblium. The approach is demonstrated for conditions involving groundwater flow in fractures at JNC's Kamaishi in-situ tests site, and is also used to estimate the travel time necessary for oxidizing surface waters to migrate to the level of a HLW repository in crystalline rock. The question of whether local equilibrium is a reasonable assumption must be addressed using an appropriate modeling approach. To be appropriate for conditions at the Kamaishi site using the modeling approach noted above, the fracture fill must closely approximate a porous medium, groundwater flow must be purely advective and diffusion of solutes across the fracture-host rock boundary must not occur. Moreover, the ...

Geochemical Site-Selection Criteria for HLW Repositories in Europe and North America

Savage, D.*; Arthur, R. C,*; Sasamoto, Hiroshi; Shibata, Masahiro; Yui, Mikazu

JNC TN8400 2000-003, 56 Pages, 2000/01

JNC-TN8400-2000-003.pdf:1.96MB

Geochemical as well as socio-economic issues associated with the selection of potential sites to host a high-level nuclear waste repository have received considerable attention in repository programs in Europe (Belgium, Finland, France, Germany, Spain, Sweden, Switzerland and the U.K.) and North America (Canada and the United States), The objective of the present study is to summarize this international experience with particular emphasis on geochemical properties that factor into the adopted site-selection strategies. Results indicate that the geochemical properties of a site play a subordinate role, at best, to other geotechnical properties in the international site-selection approaches. In countries where geochemical properties are acknowledged in the site-selection approach, requirements are stated qualitatively and tend to focus on associated impacts on the stability of the engineered barrier system and on radionuclide transport. Site geochemical properties that are likely to control the lomg-term stability of geochemical conditions and radionuclide migration behavior are unspecified, however. This non-prescriptive approach may be reasonable for purposes of screeing among potential sites, but a better understanding of site properties that are most important in controlling the long-term geochemical evolution of the site over a range of possible scenarios would enable the potential sites to be ranked in terms of their suitability to host a repository.

Alteration scenario for bentonite under hyperalkaline conditions with regard to mineral paragenetic uncertainty

Oda, Chie; Sasaki, Ryoichi; Arthur, R. C,*; Savage, D.*; Honda, Akira

no journal, , 

Bentonite materials are expected to be used as the main hydraulic barrier and cementitious materials are expected to be used for structure support in low level radioactive waste disposal systems. Interaction between bentonite and hyperalkaline fluids arising from cementitious materials has been recognized as a concern due to the close proximity of these materials in repository designs. The interaction could cause the alteration of mineralogy and associated hydraulic property of bentonite and have a deleterious influence on the function of bentonite as a hydraulic barrier, and may lead to adverse impact on the long-term safety of the repository. Many researches on such interaction have been performed to establish the bentonite alteration scheme, however, uncertainty in our understanding of the precise alteration scheme, especially mineral paragenetic sequences during the alteration, still remains. Therefore, it is important to take such uncertainty into account in estimating how the mineralogy and the hydraulic property of bentonite evolves subject to alteration by hyperalkaline fluids. In this work, multiple scenarios for mineralogical alteration of bentonite was investigated to limit the mineral paragenetic uncertainty and to identify appropriate assumption for numerical simulation of chemical and mineralogical evolution of bentonite under hyperalkaline conditions over the long-term.

Groundwater patterns in a Playa Lake Basin, California; Analogue studies for geological disposal

Gallardo, A.*; Savage, D.*; Benbow, S.*; Takase, Hiroyasu*; Oda, Chie

no journal, , 

A numerical model was constructed to understand the groundwater flow dynamics and salinity distribution in the Searles Lake, California, a dry and alkaline playa lake, as a natural analogue for processes in a potential geological repository. The result shows that vertical flow would prevail at the center of the lake, and under this condition, salts would be dissolved from shallow evaporitic horizons being redistributed downwards and away from the lake surface. Concentrations up to 220 g/L would occur within an area approximately 6 by 4 km, extending to a depth of 200 m after 200,000 years. Although several uncertainties remain, the model is consistent with many observations, and provides a framework to address geochemical processes in the basin.

Reactive transport modelling of iron-bentonite interface evolution

Wilson, J.*; Watson, C.*; Benbow, S.*; Savage, D.*; Sasamoto, Hiroshi

no journal, , 

Iron steel overpack will be surrounded by bentonite buffer as a engineering barrier system for the high-level radioactive waste disposal. It is concerned that iron-bentonite interactions result in the alteration of montmorillonite to non-swelling Fe-rich minerals. In the present study, reactive transport modeling of iron-bentonite interface evolution has been conducted to evaluate the long-term behavior of bentonite stability. A number of model cases were produced in order to assess which processes are likely to dominate at iron-bentonite interfaces.