Radioactive waste

FOREWORD

Radioactive waste originates mainly from the nuclear industry but also from other industrial and medical activities. The most internationally accepted solution for the long-term isolation of high-level waste (half lives > 30 years) is the geological disposal at depth (500 -1000 m) in suitable rock formations (low permeability). Low- and intermediate-level waste are disposed either at depth or in surface installations designed for such purpose. The Spanish site for the disposal of low-level radioactive waste is located in El Cabril (Córdoba).

Hydrogeological and geochemical processes can affect the eventual mobility of radionuclides and also the properties of the rocks hosting the repositories. Investigation of theses processes are usually performed within international projects, and they cover both engineering barriers (near field) and rock formations (far field). Our group has been active in these projects since the 1990’s.

Management of a low-level facility must take into account several risk vectors, including the potential release of radionuclides into the subsurface. The long-term behavior of the system once active monitoring ends is also a concern.

Topics

Diffusion and retention in clay rock
Diffusion and retention in crystalline rock
Interaction between Portland cement and rock; high-pH solutions
Flow in fissured rocks
Flow upscaling in anisotropic complex geological media
Multiphase flow and transport
Transport of radionuclides facilitated by colloids

EXAMPLE (Appl. Geochem. 26, 1115-1129; 27, 2096-2106)

Interaction between water and a cement-grouted fracture

Grouting of water-conducting fractures with low-alkali cement is foreseen by Posiva (Finnish nuclear waste management agency) for the potential future repository for high-level nuclear waste in Finland (ONKALO). A possible consequence of the interaction between groundwater and grout is the formation of high-pH solutions which will be able to react with the host rock and engineering-barrier materials, altering their mineralogy and porosity. Calculations were performed simulating the interaction between flowing water and grout and the alteration of the host rock (gneiss) as this water flowed beyond the grouted section of the fracture. The calculations included the hydration and simultaneous leaching of the grout through diffusive exchange between the porewater in the grout and the flowing water in the fracture. The formation of an alkaline plume was extremely limited when the low-pH grout was used. And even when using a grout with a lower silica fume content the extent and magnitude of the alkaline plume were rather minor.

Results show that after grouting with low-pH cement, the duration of the initial high-pH peak is short (< 0.5 a), which compares well with observations at a test borehole. Mg in the groundwater induces the precipitation of brucite at the grout-fracture interface, which consumes OH-. In the longer term, the results show a gradually decaying pH tail (pH < 9) controlled by the precipitation of calcite at the grout-fracture interface. The duration of this tail correlates inversely with the carbonate content of the inflowing groundwater. A major outcome of this study is that mineral precipitation controls the formation of a potential high pH plume by consuming alkalinity and limiting diffusive solute exchange between the grout and the circulating groundwater.

EXAMPLE (Eur. J. Min., in press)

Interaction between concrete and clay rock in a borehole

Hydrated cement and concrete are major components of the engineered barrier system in proposed underground repositories for radioactive waste. Concrete was in contact with a clay-rich rock during 15 years in a borehole at the Tournemire Underground Rock Laboratory in France. Overcoring of the borehole and mineralogical analyses have shown a reduction of porosity at the interface due to the precipitation of ettringite, C-S-H/C-A-S-H and calcium carbonate, together with dissolution of portlandite in the cement.

In the framework of the GTS-LCS project (JAEA, Japan; NAGRA, Switzerland; NDA, UK; POSIVA, Finland; SKB, Sweden), new reactive transport modeling (solute diffusion + mineral reaction) has been performed, including a sensitivity analysis with respect to several compositional and kinetic parameters. Results using the CrunchFlow code show sealing of porosity at the rock side of the interface (mm scale) due to the precipitation of C-A-S-H (calcium aluminum silicate hydrate), calcite and ettringite, together with some clay dissolution. The location of sealing is influenced by cation exchange. Inclusion of cation exchange results in sealing at the rock side of the interface. Without cation exchange, sealing is at the concrete side of the interface.

Calculated alteration profiles along cm-scale fractures show increased alteration distances. Sealing of porosity is at the concrete side of the interface, due to the smaller effect of cation exchange.

EXAMPLE (Appl. Geochem. 33, 191-198)

Modeling of Cs+ diffusion and retention in the Opalinus Clay

In the DI-A2 experiment at Mont Terri several non-reactive and reactive tracers were injected as a pulse in a packed-off borehole in the Opalinus Clay. Unlike the previous DI-A1 test, the design of the Teflon filter in the injection borehole forced the water to flow through the filter and the open space between the filter and the borehole wall (the filter itself did not act as a diffusion barrier between the circulating solution and the rock). The decrease in tracer concentration in the liquid phase was monitored during a period of one year. Afterwards, the borehole section was overcored and the tracer profiles in the rock were analyzed. A main interest of this experiment was to understand the chemical behavior of sorbing tracers: Cs+ (stable), 85Sr2+, 60Co2+ and Eu3+ (stable). The complete dataset (except for Eu3+ because of strong sorption to experimental equipment) was analyzed in a previous study with a 2D diffusion-reaction model and the derived diffusion and sorption parameters were compared with laboratory data. As in DI-A1, a difference by a factor of about 2 was obtained for the sorption capacity of Cs+ between in-situ and laboratory batch sorption experiments.

Recent experimental and modeling studies have shown equivalent Cs+ sorption on intact and disaggregated Opalinus Clay samples. In view of these developments, new modeling of Cs+ diffusion and retention in the DI-A2 experiment has been performed using CrunchFlow. The calculations include transport by diffusion and a cation exchange model to account for the retention of Cs+. The new results show that upscaling of Cs+ sorption from laboratory to field is not required any longer.

Modeling a complex hydrogeological system in fractured media

Long-term behavior of covers

Transport of radionuclides facilitated by colloids

We developed a formulation for colloid-facilitated transport with transient colloidal concentrations and non-linear equilibrium sorption of contaminants on colloids. It acknowledges the finite number of sorption sites on colloid surfaces and the temporal and spatial variations of carrier concentrations. Mass transfer mechanisms between soil matrix/colloids, and soil matrix/radionuclides are linear. This formulation shows that the non-linear equilibrium model, which accounts for the temporal and spatial variations of colloid concentrations, can simulate the double peak (colloidal and dissolved) as well as the reduced contaminant retardation factor often observed in contaminant breakthrough curves.

RECENT PUBLICATIONS

Soler J. M. (2013) Reactive transport modeling of concrete-clay interaction during 15 years at the Tournemire Underground Rock Laboratory. European Journal of Mineralogy, accepted. Soler J. M., Wersin P., Leupin O. X. (2013) Modeling of Cs+ diffusion and retention in the DI-A2 experiment (Mont Terri). Uncertainties in sorption and diffusion parameters. Applied Geochemistry 33, 191-198.
Jokelainen L., Meski T., Lindberg A., Soler J. M., Siitari-Kauppi M., Martin A., Eikenberg J. (2013) The determination of 134Cs and 22Na diffusion profiles in granodiorite using gamma spectroscopy. Journal of Radioanalytical and Nuclear Chemistry 295, 2153-2161.
Soler J. M. (2012) High-pH plume from low-alkali-cement fracture grouting. Reactive transport modeling and comparison with pH monitoring at ONKALO (Finland). Applied Geochemistry 27, 2096-2106.
Yi S., Samper J., Naves A., Soler J. M. (2012) Identifiability of diffusion and retention parameters of anionic tracers from the diffusion and retention (DR) experiment. Journal of Hydrology 446-447, 70-76.
Yi S., Samper J., Naves A., Soler J. M. (2012) Inverse estimation of the effective diffusion of the filter in the in situ diffusion and retention (DR) experiment. Transport in Porous Media 93, 415-429.
Marty N. C. M., Cama J., Sato T., Chino D., Villiéras F., Razafitianamaharavo A., Brendlé J., Giffaut E., Soler J. M., Gaucher E. C., Tournassat C. (2011) Dissolution Kinetics of Synthetic Na-Smectite. An Integrated Experimental Approach. Geochim Cosmochim. Acta 75, 5849-5864.
Soler J. M., Vuorio M., Hautojärvi A. (2011) Reactive Transport Modeling of the Interaction between Water and a Cementitious Grout in a Fractured Rock. Application to ONKALO (Finland). Applied Geochemistry 26, 1115-1129.
Savage D., Soler J. M., Yamaguchi K., Walker C., Honda A., Inagaki M., Watson C., Wilson J., Benbow S., Gaus I., Rueedi J. (2011) A Comparative Study of the Modelling of Cement Hydration and Cement–Rock Laboratory Experiments. Applied Geochemistry 26, 1138-1152.
Soler J. M., Mäder U. K. (2010) Cement-Rock Interaction: Infiltration of a High-pH Solution into a Fractured Granite Core. Geologica Acta 8, 221-233.
M. Gran, J. Carrera, M.W. Saaltink, C. Ayora, C. Bajos and M. Rey. Design and Monitoring System of a Pilot Waste Cover, 8th ICARD Int. Conference 2009, Skelleftea, Sweden.