Do elevated temperatures and organic matter influence the U(VI) diffusion through argillaceous rock?

The suitability of argillaceous rock as host rock and backfill material in a nuclear waste repository is discussed worldwide. In a nuclear waste repository several factors have to be considered for safety assessment. Beside high radiotoxic nuclides, such as neptunium and plutonium, the finally stored high-level radioactive waste will contain also a high amount of uranium, which originates mainly from spent nuclear fuel. 1) Due to radioactive decay of the embedded radionuclides higher temperatures are expected close to the waste containers (argillaceous rock ≤ 100 °C (Brasser et al. 2008)). 2) Argillaceous rock contains also a certain amount of organic matter such as humic acids (Claret et al. 2003), which can be leached by groundwater. Humic acids (HA) have a variety of functional groups, thus, they are able to complex metal ions such as actinides and to form stable colloids. Hence, they can influence the actinide sorption and diffusion. For performance of safety assessment, it is necessary to know, how the migration of the different actinides are influenced by elevated temperatures and the presence of organic matter.
In this study the U(VI) diffusion in the argillaceous rock Opalinus Clay (OPA) from Mont Terri, Switzerland was investigated at 25 and 60 °C both in the absence and presence of HA. As background electrolyte synthetic OPA pore water (Pearson 1998) was applied (pH 7.6, I = 0.36 M). The experimental set-up used for the diffusion experiments at 25 °C and 60 °C is described in Van Loon and Soler (2004) and Joseph et al. (2012), respectively. OPA bore core samples (diameter: 2.55 cm, thickness: 1.1 cm, dry bulk density: ~ 2400 kg/m^3) were placed in four diffusion cells. Each was connected with a tracered source reservoir and a non-tracered receiving reservoir, all reservoirs were filled with OPA pore water. Two cells were tempered at 25 °C (cell 1, cell 2) and 60 °C (cell 3, cell 4), respectively. All experiments were performed under anaerobic conditions (N2, 0 % CO2). The pressure adjusted on the OPA samples amounted to 5 MPa. At first, in all four cells through- and out-diffusion of non-sorbing HTO was studied for determining the transport porosity of the clay samples. The results were in agreement with literature data (Van Loon and Soler 2004). Subsequently, the U(VI) diffusion in OPA was investigated at 25 °C (cell 1) and 60 °C (cell 3). The simultaneous diffusion of U(VI) and HA in OPA was studied at 25 °C (cell 2) and 60 °C (cell 4). Thereby, 233U(VI) (c0 = 1E-6 mol/L) and 14C-labeled HA (c0 = 10 mg/L) were applied as tracers. After three months the experiments were terminated. In all receiving reservoirs no 233U(VI) could be detected. However, diffused HA molecules were found. The diffusion profiles of U(VI) and HA in the OPA samples were determined with the help of the abrasive peeling technique described by Van Loon and Eikenberg (2005). The obtained diffusion profiles were evaluated using the commercial software COMSOL Multiphysics 3.5a (COMSOL 2008).
In Figure 1a the U(VI) diffusion profiles in OPA at 25 and 60 °C in the absence of HA are shown. At 25 °C, the Kd value determined for the interaction of U(VI) with OPA by diffusion experiments clearly confirms the Kd value determined by means of sorption experiments (Joseph et al. 2011). A reduction of U(VI) to U(IV) was excluded. The value for the apparent diffusion coefficient Da of U(VI) was comparable to that of Np(V) determined by Wu et al. (2009). Thus, a similar migration behavior of both actinides through OPA was assumed. At 60 °C, the experimental data could be fitted only by assuming two diffusing U(VI) species resulting in two diffusion profiles. These two species were identified by means of laser-induced fluorescence spectroscopy and scanning electron microscopy with energy dispersive X-ray detector. The aqueous Ca2UO2(CO3)3 complex and so far, a not closer assignable colloidal U(VI) carbonate species were detected. In OPA the colloids diffused only about 500 µm, the aqueous U(VI) species was found up to a depth of about 2.5 mm. At 60 °C, the Kd values for both species were higher than that of U(VI) at 25 °C. Furthermore, the value for the effective diffusion coefficient De for the aqueous U(VI) species was increased. Both values compensate each other to almost equal Da values for U(VI) at 25 and 60 °C (only aqueous U(VI)). This shows, that the migration of U(VI) through OPA was not significantly influenced by the investigated higher temperature.

In Figure 1b the U(VI) diffusion profiles in OPA in the absence and presence of HA at 25 °C are presented. The profiles show, that in the presence of HA U(VI) penetrates the clay less than in the absence of HA. However, considering all the uncertainties a comparison of the respective Kd and De values verifies, that HA has no significant influence on U(VI) diffusion. This confirms the observations made by former batch sorption experiments for the system U(VI) / HA / OPA (Joseph et al. 2011). At 60 °C, also no influence of HA on the U(VI) diffusion was observed.
The study shows, that both investigated factors, an elevated temperature of 60 °C and the presence of HA, have no major influence on U(VI) migration through OPA.

References:

Brasser, T., Droste, J., Müller-Lyda, I., Neles, J.M., Sailer, M., Schmidt, G., Steinhoff, M. 2008. GRS - 247. Öko-Institut and Gesellschaft für Anlagen- und Reaktorsicherheit (GRS), Braunschweig.
Claret, F., Schäfer, T., Bauer, A., Buckau, G. 2003. Sci. Total Environ. 317, 189-200.
COMSOL 2008. Multiphysics 3.5a. Finite-element software package. http://www.comsol.com.
Joseph, C., Schmeide, K., Sachs, S., Brendler, V., Geipel, G., Bernhard, G. 2011. Chem. Geol. 284, 240-250.
Joseph, C., Van Loon, L.R., Jakob, A., Schmeide, K., Sachs, S., Bernhard, G. 2012. Geochim. Cosmochim. Acta, submitted.
Pearson, F.J. 1998. PSI Internal report TM-44-98-07. Paul Scherrer Institut, Villigen PSI, Switzerland.
Van Loon, L.R., Eikenberg, J. 2005. Appl. Radiat. Isot. 63, 11-21.
Van Loon, L.R., Soler, J.M. 2004. PSI-Bericht Nr. 04-03. Paul Scherrer Institut, Villigen, Switzerland.
Wu, T., Amayri, S., Drebert, J., Van Loon, L.R., Reich, T. 2009. Environ. Sci. Technol. 43, 6567-6571.