Mobility of Radionuclides in Fractured Carbonate Rocks: Lessons from a Field-Scale Transport Experiment

Abstract

Current research on radionuclide disposal is mostly conducted in granite, clay, saltstone, or volcanic tuff formations. These rock types are not always available to host a geological repository in every nuclear waste-generating country, but carbonate rocks may serve as a potential alternative. To assess their feasibility, a forced gradient cross-borehole tracer experiment was conducted in a saturated fractured chalk formation. The mobility of stable Sr and Cs (as analogs for their radioactive counterparts), Ce (an actinide analog), Re (a Tc analog), bentonite particles, and fluorescent dye tracers through the flow path was analyzed. The migration of each of these radionuclide analogs (RAs) was shown to be dependent upon their chemical speciation in solution, their interactions with bentonite, and their sorption potential to the chalk rock matrix. The brackish groundwater resulted in flocculation and immobilization of most particulate RAs. Nevertheless, the high permeability of the fracture system allowed for fast overall transport times of all aqueous RAs investigated. This study suggests that the geochemical properties of carbonate rocks may provide suitable conditions for certain types of radionuclide storage (in particular, brackish, high-porosity, and low-permeability chalks). Nevertheless, careful consideration should be given to high-permeability fracture networks that may result in high radionuclide mobility.

Figure 1

(A) Location of field site in Israel. (B) Location within the industrial site, (C) map of the field experiment site showing injection borehole RH11C and pumping borehole RH11A. Additional boreholes not used in experiment are marked by triangles. (D) Diagram of experimental setup showing injection and pumping boreholes, location of pumps, and the injection site.

Figure 2

Bulk and aqueous fractions of (A) Cs, (B) Ce, (C) Sr, and (D) Re at the pumping borehole throughout the full duration of the experiment. Dotted vertical lines show the injection time of low ionic strength water and naphthionate dye as described in Section 3.3.

Figure 3

Field and modeled data of Re breakthrough during the first 120 h, depicting the three contributing pathways used to achieve a reasonable fit.

Figure 4

Breakthrough curves and RELAP model fits for (A) Cs, (B) Ce, and (C) Sr. Note that all uranine data refer to the secondary y-axis.

Figure 5

Normalized concentrations of (A) bulk (closed symbols) and (B) aqueous fractions (open symbols) of RA at the pumping borehole during the freshwater injection at hour 95. The injected solution tracer naphthionate is plotted in both figures.