Radionuclide transport through fractured chalk under abrupt variations in ionic strength

Radionuclide migration through saturated fractured chalk was studied in the context of predicting potential risks to groundwater in the vicinity of nuclear repositories. The aim of the present study was to examine the effect of salinity changes which might result from a sudden rainstorm leading to freshwater infiltration on the mobility of radionuclides in fractured carbonate rocks. A tracer mixture, simulating radioactive contaminants related to spent fuel (SF), including U, Sr, Ce (simulant for redox active actinides) and Re (simulant for Tc) was injected into a naturally fractured chalk rock in the laboratory. Uranine, a fluorescent dye, served as a conservative tracer. Two sets of experiments were carried out in which tracers were added to solutions of different ionic strength (IS) represented by total dissolved solid (TDS) values (Cl^- and HCO₃^- as major anions): (1) low IS artificial rainwater (TDS of ca. 10² mg/L,); and (2) high IS artificial groundwater (TDS of ca. 10⁴ mg/L). In both sets of experiments, the tracer mixture was introduced into a fractured chalk core, followed by the injection of tracer-free solution at the same IS. Next, the opposite (low/high IS) tracer free solution was introduced into the core to induce salinity variation. The behavior of the simulants was investigated under swift changes in background (BG) solution salinity. In all cases, Re breakthrough curves (BTCs) were unaffected by the change in BG solution and exhibit conservative behavior in comparison to that of the Uranine. Cerium was transported as intrinsic colloidal carbonate complexes, in agreement with previous studies, and remained unaffected by the abrupt change in BG solution. Uranium and Strontium BTCs were influenced by the abrupt change in IS, as their recovery significantly increased when high IS solution was injected into the core and reduced when low IS solution is introduced, regardless of the injection order. This indicates that U and Sr sorption to fractured surfaces is enhanced at low salinity, a phenomenon attributed to the replacement of Sr and U ions by Ca and Na at the adsorption sites at elevated IS conditions. The variable mobility of radionuclides found in this study should be considered in the design of natural and engineered barriers for SF disposal, especially in regions where seasonal rains or flooding may cause abrupt changes to groundwater ionic strength.