Are fractured carbonate rocks an appropriate geological medium for nuclear waste disposal? Lessons from a field-scale transport experiment

Deep geological disposal is the internationally-accepted solution to permanently store nuclear waste such that it can safely decay without negatively impacting the biosphere. Most nuclear waste-producing countries are investigating disposal sites in granite, clay, saltstone or volcanic tuff formations. However, in certain countries such as Israel, the most feasible sites for building such a repository are fractured carbonate rocks. To investigate potential radionuclide-rock interactions under local conditions, a field-scale reactive transport experiment was conducted. The experiment investigated transport of Re, Ce, Sr and Cs in the presence of bentonite colloids through a 47-m flow path through fractured chalk located in the northern Negev Desert of Israel. A 100-L tracer solution containing known concentrations of each metal, the fluorescent dye uranine, and bentonite colloids was injected into an observation well, while another well was pumped to induce a forced gradient throughout the full 8-day duration of the experiment. After five days of pumping, a second injection of 500 L tap water containing sodium naphthionate, a different fluorescent dye, was injected into the observation well to investigate the impact of changing the ionic strength. Breakthrough curves of total and aqueous fractions of each metal were modeled to determine the hydrological parameters of the flow path and show the mobility of each metal under the site conditions investigated. Rhenium exhibited conservative behavior, attributable to its ReO4- anionic form. Strontium was transported primarily as an aqueous species, but SrCO3 colloids appeared to be mobilized following the freshwater injection. Cesium migrated entirely as an aqueous species throughout the experiment. Cerium migrated as an intrinsic colloid precipitated with bicarbonate, and was somewhat remobilized following the freshwater injection. Each metal's transport properties were successfully modeled to demonstrate its geochemical properties and mobility. This study suggests that although careful consideration should be given to site-specific hydrologic and geochemical conditions, fractured carbonate rocks can potentially serve as a suitable barrier to radionuclide transport from a repository.