Simulation of iodine migration at Cigar Lake – A natural analog study

Natural analog studies have long been recognized to support safety cases for the geological disposal of radioactive waste. The uranium ore deposit at the Cigar Lake site in Canada is highly enriched (>55 wt.% U), is located around 450 m below the surface and remained stable since its deposition 1.3 billion years ago. This is related to the presence of a clay‑rich layer, which isolates U and its daughter nuclides from the surface environment. This clay-rich zone mainly consists of illite, a main component of many argillaceous formations considered as suitable host rocks for radioactive waste disposal. Cigar Lake provides a strong and unique natural analog to the multi-barrier system of many disposal concepts.

Iodine, particularly the isotope ¹²⁹I, is a radionuclide of concern in safety assessments due to its long half-life of 15.7 million years, high mobility, and potential to accumulate in the human body. Naturally, the isotope ¹²⁹I is produced by neutron‑induced fission of ²³⁵U and spontaneous fission of ²³⁸U. At Cigar Lake, concentration depth profiles of ¹²⁹I and U are available over the entire 450 m covering the ore body, clay-rich zone, and surrounding formations. Three zones of U and ¹²⁹I enrichment were identified. One is located at the depth of the ore body. The other two are at depths of around 270 m and 150 m below the surface. However, measured ¹²⁹I concentrations do not correspond to the expected theoretical maximum value resulting from the source term calculation for the U isotopes. Compared to the calculated theoretical maximum value, measured ¹²⁹I concentrations are too low inside (high U concentrations) and too high outside the ore body (low U concentrations). The discrepancy between expected and measured concentrations indicate transport from the ore body through the overburden, via diffusion or through faults. Cigar Lake offers the unique opportunity to investigate ¹²⁹I migration in an illite‑rich unit on geological spatial and temporal scales giving implications for the near- and far-field of a potential repository.

Quantification of ¹²⁹I migration processes at Cigar Lake with transport simulations are in the focus of the ANALOG task of DECOVALEX-2027. A step-wise approach is used to identify the underlying processes. First, uncertainties of parameters required for the source term calculation are investigated. This step revealed that the discrepancy between theoretical maximum values and measurements cannot solely be attributed to the source term calculation. Second, migration of ¹²⁹I from the ore body is therefore modelled with a one‑dimensional diffusion model covering the ore body and the surrounding clay-rich unit. Diffusion and sorption parameters are varied within plausible ranges based on literature values. Results indicate that ¹²⁹I migration from the ore body must be retarded, either due to very low diffusion or sorption, presumably on minerals other than illite, such as Cu-sulphides. Third, the entire profile of 450 m is modelled including sorption, diffusion, and advection. This integrated approach can help to reduce the uncertainties associated with radionuclide migration and provide a more robust basis for decision-making in the context of radioactive waste disposal.