On the long-term migration of uranyl in bentonite barrier for high-level radioactive waste repositories: The effect of different host rocks

Abstract

Repositories for high level radioactive waste hosted in argillite or granite with bentonite backfill are the two disposal concepts that been extensively studied. A general perception is that granite has very low sorption capacity and therefore is disadvantageous in retarding the migration of radionuclides in comparison with argillite. However, when there is bentonite backfill, does the sorption capacity of granite really matter for the migration of radionuclides? How different host rocks would affect the migration of U(VI) through the bentonite backfill of an engineered barrier system (EBS), and which properties of the host rocks cause those differences in migration, given that migration of radionuclides is a critical measure in assessing the performance of a repository? Here we present two coupled thermal, hydrological, and chemical models for the transport of U(VI), each with an identical setup except for the type of host rock (argillite vs granite rock). Comparisons between our models show that the different host rocks exert their influence on migration of U(VI) via regulating the chemical conditions in the bentonite, which affect the concentration of U(VI) at the source and the adsorption of U(VI) in the bentonite. The key chemical conditions are pH, Ca + , and HCO 3-concentration in pore-water, which are strongly affected by soluble carbonate minerals. Our models also show the occurrence of illitization (dissolution of smectite and precipitation of illite) in the bentonite, which affects the migration of U(VI) through changing pH and the quantity of adsorbents. Although generalization of current model results for granite and argillite should be done carefully, the simulations highlight the importance of pore-water chemistry in host rock, as well as bentonite-host rock interactions, when assessing the performance of a repository. In comparison to properties such as thermal conductivity, permeability, and adsorption capacity, properties that have been studied intensively, the pore-water chemistry of host rocks deserve equal or even more attention because of its strong influence on radionuclide migration.

Harnessing Geothermal Energy Potential from High-Level Nuclear Waste Repositories

“…The thermal and hydrodynamic parameters used in the model are listed in Table 1. We assume that the EBS buffer is composed of FEBEX bentonite and that the host-rock properties of argillite are adapted from the properties of Opalinus clay [21]. In the proposed repository, bentonite serves a dual purpose: (1) as a heat conductor (grout), aiding energy transfer, and (2) as an engineered barrier, retarding the radionuclide migration through the sorption to clay minerals.…”

“…In this simplified domain, the canister is modeled as a heat source with steel's mechanical properties. The mesh (Figure 3) used in the numerical simulation was taken from Cao et al [21,23,24]; the applicability of this model is limited to the interpretation and prediction of the behavior of TH, primarily in the vicinity of the canister where thermal effects are significant. We adopted an axisymmetric mesh to represent the symmetrical heating process of the EBS.…”

Computational Framework for Radionuclide Migration Assessment in Clay Rocks

“…Furthermore, the adoption of software containers by numerical modelling software has gained relevance in recent years, such as dfnWorks (Hyman et al, 2015) and FEniCS (Alnaes et al, 2015). Finally, the development of new software or the extension of already existing codes should provide quality assurance process including state-of-the-art scientific knowledge, testing and benchmarking (Cao et al, 2019;Águila et al, 2021;Montoya et al, 2022).…”

“…On one hand, one numerical methods consider, a mechanistic approach taking into account surface complexation and cation exchange processes with a realistic composition of the porewater of the clay rock (Appelo and Wersin, 2007). Although mechanistic approaches have benefits in terms of process understanding, it does so at the cost of a significant increase in computational expense, sometimes resulting prohibitive for large-scale simulations (Appelo and Wersin, 2007;Cao et al, 2019;Águila et al, 2021;Montoya et al, 2022). Mechanistic sorption models are typically obtained from batch experiments, where the sorption behavior of one radionuclide is studied as a function of radionuclide concentration (Bradbury and Baeyens, 2005).…”

Effect of temperature on the complexation of triscarbonatouranyl(vi) with calcium and magnesium in NaCl aqueous solution

“…The nuclear waste repository isolation system consists typically of a natural barrier, mainly in clay or granite, depending on the country-specific rock formation, and an engineered barrier including in general waste canister, bentonite backfill, and concrete liner [8]. This multi-barrier system is extremely important in preventing the migration of actinides into the biosphere by means of pore-waters, which are similar in compositions with Cl -, SO4 2-, HCO3 -, Ca 2+ , Mg 2+ , and Na + as the main ions at pH value ca.…”