The long-term performance of the Canadian deep geologic repository (DGR) relies significantly on bentonite clay, as sealing materials intended for use in the engineered barrier system (EBS). One particular safety concern is microbiologically influenced corrosion of the used fuel containers (UFCs) which may occur if bisulfide (HS-) transports through the bentonite buffer to reach the UFC surface and corrode the copper coating. Understanding HS- sorption onto bentonite is therefore an important aspect of this problem, as HS- sorption can reduce the extent of copper corrosion. However, sorption dynamics onto bentonite are not yet well-understood. As such, this study performed laboratory batch experiments to investigate HS- sorption onto bentonite slurries as a function of temperature (10-40 °C), pH (9-11), and ionic strength (0.01 M-1 M NaCl). These conditions were aimed to reflect the range of possible DGR geochemical conditions. The experimental results showed that HS- sorption onto bentonite increased with increasing temperature but decreased with increasing pH and ionic strength. A 3-way ANOVA (analysis of variance) showed that the variables' individual and 2-way interaction effects are statistically significant, which implies that they should be incorporated into a sorption mechanism. A thermodynamic-based sorption model was also developed in PHREEQC assuming that sorption was driven by three key processes: (i) redox reaction with the structural Fe3+ sites, (ii) surface precipitation as FeS (mackinawite), and (iii) surface complexation reactions with surface hydroxyl group (OH) at the edge sites of montmorillonite. The model successfully described the main experimental trends and provided valuable insights into the relative contribution of these processes to the total HS- sorption mechanism. Altogether, this study provides novel insights from experimental and numerical modelling findings that enhance the understanding of HS- sorption onto bentonite, in the context of Canadian DGR design as well as other nuclear repositories worldwide.
Understanding how concentrations of corrosive oxidants evolve under γ-radiation is essential for assessing long-term material performance in nuclear environments. This work integrates in situ differential pulse voltammetry (DPV) with finite-element kinetic modeling to track and interpret the decay of hypochlorite (OCl-) in aerated 0.1 M NaCl at pH ∼9.9 under a dose rate of 725 Gy h-1. Using a three-electrode setup inside a 60Co γ-cell, OCl- was quantified from calibration curves, where concentrations were validated ex situ by UV-vis spectroscopy. For an initial OCl- added concentration of 10 mM, both methods showed a decrease to ∼1 mM after 24 h. Kinetic analysis based on a radiolysis/halogen reaction scheme indicates that HOCl consumption is dominated by OH• abstraction (∼71%), with a secondary contribution from O2 •- reduction (∼22%) and a minor Cl--mediated pathway (∼7%); contributions from e- and H• are negligible at the steady state. The added HOCl drives a pronounced rise in OCl• that feeds back to lower the steady-state OH• level and modulates H2O2 formation. From these kinetics, it is estimated that g HOCl ≈ -0.53 μmol J-1. Beyond validating DPV as an in situ radiation electroanalytical method, the combined measurements and modeling provide a mechanistic basis for OCl- transformation in saline waters, establishing a general radiation-electrochemistry platform for time-resolved detection of radiolysis products relevant to nuclear-waste containment.
ABSTRACT Copper‐coated used fuel containers (UFC) may be exposed to Cl − ‐containing bentonite pore waters under oxic conditions. A mixed‐potential model has been developed to predict the time dependence of the corrosion potential ( E CORR ) and corrosion rate of the container as the near‐field environment evolves. Prediction of E CORR not only provides mechanistic insight into the evolution of uniform corrosion but also allows the assessment of the likelihood of localized corrosion and stress corrosion cracking mechanisms that are contingent on a threshold potential. Both 1‐D and 2‐D axisymmetric versions of the COMSOL‐based model have been validated against experimental measurements of E CORR of copper electrodes in which the mass transfer coefficient was varied over five orders of magnitude and the O 2 concentration varied over four orders of magnitude. Good agreement is found between the predicted steady‐state and transient values of E CORR and those measured experimentally.
Two full-scale emplacement trials were carried out at the Oakville Test Facility of the Nuclear Waste Management Organization (NWMO), one in April 2022 and one in March 2024, to assess the feasibility of the NWMO placement concept as part of Canada's deep geological repository for used nuclear fuel. The placement concept consists of copper-coated used fuel containers encapsulated in highly compacted bentonite (HCB) blocks being lowered into an underground tunnel known as an emplacement room. Gap fill material (GFM) is then augered into the annulus between the HCB blocks and the rock walls of the emplacement room to fill all remaining voids. One of the main objectives of these trials was to examine whether the as-placed dry density of the bentonite buffer could meet dry density requirements specified in the NWMO placement concept. Since the HCB boxes were previously manufactured and their dry density remained unchanged (i.e. ≥1.70 g cm −3 ), the focus was primarily on the dry density of the as-placed GFM, which was measured during these emplacement trials. The placement concept specifies a minimum average dry density of 1.40 g cm −3 for GFM. In the first emplacement trial, the mock-up emplacement room was filled with seven HCB box assemblies, and layers of GFM were deposited after the second, fourth, sixth and seventh HCB boxes were installed. In the second emplacement trial, two wall panels were removed from the mock-up emplacement room wall to simulate outbreak areas or spalls in the rock. The emplacement room was filled with five HCB dummy boxes and three layers of GFM, deposited after the second, fourth and fifth HCB boxes were installed. Various field-testing techniques were employed to measure local dry densities of the as-placed GFM. The sand-cone method and nuclear gauge were used in the first trial, while time domain reflectometry sensors were used in the second trial. The study results indicate that in the first emplacement trial the average dry density of the as-placed GFM was 1.44 g cm −3 , although individual measurements ranged from 1.35 to 1.56 g cm −3 . In the second emplacement trial no meaningful overall average for dry density was obtained, as the testing programme focused on the GFM within the spall regions, where the average dry density was 1.34 g cm −3 . As with the first trial, inhomogeneous density distributions were observed across the GFM-filled region. This inhomogeneity suggests that achieving density equilibrium in the bentonite buffer system will take significantly longer, emphasizing the need to install the GFM at the highest practical initial dry density.
Thermodynamic benchmark calculations have been performed to better understand the behavior of 75Se(VI), 99Tc(VII), 233U(VI), 237Np(V), 241Am(III), Th(IV) and 242Pu(IV)) in the evolving geochemical conditions of the Long-term In-situ Test (LIT) at the Grimsel Test Site (GTS) and corresponding mock-up experiment. It also aims to identify the status of the geochemical speciation models and databases for these elements. The experiments are simulating the near-field conditions in some radioactive waste repository concept including a bentonite engineered barrier emplaced in crystalline rock and the findings are contributing to the long-term safety assessment of these facilities.In general, the calculations of all six modelling groups who participated in the benchmark agree well and differences in results could always be traced back either to differences in input data or to modelling assumptions. Such assumptions concern for example the characteristics of solid phases, especially the selection of a crystalline, aged or fresh mineral phase, which directly impacts the predicted radionuclide (RN) solubility as illustrated in the results for thorium and plutonium. In a similar way, the inclusion/exclusion of aqueous species leads to an increase/decrease of the solubility like for the polyselenides in case of selenium. In particular, the selection of recently approved species such as ternary silica actinide complexes or ternary uranyl carbonate complexes are impacting, for example, the speciation and solubility of U, Pu, Am or Np in the systems investigated here. It is crucial that all such data selection and modelling decisions need to be transparent and well justified or at least documented.The results from the thermodynamic benchmark contribute to a better understanding of the mobility of the respective RNs in the LIT and accompanying mock-up test. The observed mobility of significant fractions of 99Tc, 237Np and 233U in the mock-up experiment, but not in the LIT, together with the results from the thermodynamic benchmark calculations, suggest that conditions in the mock-up experiment are less reducing, since they are not buffered by surrounding minerals in the shear zone as is the case in the LIT. Rather low mobility 241Am(III) and 243Pu(III/IV) redox states are predicted in the calculations for all waters in accordance with the fact that no release of these nuclides is observed in either LIT or in the mock-up experiment. Ongoing analysis of the over-cored material of the LIT, namely the characterization of RN distribution profiles in the bentonite, will contribute to a further verification of the benchmark results.
