Mineralogical Analysis of Bentonite from the ABM5 Heater Experiment at Äspö Hard Rock Laboratory, Sweden

The present study reports on the analysis of all blocks of the ABM5 test, which is a medium scale bentonite buffer deposition test. In contrast to similar tests, the ABM5 was conducted at higher temperature (up to 250 °C). The aim of the study was to characterize the chemical and mineralogical reactions and to identify the effect of the extraordinarily high temperature. Reactions observed were similar to those observed in previous and/or similar tests covering cation exchange, anion inflow, dissolution and precipitation of C- and S-phases, Fe corrosion, and Mg increase at the heater. Neither the type nor the extent of the different reactions could be related to the significantly higher temperature. However, due to the absence of lubricant used between heater and bentonite, it could be proved that the calcite previously present was dissolved and precipitated as siderite at the contact, pointing towards the importance of the presence of carbonate when considering different Fe corrosion products. Moreover, for the first time, a decrease of the Mg content at the heater was observed, which was probably because a Mg-rich clay was used. The reasons for Mg increase or decrease are still not completely understood.

The Healing Process of the Joints between Buffer Material Blocks and the Influence on Solute Migration

A set of thermo-hydro-mechanical coupling control equations was established, and the healing process of the joints between Gaomiaozi bentonite (GMZ01) buffer material blocks and the influence of the joint parameters were numerically simulated. The calculations consider the effect of joints on solute migration, the permeability and thermal conductivity of the buffer material, and the evolution of the healing effect. The effects of the joint design parameters, including the type, number, width, splicing form, and average dry density of the joint, are investigated. Studies show that, under an external water head, the joints will become hydraulic priority channels due to their higher permeability, which will shorten the saturation time of the blocks. As the bentonite gradually saturates, the swelling force compresses the joint material. This action improves the overall uniformity of the buffer material and reduces the priority channel effect. Meanwhile, the final average permeability and diffusion coefficient of the buffer material are found to mainly depend on the average dry density of the buffer material. The higher the average dry density of the buffer material is, the lower the final average permeability and diffusion coefficient are, whereas the distribution of joints and the block splicing are less affected by the average dry density of the buffer material. The findings of this study can provide a reference for the design of bentonite buffer material blocks in the repository.

Global(VI)integrating coupled THCinto PA model

The bentonite buffer long-term integrity is of significant interest in the performance assessment (PA) of nuclear waste disposal. This study aims at understanding how the initial geochemical parameters affect long-term chemical properties within the buffer, which will subsequently affect the transport. Using coupled thermal-hydrological-chemical (THC) models for migration of U(VI) in a generic repository, we performed a global sensitivity analysis (GSA) to identify the influence of each parameter on the temporal evolution of a spatially averaged distribution coefficient for the entire buffer. Such an analysis can be used in a repository-scale PA. In this work, we used the TOUGHREACT software to model coupled THC processes in a generic clay repository with bentonite buffer. In this model, U(VI) is released from a canister via schoepite dissolution, which is assumed to occur 1000 years after closure. U(VI) migrates through the bentonite buffer affected by two-site protolysis non-electrostatic surface complexation and cation exchange. GSA results showed that adsorption density on smectite, pH, volume fractions of smectite, calcite, Ca2+ aqueous concentration all play a significant role in U(VI) transport, since roughly 80% of adsorbed U(VI) is absorbed by smectite, and Ca2+ affects the aqueous complexation with U(VI). This work demonstrates the complex process models potential usefulness that can be transferred to the PA model. It also provides information needed to proceed with the development of a reduced-order model, which has the potential to optimize repository designs, site characterization, performance confirmation.

Assessing the effect of radioactive waste glass dissolution on early-stage radionuclide migration using simplified geological repository Monte Carlo transport models

The vitrification of radioactive waste within glass and subsequent disposal within a geological disposal facility (GDF) requires a comprehensive understanding of the effect of glass dissolution on GDF performance. This paper aims to analyse the effect of both high-level and intermediate-level waste (HLW and ILW) glass dissolution source terms on radionuclide release into the geosphere just above the disposal vault (the ‘crown’). Radionuclide migration was simulated in GoldSim for HLW in either granite or clay host rocks with a bentonite buffer using carbon steel or copper canisters, whereas ILW simulations considered either granite or clay host rocks, in either bentonite buffer or cement backfill, using concrete or cast-iron canisters. Glass dissolution source terms were varied by coupling GoldSim and MATLAB to modify the initial, residual, and resumption dissolution rates of the glass or by applying the analytical GRAAL model to glass dissolution. HLW glass results indicate no preference of granite over clay host rocks for a given canister type but that a copper canister is preferable to steel. ILW results suggest that a granite–bentonite–cast-iron environment yields lowest crown activities with cast-iron preferable to concrete as the canister, bentonite preferable to cement as the buffer/backfill, and granite preferable to clay as the host rock. Varying glass dissolution source terms (initial, residual, and resumption dissolution rates) had an understood effect on radionuclide migration, although changes were arguably insignificant considering peak crown activity for both HLW and ILW.