ABSTRACT Fluid release structures resulting from the interaction of igneous intrusions with sedimentary basins form an important part of the evolution of large igneous provinces. Hydrothermal breccia pipes formed in the Karoo Basin in South Africa during emplacement of igneous sills in the Karoo large igneous province represent one of the best-exposed expressions of such venting structures. Earlier work has shown that degassing of thermogenic CO 2 and CH 4 through the breccia pipes may have contributed to the Early Jurassic environmental changes. Here, we present the first detailed analysis of the distribution of breccia pipes in the western parts of the Karoo Basin. We mapped 431 pipes in a 650 km 2 area using outcrop data. The pipes are rooted in contact aureoles around four sills emplaced in organic-rich Ecca Group shale, and thermal modeling of sill cooling and contact metamorphism gives a maximum temperature of 675 °C near the sill contacts, sufficient to convert a significant fraction of the organic carbon to gas. Model estimates indicate that metamorphism in the 650 km 2 area generated 75–88 Gt of CO 2 , depending on actual sill thicknesses and emplacement levels. When further up-scaled, an area of 7400–8700 km 2 (i.e., less than 2% of the area in the Karoo Basin intruded by sills) would be required to generate 1000 Gt of CO 2 . In order to characterize the degassing pipes, their geographical positions and diameters were analyzed using several point-pattern methods. The results showed that the pipes (1) have diameters in the 11–177 m range (average 44 m), (2) are spaced with an average nearest-neighbor distance of 452 m, and (3) are overall randomly spaced but with weak overdispersion at very small scales (<50 m) and weak clusters at larger scales (400–3000 m). In contrast to studies of volcanic pipe spacing, this study on breccia pipes does not indicate that the pipe spacing is controlled by any large-scale geophysical parameters such as crustal or basin thicknesses. Conclusions point to the pipes being formed following sill emplacement and pressure increase in the low-permeability organic-rich shale, followed by rapid carbon degassing, emphasizing their important role in the Early Jurassic climate change and oceanic anoxic event.

Abstract A prediction–evaluation approach is developed to assess the propagation of parameter, conceptual and scenario uncertainties in the estimated near-field temperatures of the full-scale emplacement experiment at the Mont Terri rock laboratory. The uncertainty assessment is performed using a three-dimensional thermo-hydraulic numerical model of the full-scale emplacement experiment that represents the emplaced materials and surrounding Opalinus Clay and accounts for heat generation at the heaters. The propagation of parametric uncertainties is assessed using a first-order second-moment method supplemented by Monte Carlo simulations sampling the uncertain parameter space. The risk of uncertain parameters resulting in the failure of the maximum temperature criteria is evaluated with a first-order reliability method. Conceptual and scenario uncertainties are evaluated with deterministic simulation variants. After the conclusion of predictive modelling, a mid-term evaluation of the temperature predictions is performed through a comparison with measurements after 2.5 years of heating. The comparison indicates that the best estimates of temperature agree well with the measurements and that the 95% error bands assessed with parametric uncertainty envelope the measured values in almost all locations. Additional comparison with the measured degree of water saturation and the relative humidity is performed to assess the hydraulic behaviour and set the ground for the long-term evaluation, which will include predictions of the near-field pore pressures.

Abstract A key component of the site comparison planned for the deep geological disposal of spent fuel and high-level waste (SF/HLW) in Switzerland is the assessment of the evolution of repository-induced perturbations in the repository nearfield associated with thermal effects from heat production due to radioactive decay of radionuclides, as well as gas pressures developing in the backfilled underground structures from the anaerobic corrosion of the steel waste canisters and tunnel support materials. The assessment of such effects is integrated in the site comparison through safety indicators used to evaluate repository performance. In this context, probabilistic assessments need to integrate the uncertainty of the entire ensemble of input parameters, and estimate the propagation to these indicators in a reliable and computationally efficient manner. This paper presents the development of a methodology for an indicator-based assessment of heat- and gas-induced effects in a SF/HLW repository in Opalinus Clay integrating a probabilistic treatment of parametric uncertainty. The workflow is demonstrated using preliminary data, repository configurations and indicators. Complementary simulations are presented to demonstrate the feedback to the optimization of repository design in order to mitigate repository-induced effects that can potentially compromise the safety function of the engineered and natural barriers.

Abstract The Prototype Repository (PR) tunnel is located at the Äspö Hard Rock Laboratory near Oskarshamn in the southeast of Sweden. In the PR tunnel, six full-sized deposition holes (8.37 m deep and 1.75 m in diameter) have been constructed. Each deposition hole is designed to mimic the Swedish reference system for the disposal of nuclear fuel, KBS-3V. The PR experiment is designed to provide a full-scale simulation of the emplacement of heat-generating waste. There are three phases to the experiment: (1) the open tunnel phase following construction, where both the tunnel and deposition holes are open to atmospheric conditions; (2) the emplacement of canisters (containing heaters), backfill and seal in the first section of the tunnel; and (3) the emplacement of canisters, backfill and seal in the second section of the tunnel. This work describes the numerical modelling, performed as part of the engineered barrier systems (EBS) Task Force, to understand the thermo-hydraulic (TH) evolution of the PR experiment and to provide a better understanding of the interaction between the fractured rock and bentonite surrounding the canister at the scale of a single deposition tunnel. A coupled integrated TH model for predicting the wetting and the temperature of bentonite emplaced in fractured rock was developed, accounting for the heterogeneity of the fractured rock. In this model, geometrical uncertainties of fracture locations are modelled by using several stochastic realizations of the fracture network. The modelling methodology utilized information available at early stages of site characterization and included site statistics for fracture occurrence and properties, as well as proposed installation properties of the bentonite. The adopted approach provides an evaluation of the predictive capability of models, it gives an insight of the uncertainties to data and demonstrates that a simplified equivalent homogeneous description of the fractured host rock is insufficient to represent the bentonite resaturation.