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Arbeitsbericht NAB 22-16

BEACON / WP1.3 / Final Assessment Report

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  1. Introduction

 

The 'BEACON' (Bentonite Mechanical Evolution) project addresses key technical issues that must be tackled in order to support the implementation of planned geological disposal projects for high-level radioactive wastes across the EU. The overall objective of the project is to evaluate the performance of an inhomogeneous bentonite barrier for the entire evolution of it i.e., from an installed engineered system to a fully functioning barrier.

An increased understanding of material properties and fundamental processes that lead to homogenization as well as improved capabilities for numerical modelling were required to achieve the project goals. The output allows for a verification of the performance of current designs for buffers, backfills, seals and plugs and an improved handling of mass losses in long-term assessments.

Part of the project activities was to develop and test the tools necessary for the assessment of the mechanical evolution of an installed bentonite barrier and the resulting performance of the barrier. For some repository designs, mainly in crystalline host rock, the results can also be used for the assessment of the consequences of mass loss from a bentonite barrier on the long term.

The driver for this project is repository safety, and the demands of waste management organizations to verify that the material selection and initial state design fulfil the long-term performance expectations. For this project, the initial state refers to the period during installation of the barrier, while long-term performance refers to the period for barrier saturation and evolution of the hydro-mechanical properties, which could range from 10's to 1'000 s of years. In current and future applications for repositories, the regulators will expect the applicants to have a sufficient predictive capability of the barrier evolution from the installed to the final state.

A part of BEACON has been focused on direct application to real assessment cases in actual repository systems. A few cases from relevant repository systems have therefore been selected as test examples. The systems to be evaluated in BEACON include three cases: 1) a tunnel plug based on the Andra design, 2) a disposal cell from the Nagra concept, 3) the KBS-3 deposition tunnel backfill. These are representative of the primary areas of uncertainty in density homogeneity. These examples cover a broad range of issues, and the results should also be applicable to other concepts and systems.

One objective of this report is the syntheses of the findings in the frame of the 3 assessment cases within the BEACON project. It reports the modelling results from the different groups assessing the HM evolution of the three assessment cases and evaluates these results in view of the respective safety case. It summarizes the lesson learned from the different work packages leading to recommendations on the implementation of bentonite-based barriers.

In the last chapter requirements are formulated on the assessment of the performance of bentonite-based barriers. Since the safety-relevant performance of the bentonite is mainly defined by its chemical, mineralogical and physical properties, comprehensive sets of requirements regarding the chemical, mineralogical and physical characteristics of bentonite have been developed by the different waste management organizations for their concepts. In most cases, these requirements assume a bentonite density as a boundary condition for the requirement to be fulfilled.

 

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