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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - Beacon (Bentonite mechanical evolution)

Teaser

Summary

The Beacon project will address key technical issues that must be tackled to support the implementation of planned geological disposal projects for high-level radioactive wastes across the EU.

The overall objective is to evaluate the performance of an inhomogeneous bentonite barrier from an installed engineered system to a fully functioning barrier, achieved by developing and testing the tools necessary for the assessment of the mechanical evolution of an installed bentonite barrier and the resulting performance of the barrier. The goal is to verify the performance of current designs for buffers, backfills, seals and plugs. For some repository designs mainly in crystalline host rock, the results can also be used for the assessment of consequences of mass loss from a bentonite barrier in long term perspective.

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 at installation of the barrier, while long-term performance refers to the period for barrier saturation and evolution of the hydro-mechanical state, which could range from 10 to 1000s 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. This will require an increased understanding of material properties and fundamental processes that lead to homogenisation as well as improved capabilities for numerical modelling.

The output will be 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.

Work performed

\"The scientific-technical work in Beacon is structured in 5 WPs, dissemination and training is handled in WPs 6-7, coordination & management is covered in WP8
The objective of WP1 is to define the important issues concerning the mechanical properties of bentonite and to define how these should be treated. This will result in a number of specified assessment cases with focus on long term performance and/or repository engineering. During the first period of the project WP1 has issued, and taken care of, a questionnaire answered by all the waste management programs involved in Beacon. The survey was carried out to identify in how far bentonite heterogeneities would be included in the safety assessment of a geological repositories. Based on that a state-of-the-art report (D1.1) on the treatment of the mechanical evolution of the bentonite buffers, backfills and seals in the most recent safety assessment in the national programs has been submitted.
In WP2 the existing knowledge base is treated. During the first months of Beacon, WP2 collected and compiled data from ongoing and decommissioned large scale URL experiments, from partners and several non-partners, information and results from earlier assessments, design considerations, experiments and modelling tasks. These data are to be \"\"reused\"\" in Beacon, and no such tests are performed within the project. The information was compiled and analysed in D2.2 (submitted Oct 2017) and a database with the assembled data and information was created, available at the project web www.beacon-h2020.eu.
The strongest driver for a joint project is the current limitations in the predictive capability in the numerical models. The issue of homogenisation and swelling is challenging both from a conceptual and a numerical point of view. The purpose of WP3 is to identify and resolve the shortcomings of current models. The state of the constitutive models of the 12 different modelling teams has been presented and discussed, and assessed with reference to a set of key features of the hydromechanical behaviour of the bentonite. These descriptions and assessments have been gathered in the first Deliverable of WP3 (D3.1) submitted in November 2018. Accordingly, the bulk of the work carried out within WP3 has been the further development and improvement of the constitutive models that were available to the various modelling teams at the start of the project.
Although there is a substantial experimental database available for the project, it is necessary to perform additional experiments to support the model development in WP3 and the model testing in WP5, and the necessary experimental work is coordinated in WP4. WP4 consists of experienced experimental groups, which have the flexibility to adapt the experimental work to support the needs of WP3 and WP5. During the first period an experiment table was developed containing the essential information about the experiments planned and performed in Beacon in order to facilitate interaction between WP4 and the modelling work packages. Work has been done both in task 4.1, 4.2 and some progress was made in task 4.3.
The core component of Beacon is WP5. The main effort will be performed in this WP. The overall objective of WP5 is to simulate the assessment cases defined by WP1. In order to do this, the available models have to be tested first on results from earlier performed laboratory experiments and later on results from large scale field test to gain confidence in their predictive capability. Based on the inventory of existing data made in WP2, three tests cases were selected for task5.1 and three for task5.2. Results from the tests so far are presented in deliverables D5.1 and D5.3, submitted M13 and M18.\"

Final results

In earlier assessments of the long term performance of bentonite EBS, the mechanical evolution of the installed bentonite was neglected and an â€œidealâ€ final state was optimistically assumed. Now that several European national programs are moving towards licensing, construction and operation of repositories, this assumption is no longer sufficient.
In order to verify the performance of current designs for buffers, backfills, seals and plugs the following work is planned:
I. A well-documented and communicated collection of the available knowledge prior to the Project
II. Re-evaluation of large part of the existing database to extract the important information, to compile the qualitative and quantitative observations and to develop the conceptual understanding
III. Enhanced, robust and practical numerical tools, firmly grounded on a good conceptual understanding, that have the required predictive capabilities concerning the behaviour of engineered barriers and seals
IV. A complete experimental database for the need of the assessment models
V. Verified models based on experimental results from experiments in different scales
VI. Workshops dedicated to the mechanical issues in bentonite open to the scientific community
The Beacon project is needed for the pan-European aims at building confidence amongst regulators and stakeholders regarding the performance of safety barriers in a geological repository. It is also cost- and time-effective to progress development of understanding regarding bentonite behaviour in a collaborative manner, and the pooling / sharing of precedent information enhances efficiency of overall process.