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Cement-based materials, properties, evolution, barrier functions

Periodic Reporting for period 3 - Cebama (Cement-based materials, properties, evolution, barrier functions)

Reporting period: 2018-06-01 to 2019-05-31

The HORIZON 2020 EURATOM Collaborative Project “Cement-based materials, properties, evolution, barrier functions (CEBAMA)” was developed to support the implementation of nuclear waste disposal in deep underground facilities. Radioactive waste poses potential health hazards and risk to the biosphere including humans. The best way to handle and dispose this material is a topic of broad public debate and concern. Supporting safe options for the long-term disposal of nuclear waste is therefore a key component in developing sustainable strategies to implement nuclear energy as part of the energy mix in Europe but also within decisions to finally phase out the use of nuclear energy.

Cement-based materials are highly relevant for the nuclear waste disposal Safety Case, because they are widely used in a repository, e.g. as waste matrix, liners and structural components or sealing materials. In order to make reliable assessments of the potential evolution and performance of a repository with time, it is important to understand the specific chemical and physical processes affecting cement materials and their effect on radionuclide behaviour and migration. Specific technical questions tackled within CEBAMA were:

- How do cement-based materials affect the isolation properties of other barriers, like the host rock and the clay backfill material? Experimental studies were performed to understand the interface processes between cement-based materials and the host rocks (crystalline rock, Boom Clay, Opalinus Clay, Callovo-Oxfordian) or bentonite backfill and assess the impact on physical and geochemical properties.
- How do specific radionuclides or toxic elements of interest behave in the presence of cement-based materials, or in media altered by the presence of these materials? Experimental studies were performed with relevant elements (Be, C, Cl, Ca, Se, Mo, I, Ra) in cement-driven environments.
- How well are we able to predict changes in transport properties coupled with chemical and physical processes on the cementitious matrix or in the cement host rock interface? Modelling work performed in CEBAMA supported advanced data interpretation and process modelling, covering mainly physical and chemical processes responsible for the changes in transport properties and extrapolate the models to different scales for application in Safety/Performance assessment.
As main outcome and key impact of the scientific studies carried out in the CEBAMA project, advanced modelling approaches were developed which allow predicting the performance of cement-based materials in contact with the engineered and natural barriers of repositories in crystalline and argillaceous host rocks and the retention of radionuclides by cement-based materials. These improved models may be applied for high level waste disposal but also for scenarios in low and intermediate level waste disposal, currently implemented in several countries. CEBAMA has enhanced the publicly available knowledge on the performance and reliability of the engineered barrier systems (EBS) for nuclear waste repositories. This has impact on the public debate on nuclear waste disposal, also by keeping non-scientific stakeholders informed.

CEBAMA established cooperative international research for basic understanding of EBS systems, with main highlights addressing design issues, safety assessment issues, radionuclide retention and modelling. CEBAMA influences several design issues (which thus (i) impact optimisation of repository dimensioning; (ii) aid specification/selection of material parameters, material compatibility, evolution; (iii) aid specifications for experimental methods, i.e. for material quality control). CEBAMA has likewise advanced several safety assessment issues (which thus (i) provide evidence that interfaces (concrete-bentonite-host rock) can co-exist safely; (ii) provide better understanding of impacts from material interface processes for realistic description of the system performance affecting strength, flow properties, etc. and transport processes variation with time; (iii) evidence porosity changes – if and when clogging occurs; (iv) offer improved accuracy in robustness and weighting of safety functions; (v) increased modelling accuracy with new data and process understanding (WP3)), accounting for evolution of the system.

With the view on radionuclide retention the results can be used for the evaluation and assessment of radionuclide migration in cementitious repository near fields by (i) decreasing uncertainties and increasing safety margins with respect to relevant radionuclide retention processes, (ii) substantiating and justifying assumptions made with respect to the radionuclide migration behaviour in Safety Assessments, and (iii) improving sorption databases for cementitious environments for so far less studied systems and different stages of system evolution (i.e. cement-based material degradation) in the long-term. Furthermore, there is a direct input to case studies like for the LLW-ILW repository Bratrství (Czech Republic) and the licensing process for the near surface repository in the proximity of Cernavoda NPP (Romania).

Regarding modelling, CEBAMA increases the level of confidence in reactive transport models for further use in Safety Cases for near-field applications. The project provided improved reactive transport modelling tools, available as open source or upon request, to quantify how bentonite barrier or clayey host rocks could affect the integrity of normal and low-pH cementitious materials. The developed models can be used by end users to study the impact of reactive transport processes, but also other THMC coupled processes on the long-term performance of the near-field, including low-pH cement-based materials. Pore-scale reactive transport models can be used as process models in support of the Safety Case to enhance understanding of the impact of alteration of cement-based materials on their transport properties.
Besides the scientific and technical based impacts in terms of generating specific knowledge and decreasing uncertainties described above, the enhanced cooperation/exchange and knowledge transfer between different institutions on an international level was highly valuable. This is including cooperation between research institutions in different fields (i.e. Geosciences, Environmental Engineering, Radiochemistry and Computational Sciences) with access to specialized state-of-art analytical equipment and modelling tools. CEBAMA Consortium members thus became aware of the respective complementary competencies, and, based on this experience, may likely tackle future challenges in a focused, cost-saving, collaborative approach.

The experimental and modelling work in CEBAMA was to a significant extent performed by young researchers and within PhD theses. The dedicated support of young talent actively involved in the CEBAMA project contributed to the maintenance of competences, aiming to ensure continued availability of highly trained specialists for implementers and regulators.

Finally, CEBAMA has contributed to European integration by bringing together experts from several European member states and Japan. The involvement from experts coming from countries at very different stages of implementation likewise poses a positive achievement, for instance in view of sharing of expertise and resources and integrating new member states.