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Fission and radiation protection

Understanding and Physical and Numerical Modelling of the Key Processes in the Near-field and their Coupling for Different Host Rocks and Repository Strategies

Resolving the issue of high-level radioactive waste is a strategic policy objective of European research. Disposal of the waste in deep geological repositories is currently the most advanced disposal concept. Integrated Project NF-PRO brings together a wide spectrum of European expertise to address outstanding issues in the understanding of near-field (the area immediately surrounding the stored waste) processes to assist modelling studies. NF-PRO results will be used in safety assessments and to optimise repository designs.

Understanding the near-field in repositories

FEBEX in situ THM test in granite (Grimsel, CH) with dummy SF canister and bentonite buffer provides experimental data to NF-PRO © Courtesy: NAGRA
FEBEX in situ THM test in granite (Grimsel, CH) with dummy SF canister and bentonite buffer provides experimental data to NF-PRO
© Courtesy: NAGRA
The near-field of a geological repository includes the area surrounding the disposed waste, including the waste forms and the engineered barrier system (EBS). In all repository concepts, the EBS fulfils an essential safety function and is tailored to the specific disposal environment in order to prevent or to delay the release of long-lived radionuclides from the waste to the host rock over the extended periods of time necessary for safe disposal. The Integrated Project NF-PRO will investigate the performance of the near-field by studying dominant processes affecting the isolation of radioactive waste.

Experimental work within NF-PRO, in parallel with modelling of near-field processes, will identify and provide data that are fundamental for the understanding of the total system behaviour of the near-field of a geological repository for high level long-lived radioactive waste disposal but which have not yet been investigated in sufficient detail.

Interaction of theory, experiment and performance assessment

Among the repository designs being considered within the EU, the near-field plays an important role as regards safety. However, the near-field is a complex environment consisting of various components including the waste form, the waste canisters, backfills, seals, plugs and the disturbed zone of the host rock component. The detailed evaluation of overall EBS behaviour in both time and space and the application of results in performance and safety assessments is one of the major scientific challenges in R&D concerning radioactive waste disposal. As such it requires an integrated and multidisciplinary approach. Different repository concepts currently under investigation in EU Member States will be addressed in NFPRO, as will the various host rocks – salt, granite and clay.

NF-PRO aims to provide a comprehensive assessment of all safety-related issues concerning the near-field, and will concentrate on the most important outstanding issues. A strong link will be established between laboratory and in situ experiments, modelling and performance assessment. Specific research components will address key processes controlling dissolution of and release from the waste matrix, chemical processes taking place in the EBS, the thermohydro mechanical (THM) evolution of the EBS, and the characteristics and evolution of the excavation disturbed zone (EDZ). How these processes interact and their integration in performance assessment is an essential goal.

The 40 organisations in the NF-PRO consortium include radioactive waste agencies and end-users, large nuclear research organisations, and a range of universities, industrial and government partners which will provide specialist skills and facilities. Broad participation will be a decisive factor in the success of the project given the large number of complementary scientific disciplines that are required in investigations related to near-field processes.

Scientific basis for evaluation, outstanding issues addressed

NF-PRO will establish a comprehensive scientific basis for evaluating and modelling the safety function ‘containment and minimisation of release’ for the near-field in geological waste repositories. The main uncertainties and issues concerning the EBS system relate to the thermo-hydromechanical-chemical-biological (THMCB) properties of clay-based buffers and backfills, the evolution of properties and parameter values over long time periods, issues in relation to gas generation through corrosion of waste canisters, EBS degradation rates and interaction with the host rock or between EBS materials (for example, interactions between cement and bentonite), canister corrosion and possible canister defects, and radionuclide retention properties of the buffer and backfill materials. All these issues will be addressed by NF-PRO.

In particular, the project will resolve outstanding issues with respect to the key processes controlling the dissolution of the vitrified waste/spent fuel matrix material, including processes related to the release of radionuclides from the waste matrix to the geological environment. It will establish a comprehensive insight into the chemical processes and materials interactions taking place in the near-field and investigate the evolution of the thermal, hydrological and mechanical processes taking place in the near-field along with their influence on the total system. It will assess the impact of these processes on the EDZ on the physicochemical conditions of the near-field, from repository construction until equilibrium is achieved. These key data will identify the critical processes and their associations to determine the evolution of the near-field and how this affects radionuclide release to the outside environment. The results will be provided as concise but accurate models and data for input to performance assessments of repository systems.

Improved safety assessments, optimised designs

NF-PRO will integrate European R&D on key processes in the near-field by bringing together scientific disciplines and research teams that currently work independently, as well as by strengthening interactions between major R&D organisations and radioactive waste management agencies/implementing organisations throughout Europe. This will assist the definition of common policies and techniques for disposal.

Conclusions from NF-PRO will be used in safety assessments as well as for optimising repository designs, with the aim of enhancing safety disposal. The project will have a strategic impact as it will strengthen the scientific-technical basis for geological disposal and contribute to progress in safety assessments and the optimisation of repository designs under investigation in the European Union.

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