Dissolution and release from the waste matrix (RTDC1)
In a geological repository for the disposal of high-level vitrified waste and spent fuel, the waste matrix constitutes the first barrier confining the disposed radionuclides. Various factors and processes such as radiation, thermal, hydraulic and geomechanical processes influence the stability and the evolution of the waste matrix. For example, interactions with repository groundwater as well as with engineered barriers forming part of the disposal system might affect the isolation capacity of the waste matrix and may lead to the slow dissolution of the waste matrix and the release of radionuclides. NF-PRO’s research component 1 investigates key processes having an effect on the isolation capacity of the waste matrix Work performed under NF-PRO concentrates on two main waste types: vitrified radioactive waste and spent fuel.
The largest fraction of radionuclides present vitrified high level waste or spent nuclear fuel is incorporated in the waste matrix (structure of the solid phase) . In spent fuel, a small portion of radionuclides is not embedded in the waste matrix but deposited on surfaces or grain boundaries.
Upon contact with groundwater, the waste matrix will gradually dissolve and undergo alterations leading to the release of radionuclides. Accordingly, the behaviour of the disposed vitrified waste and spent fuel in the presence of groundwater is a key element when evaluating the performance of the disposal system over long periods of time. The containment properties of the waste matrix are controlled by the waste matrix characteristics as well as by various factors such as radiation, thermal, hydraulic and geochemical processes and the presence of engineered barriers. Key performance indicators are waste matrix dissolution rates, the mobile fraction of radionuclides, and radionuclide retention parameters. The time-dependent rate at which radionuclides are released from the disposed waste is known as the source term. Processes having an effect on the time-dependent evolution of the waste matrix (influencing the isolation capacity of vitrified high-level radioactive waste and spent fuel) have been studied for many years as part of both national and Community-supported R&D research projects. The Integrated Project NF-PRO builds on scientific results from these programmes. In doing so, NF-PRO's research efforts aim to resolve outstanding key issues and reduce experimental and modelling uncertainties related to the long-term evolution of the vitrified waste and spent fuel matrix under specific repository conditions.
The overall purpose of RTD component 1 is to further develop and refine the scientific basis with the purpose of quantifying and predicting key-processes affecting matrix dissolution and radionuclide release from vitrified HLW and spent nuclear fuel, taking into account the relevant near-field processes, their couplings and their evolution.
NF-PRO's RTD Component 1 includes experimental and modelling studies. In particular, the dissolution of the glass and spent fuel matrix and associated release of radionuclides are studied. This includes experimental studies investigating the dissolution of and the radionuclide release from the vitrified waste in presence of near field materials (WP 1.2) and the development of models allowing to describe and to understand quantitatively experimental results and to predict geochemical and kinetic processes controlling glass dissolution and associated radionuclide release under near field conditions (WP 1.3). Key processes governing the source term for spent nuclear fuel are studied in work package 1.4 and 1.5. Work package 1.4 investigates the evolution of the spent fuel matrix in the normal evolution and the early failure scenario and impact on radionuclide release. Work package 1.5 studies selected key processes having an effect on the spent fuel matrix dissolution under representative near-field repository conditions. Finally work package 1.6 intends to develop approaches for ensuring integration of the results derived from RTD component 1 in an integrated near field performance assessment (RTD Component 5).