Deliverables Conceptual model for the description of the migration of radionuclides in a reducing, organic rich clay environment that can be implemented by performance assessment The aim is to come to conceptual models describing the migration of RN in reducing, organic, rich sediments. These models should be scientific underbuilt and directly usable for performance assessment. Status: in progress and continuously updated whenever new insights are obtained Results: Conceptual models were described for the studied radionuclides and can be summarises as follows: The transport mechanism for the studied radionuclides (except for SeO42-) is dominated by an immobilisation process with a constant release to the aqueous phase. In case of U and Se the constant release is controlled by a solubility limit of the precipitated phases and the transport is independent of natural organic matter (NOM). For Am - as representative of trivalent actinides/lanthanides - the constant release is due to a strong interaction with the solid phase (immobile OM?) and not determined by a solubility limit. The transport is correlated with NOM. For Pu the underlying processes are not understood but resemble that of Am. PA calculations can therefore be performed with a limited set of parameters: concentration limit (or solubility limit), Dpore, R and diffusion accessible porosity. Values can be selected from experimental observations and interpretations. Software code for transport calculations of complexed organic species The aim is to refine or develop scientific software, which can be used to calculate migration of organic species and radionuclides complexed by organic species. The software is based on fundamental thermodynamic principles and conceptual mechanisms, which are established during the project. The software can then be used to model, interpret and simulate the migration behaviour of radionuclides (RN) through reducing organic rich sediment at scales relevant for Performance Assessment. The kinetics of radionuclide complexation to, and destabilisation from, organic matter, are potentially significant factors in radionuclides migration. The model, POPCORN, which evaluates the effects of POPulations of COlloids on RadioNuclide transport, has been developed to study these kinetic effects. Status: in progress - the POPCORN model has been developed and tested, and initial applications have been made, in the TRANCOM II project. Results: POPCORN has been used to evaluate organic matter (OM) diffusion experiments and 241Am-14C-OM migration experiments. Model application at the repository performance assessment level may also be possible. Key model features are: representation of dissolved radionuclides and radionuclides associated with mobile and immobile OM; linear kinetic representations of radionuclide adsorption (complexation) to, and desorption (destabilisation) from, mobile and immobile OM; and representation of rates of attachment of mobile OM to the rock matrix and rates of detachment of immobile OM from the rock matrix. The model reproduces the observations from the diffusion and migration experiments. The diffusion experiments were simulated assuming diffusion and filtration of OM by attachment to the surface of the clay matrix. The instability of the 241Am-OM complex was successfully characterised by kinetic adsorption constants for mobile and immobile OM, with the interaction of OM with the clay rock providing filtering. A sub-population of the original 241Am-OM complex in the larger size range, and characterised by a low dispersion and a greater potential for filtering, appears to be the most stable. Relationships between the inverse of the rate terms that describe OM attachment and adsorption kinetics and the estimated peak advection-driven OM breakthrough time have been identified. The relationships offer potential means of extrapolating parameter values for model application to repository-scale problems. TRANCOM-II: Migration case study: Transport of radionuclides in a reducing clay sediment For deep geological disposal of radioactive waste in clay, it is essential to consider the mobility of radionuclides. This project addressed the migration behaviour of radionuclides (U, Pu, Se and Am), identified as critical by Performance Assessment, in a reducing clay environment, with special emphasis on the role of the organic matter. Considerable attention was devoted to developing and demonstrating concepts describing radionuclide migration and the acquisition of the model parameters. An equal amount of effort was devoted to batch experiments, aimed at understanding the mechanisms behind the migration. The following conclusions concerning the transport behaviour of the studied radionuclides in the OM rich Boom Clay could be made: - Only for Pu and Am an increase in solubility was observed due to interaction with NOM. Colloids were observed in many cases and are very prominent in case of Uranium. - Despite the possible enhanced solubility due to OM complexation and the increased radionuclide concentrations in solution due to colloid formation, no increased radionuclide concentrations after migration through clay cores were observed. - The measured constant concentrations are of the same order of magnitude as the predicted solubility values or lower. - In all cases, an immobilisation process governed the transport of the radionuclides, with one important exception: SeO4(2)- which is not immobilised and moves unretarded. - Only for Am we observed undoubtedly that the transport of the mobile Am fraction was linked with mobile OM. These observations were translated into conceptual models: - Selenium The selenate transport mechanism can be described as a diffusion process of an unretarded anionic species. The selenite transport mechanism can be described as an immobilisation, by a kinetic controlled sorption-reduction followed by a solubility limited release of Se which diffuses unretarded and independent of NOM through the BC. The selenide transport mechanism can be described as an immobilisation/solubility limited release of Se which diffuses unretarded and independent through the BC. - Uranium The uranium transport mechanism can be described as an immobilisation, precipitation of an U(IV) solid phase, followed by a solubility limited release of U for which the diffusion is independent of NOM but retarded due to sorption onto the BC solid phase. - Americium (as representative of trivalent actinides and lanthanides) The Am transport mechanism can be described by an immobilisation (quasi irreversible interaction of Am with the solid phase) followed by a constant release of Am, which migrates associated with mobile OM. This immobilisation/constant leaching resembles solubility like mechanism and can be implemented as a constant concentration source, and transport of the mobilised Am through the Boom Clay occurs with a similar Dapp and R as the mobile OM. - Plutonium The Pu transport mechanism can be described by an immobilisation (solubility or quasi irreversible interaction with the solid phase) followed by a constant release of Pu. This immobilisation/constant release resembles solubility like mechanism and can be implemented as a constant concentration source. Due to the observed association with NOM, we consider that the mobilised Pu migrates through the Boom Clay with similar Dapp and R as for the mobile OM. Internet site with the results of the project Emphasis is put on communication of our results to the public. In this respect an internet site will be developed. All information and data will be made available for other research teams for further interpretation, comparison, modelling. Dissemination of knowledge will further be done through accurate visualisations (simulations of diffusion patterns in the clay core) and sensitivity analysis by PA people allowing to estimate the importance of processes identified in this project. Status: In progress. Currently all information is available on an ftp-server. This will be transferred to a project web site after approval of the final EC report. C-14 labelling technique for natural organic matter Natural organic matter extracted from Boom Clay, has been radio-labelled with 14C-methylamine and used in laboratory and field investigations for observing the migration behaviour of organic matter and organic matter complexed with RN. An alternative method of radio-labelling the organic matter using 14C-formaldehyde has now been developed and the stability of the label verified. This product has also been used in lab migration and batch reaction studies. Internal coherent database for the interaction of radionuclides with organic matter and clay under reducing conditions For the purpose of geochemical modeling, high quality database is essential to avoid GIGO (Garbage In - Garbage Out). The aim is therefore to pursue a good quality chemical thermodynamic database. To that end, MOLDATA - a database intended to be specific to clay environments is initiated with two objectives: (1) the database should reflect the most up-to-date scientific research in the field; (2) the newly collected experimental data should be implemented in a coherent way considering other database generally accepted in the field. Status: in progress (continuous updating) Results: The first version of MOLDATA has been established. Major achievements include: - Implementation of newly updated NEA-TDB values;- Implementation of data collected from the first and the current phase of the TRANCOM project; - Establishment of a Boom Clay specific ion exchange sub-database;- Review and re-implementation of data for aluminous minerals and sulphides that are known to be of poor quality in the original database but important for a clay environment. Searching for OpenAIRE data... There was an error trying to search data from OpenAIRE No results available
