Quantification of long-term entrapment of key radionuclides in solid phases to inform reactive transport models and the influence of redox
Major highlights (June 2022 - May 2023) and forward look
Initial project program was allocated for 48PM. Despite obvious difficulties during the COVID lockdown most of the partners have managed to catch up with the schedule and finalize the experimental activities within the initially planned project period. These scientific achievements and the optimization of interaction within the WP and with the end-users are well in-line with the KPIs defined for the work package. The remaining project period covered by cost free extension is devoted to the finalization of reporting, preparation of the final SOTA report and dissemination of results in peer reviewed journals.
On the scientific side, project has successfully achieved the main research targets and scientific objectives. To mention few specific highlights relevant of the repository safety, the experiments conducted in Task 2 confirm that electrostatic properties of clay mineral surface have quantitative impact on the mobility of the cations absorbed by cation exchange in the interlayer or electrical double layer (mobile surface species are localized on the planar surfaces). It could be demonstrated that the sorption competition is important factor to be considered when interpreting data on natural samples as had been successfully shown for different clay rocks (e.g. COx). Using state of the art spectroscopic technics, strong evidences could be provided that sorption reversibility play an important role under high loading of the samples. In the same time surface complexation at trace concentration leads to formation of strongly bound surface complexes having features of structurally incorporated ions with very limited sorption reversibility.
The combination of advanced spectroscopic techniques and numerical simulations in Task 3 provided new insight on the surface reactivity of redox sensitive elements. Thus, EXAFS data of fresh and on aged sample combined with DFT calculations provided evidence that TcO2·xH2O forms an inorganic polymer with zigzag chains of edge-sharing Tc(μ‑O)4(H2O)2 octahedra, correcting the previously proposed model of linear chains. The results also indicate an aging process consistent with elongation of the chains and formation of bonds between parallel chains.
The experimental data obtained in the project provide a solid basis for the calibration of the sorption and transport models at reference chemical environment and ambient (equilibrium) conditions. However, possible evolution scenario of real repository may include elevated temperature phase and chemical gradients. These aspects have not been covered in this project and is worth future investigations. Further important aspects include small scale heterogeneities on the natural rocks.
In the collaborative spirit of joint programming a highly successful winter school on reactive transport simulations was organized by ACED-FUTURE-DONUT in Switzerland at the University of Berne on 2-10 February 2022. These event had received a highly positive feedback from various experts in the field.
This WP aims at realizing a step change in quantitative mechanistic understanding of radionuclide retention in the repository barrier system, the key mission of any repository for radioactive waste. In consequence, the raison d’être of this WP concerns the identification of constraints and the increase in predictability of RN migration properties in “real” clay and crystalline rocks, quantifying the influence of key parameters of the heterogeneous rock/water system such a rock structure, redox interfaces, water saturation, reversibility etc. with the goal to develop multicomponent mechanistic sorption models, fracture and/or pore scale simulations of radionuclides transport in both in crystalline clay rock considering the combined analysis of reactivity, structure, flow field, and RN mobility/retention.
“ Radionuclide mobility” has been identified by the mandated actors of WMO, TSO and RE as one of the key themes (4) of the EJP, the SRA and its concretization in the roadmap. It is a key theme in all radioactive waste management countries in Europe, a cornerstone for any proof of safety of nuclear waste disposal concepts. Hence, it was evident to all actors that this theme should also be part of the first EJP, acknowledging that there has been research on the various topics of radionuclide migration for more than 30 years, often funded by the European Commission, but realizing as well that various key themes have not been addressed in previous European projects (e.g. FUNMIG, SKIN, RECOSY) with sufficient depth and with sufficient potential for applicability on the real repository systems in clay or crystalline rock. The results of the project are expected to reduce uncertainties and over-conservatism of current approaches and improve the scientific basis, the realism and credibility for the safety case of deep geological disposal in clay and crystalline rock.