Long-Term Impact on Humans from a Repository for Radioactive Waste: Experimentally Verified Radioecological Biosphere Model

Transport processes of long-lived radionuclides potentially released from a repository for radioactive waste in the far field must be known for the long-term prognosis of a repository system. They represent an integral part of radioecological biosphere modelling. For this, a proper description how the radionuclides migrate from the ground water level via the soil into plants and thus into the human food chain is needed. Data from well-defined experiments represent a valuable basis for the model parametrization. The joint project TRAVARIS, funded by the German Federal Ministry of Education and Research, focuses on the experimental investigation of water and radionuclide transport in the soil and the uptake mechanisms of radionuclides in plants that are interlinked with the model development. The overall objective of the joint project two-fold. Firstly, we aim to deepen the mechanistic understanding of micro- to mesoscale mobility processes of selected radionuclides in the pedosphere and rhizosphere including uptake mechanisms in plants. Secondly, we integrate these findings into the PHREEQC calculations of sorption processes and into the macroscale probabilistic biosphere modeling with dose estimation. In coordination with the project partners, the experiments are set up and the measured values are recorded and included in the model approaches at different scales of the process considerations. The measurement results (parameter values) are used to calibrate the model approaches.

In our contribution, we will present the radioecological biosphere model developed with the software AFRY Intelligent Scenario Modelling. It is a compartment model that consists of three main model parts. The model part “soil transport” describes water level fluctuations in the pore space under the influence of evapotranspiration along several soil horizons and the associated interaction of the radionuclides in the pedosphere. The implementation of water movement and hysteresis effects due to water level fluctuations as well as radionuclide migration is based on the results from lysimeter, and column experiments carried out by the project partners. To describe the sorption processes, the K_d-value variability due do different mineral phases, the organic matter content and the pH value is implemented in the transport model based on the “Smart-Kd concept”. The description of the K_d-value variability is also linked with the results of the PHREEQC calculations of sorption processes. The model part “plant” takes into account the microscale accumulation processes of radionuclides in the root area and the distribution in plant compartments. Here, parameter values resulting from the lysimeter experiments with planting and the rhizoboxes are considered as well as the results of the laboratory investigations on the role of the plant transporters and exudates in the rhizosphere. This is followed by the dose estimation over long periods of time and supplemented with a subsequent statistical analysis of the uncertainties in the determination of the exposure using Monte Carlo simulations (model part “dose estimation”). A sensitivity analysis investigates the influence of individual parameters on the overall result of the biosphere model - the dose to humans.