Ph.D. topics

Geochemical heterogeneities and parameter variances for radionuclide retention in crystalline rocks - relevance, relation and reduction

Promotionsstudent:

Alexandra Duckstein

Betreuer:

Prof. Dr. Thorsten Stumpf, Prof. Dr. Vinzenz Brendler, Dr. Solveig Pospiech (HZDR)

Abteilung:

Acinides thermodynamics

Zeitraum:

06/2023–05/2026

A large number of complex factors play a role in the search for a suitable repository for radioactive materials, including in the field of geosciences. In order to find the best site, it is necessary to understand the importance of the relevant factors and the possible interactions between them. It is essential to consider not only the conditions in the repository itself, but also in the surrounding host rock. In the event of radionuclides leaking from the repository into the host rock, they can be transported onwards via a liquid phase. Retention mechanisms such as the sorption of radionuclides on the mineral phase along the pathways are important in this respect.

When modelling radionuclide retention, many different parameters, which carry uncertainties, must be taken into account:

  • Mineralogy of the host rock: mineral inventory, permeability, porosity, aperture of pore spaces...
  • Groundwater composition: concentration of radionuclides, pH value, concentration of dissolved inorganic carbon, complexing agents and competing ions...
  • Thermodynamic sorption data of all reactants under consideration

The aim of this PhD project is to identify the important model parameters and to understand their uncertainties and interactions. Ideally, this will allow an improvement in the predictive quality of the safety case for a potential repository and more efficient modelling (reduction of the model).

One focus is on the geostatistical simulation of the mineralogical composition of the host rock in 2D. This allows a quantification of the parameter uncertainties regarding the host rock which are essential for subsequent sensitivity analyses of the geochemical simulation, e.g. for the determination of Kd values (distribution coefficients).

The individual steps are to be established in a modular workflow. Firstly, a program-independent formulation is to be made and the individual steps subsequently implemented in Python.

The work presented here is part of the BMUV-funded joint project SANGUR (Systematic sensitivity analysis for mechanistic geochemical models using field data from crystalline rock). Its aim is to gain a better understanding of parameter uncertainties in the context of nuclear waste disposal. The project focuses on the question of the long-term safety of future repository systems in deep geological formations, especially in the crystalline host rock granite.

As part of this project, the project partner TU Clausthal is also further developing the methods used here for the sensitivity analysis of geochemical models and optimizing them with regard to the research question. Thanks to collaboration with the project partner ÚJV Řež, field data and its interpretation from the Czech underground laboratory Bukov can be used in the project.