RULET - "Retention and solubility of dose-relevant radionuclides under the reducing near-field conditions of a repository in clay or crystalline rock"

Funding source:

German Federal Ministry for the Environment, Climate Action, Nature Conservation, and Nuclear Safety (BMUKN)

Duration:

01.11.2024 till 30.04.2028

Partner:

• Karlsruhe Institute for Technology, Institute for Nuclear Waste Disposal (KIT-INE, Karlsruhe, Germany)
• Gesellschaft für Anlagen- und Reaktorsicherheit (GRS) GmbH (Braunschweig, Germany)   
• Institut für Energie- und Klimaforschung Nukleare Entsorgung und Reaktorsicherheit (IEK-6, Jülich, Germany)

People:

Zarina Salkenova (PhD student), Mario Löw (Postdoc), Natalia Mayordomo, Katharina Müller

Grant number:

02 E 12224B

Project management:

Projektträger Karlsruhe (PTKA)

Short description:

The joint project RULET deals with the safety-relevant physicochemical properties and the retention of ⁹⁹Tc, ⁷⁹Se, ²²⁶Ra and ¹⁴C in the near field of a repository. The focus is on solid phases, oxidation states and solution species as they are to be expected in the immediate vicinity of the waste containers. Special emphasis is placed on chemical reactions and equilibrium processes with secondary corrosion phases containing iron(II). The findings of the investigations of all project partners are summarized and used in a simplified material dispersion model for a generic repository. This will be applied to check whether the new findings could contribute to a reduction in conservativities or whether new mobility-increasing processes could occur. Based on the findings, recommendations are derived as to how the retention and mobilization mechanisms found should be taken into account in safety assessments. In addition, the results obtained extend the physico-chemical data basis required both for thermodynamic calculations of the radionuclide source term and for reactive mass transport models.

Research at HZDR focuses on the chemistry of technetium-99 (⁹⁹Tc), a long-lived fission product with a half-life of 2.13 × 10⁵ years. The investigated boundary conditions are defined by the near- and far-field environments of a nuclear waste repository in Germany.

Under environmentally relevant conditions, technetium occurs predominantly in the oxidation states Tc(VII) and Tc(IV). Tc(VII) is present mainly as the pertechnetate anion (TcO₄⁻). Owing to its high solubility and weak interaction with mineral surfaces, pertechnetate is commonly regarded as behaving conservatively; consequently, retardation processes are often neglected in long-term safety assessments. In contrast, Tc(IV) typically forms sparingly soluble solids, most notably TcO₂ and Tc₂S₇, and its formation significantly limits technetium mobility in groundwater systems. Therefore, the reduction of Tc(VII) to Tc(IV) represents a key process for technetium immobilization and environmental remediation. However, the mechanisms governing this multi-electron reduction is not fully understood.

To address these knowledge gaps in technetium thermodynamics and speciation, our work is divided into two complementary research areas, with the overarching goal of improving the understanding of technetium retention.

I) We investigate the interaction of Tc(VII) with Fe(II) carbonate minerals relevant to repository environments, such as siderite (FeCO3) and ankerite (Ca(Fe,Mg,Mn)(CO₃)₂). The focus lies on identifying the conditions that maximize technetium retention and on elucidating the underlying immobilization mechanisms, including surface complexation, precipitation, and structural incorporation. The applied analytical techniques include liquid scintillation counting, vibrational spectroscopy, UV–vis spectroscopy, X-ray photoelectron spectroscopy (XPS), as well as X-ray absorption spectroscopy (XAS).

II) We study the electrochemical reduction of technetium using spectroelectrochemical methods under controlled potentials and in the presence of environmentally relevant ligands (NO₃⁻, HCO₃⁻). This approach aims to derive thermodynamic parameters for technetium reduction and complex formation, and to assess the conditions under which less common technetium oxidation states (beyond Tc(IV) and Tc(VII)) may occur in aqueous solution.

Events

Joint TecRad-RULET Autumn School on Technetium and Other Fission Products (24-27.11.2024, HZDR)

The school focused on synthetic radiochemistry, radioecology, and final storage, with a particular emphasis on analytical methods for investigating fission products. In addition to scientific lectures, the focus was on exchange and networking, e.g., poster sessions, discussion of case studies, laboratory tours, and career discussions with representatives from science, industry, and German authorities. The school was organized together with the young investigator group TecRad. We thank all contributors for excellent talks and posters. Funding from BMUKN and BMFTR are highly acknowledged. Poster Awards were kindly sponsored by NMR Service! 

Selected Publications

2025