Chemical reactions of organic matter induced by elevated temperature and ionising radiation in different geologic repository host rocks, and thermochemical sulphate reduction
Country / Region: Germany
Begin of project: October 24, 2006
End of project: December 31, 2022
Status of project: April 19, 2022
This project investigates the conversion of organic matter under the influence of energy input. This extra energy can be a result of a thermal event (increased temperature), or of ionizing radiation in areas affected by the emplacement of high-level radioactive waste (cf. FEP 3.2.1 Thermal processes repository). The laboratory experiments are focussed on natural organic material present in possible geologic repository host rocks, and in the excavation disturbed zone in the subsurface repository. The investigations mainly look at processes and/or reactions, which have already been defined in the safety analysis as FEP/international FEP (features, events and processes) (cf. e.g. Stark et al. 2016, NEA, 2019), or which affect these directly.
Fig. 1: Schematic diagram showing selected reactions of organic matter, and the processes they affect in host rocks, in connection with the influence of temperature and ionizing radiation. Gases released in the aqueous phase are depicted with the subscript (aq)
Source: BGR
Organic matter can occur in host rocks in the solid host rock, in aqueous solutions in its porespace or as dissolved or free gas phase in its porespace (FEP 4.1.8 Geochemical characteristics and properties). The organic material in a host rock can be altered via the FEP 4.2.4 chemical processes into water-soluble organic compounds, or into dissolved gases such as CH4(aq), H2(aq) or CO2(aq) (shown by the brown arrows in Fig. 1). The dissolved gases formed in this way can partition into an existing separate gas phase – or one forming as a result of oversaturation (shown by yellow arrows in Fig. 1) and might induce migration (FEP 4.3.2 Gas-mediated migration – geosphere) even into the repository. The dissolved gases as H2(aq) can influence the redox environment (Eh) in the aqueous solutions in host rocks and the adjacent repository (FEP 3.2.4.2 Evolution of redox conditions - repository), or influence the pH by e.g. CO2(aq) (shown by the dashed grey arrows in Fig. 1, FEP 3.2.4.1 Evolution of pH conditions - repository). Similarly, they can also influence mineral dissolution processes and mineral precipitation processes, such as e.g. the formation of additional carbonate (FEP 4.3.1.5 Dissolution, precipitation, and crystallization - geosphere shown by black arrows in Fig. 1). In addition to the formation of gases from organic materials, the gases can also form in host rocks by the radiolytic splitting of water (blue arrow in Fig. 1). The water-soluble organic compounds, as well as organic materials in solid form present on the surfaces in host rocks, can interact with dissolved radionuclides via complexation and sorption processes (shown in Figure 1 by red arrows; FEP 4.3.1.6 Speciation and solubility and/or FEP 4.3.1.7 Sorption and desorption - geosphere). Sorption processes can also occur on mineral surfaces. Microorganisms can utilise water-soluble organic compounds (shown by green arrows in Fig. 1, FEP 3.2.5.2 Microbially/biologically mediated processes - repository) with the formation of CO2(aq), and in many cases, also by utilising dissolved H2(aq). The process FEP 3.2.6.2 Radiolysis - repository can affect the organic matter as well as the water (shown in Figure 1 by the excited water molecule in blue), and can result e.g. in the formation of hydrogen H2(aq). In addition, investigations are carried out in context of the process of thermochemical sulphate reduction (cf. FEP in Stark et al. 2016) not yet included in the international FEP list.
Fig. 2: Example of a high-pressure reactor system (right side of the picture) in the BGR high-pressure experimental laboratory, which enables diverse experiments under in-situ pressure and temperature conditions. The gaseous products can be monitored directly by online analytical gas chromatography systems (left side of the picture)
Source: BGR
Experiments are carried out for these aims in the laboratory mainly in high pressure reactors (see Fig. 2). Conditions can be recreated here which correspond to the in-situ pressure and temperature conditions which occur underground. Comparative investigations on various potential host rocks allow the determination of parameters, which then can be incorporated in subsequent safety analyses. Currently, an experimental system is being developed that allows the parallel determination of gas formation from several host rock cores under elevated temperature and pressure conditions. This is being designed in such a simple way and documented in such detail that it could be set up at various institutions and used effectively during future surface and underground exploration in the course of the site search.
The investigations undertaken in the BGR laboratories are conducted in close co-operation with projects undertaken by and with other partners in in-situ underground laboratories, such as in Mont Terri (Switzerland), to verify the transfer of the findings derived from the BGR laboratory investigations, and to support the interpretation of the experiments carried out in the in-situ rock laboratories.
Literature:
Stark, L., Jahn, S., Jobmann, M., Lommerzheim, A., Meleshyn, A., Mrugalla, S., Reinhold, K. & Rübel, A. (2016): FEP Katalog für das Endlagerstandortmodell SÜD - Dokumentation. Bundesanstalt für Geowissenschaften und Rohstoffe (BGR), DBE Technology GmbH, Gesellschaft für Anlagen- und Reaktorsicherheit (GRS) mbH, Anlage zum Bericht, TEC-16-2016-TB, Projekt ANSICHT – Methodik und Anwendungsbezug eines Sicherheits- und Nachweiskonzeptes für ein HAW-Endlager im Tonstein; Peine.
NEA (2019): International Features, Events and Processes (IFEP) List for the Deep Geological Disposal of Radioactive Waste Version 3.0. - Radioactive Waste Management and Decommissioning, report NEA/RWM/R(2019)1; https://www.oecd-nea.org/upload/docs/application/pdf/2020-01/dir1/rwm-r2019-1.pdf