Generation of gases and low-molecular-weight organic acids in Opalinus Clay at elevated temperatures

Due to their low permeability, high retention capability for radionuclides, and self-sealing of fractures by swelling, claystones have beneficial properties as natural barrier for high-level heat-emitting nuclear waste (HLW). Temperatures at the surface of HLW containers in claystone formations are expected to be in the range of 90–150 °C. Elevated temperatures can cause clay mineral transformation reactions reducing the radionuclide retention potential of the host rock.

In this thesis, the thermal transformation of organic matter in a host rock for HLW disposal, the Opalinus Clay (OPA; Mont Terri, St. Ursanne, Switzerland), was investigated in hydrous pyrolysis experiments at temperatures from 80–200 °C – and beyond. Geochemical reactions were monitored over periods of several weeks. The experiments demonstrated the generation of gaseous CO2 and C1–C4 hydrocarbons. CO2(g) was the dominant gas. Gas yields increased with increasing temperature. CO2(g) originated predominantly from the dissolution of carbonate mineral phases in the OPA sample material and, to a lesser extent, from the thermal transformation of OPA organic matter. Analyses of aqueous fluid samples showed the generation of LMWOA, predominantly acetate, followed by oxalate and formate. The yields of the acids increased with increasing temperature, with the exception of significantly lower quantities of oxalate and formate at temperatures ≥160 °C.
The application of kinetic data on thermal LMWOA decomposition (e.g., decarboxylation) from the literature demonstrated that simultaneous generation and decomposition reactions of LMWOA affect the prediction of gas generation (e.g., CO2(g) + CH4(g)) by decarboxylation.

It is concluded that the quantities of generated gases and LMWOA, when confronted with geochemical reactions decomposing and/or immobilizing some of them, are unlikely to affect the integrity and long-term safety of a HLW repository in the Opalinus Clay.