BN Experiment: Status report of phase 26: Selenate injection tests in interval 3: Technical Note 2021-89. Status October 2021

Clay formations are considered as the host rock for the geological disposal of bituminised low-level and intermediate-level long-lived radioactive waste, as they exhibit favourable hydromechanical and geochemical characteristics, which are expected to retard the migration of radionuclides. Along with radionuclides, certain classes of bituminised radioactive waste contain high concentrations of NaNO3, dispersed into the hydrophobic bitumen matrix used to stabilise the waste. After saturation of the disposal gallery, bituminised waste will take up water, resulting in the leaching of significant amounts of NaNO3 and low amounts of soluble organic compounds from the bitumen degradation, i.e. bitumen degradation products (BDP), into the clay pore water. This nitrate plume could initiate several geochemical and biochemical processes in the clay surrounding the waste disposal cells, possibly affecting the barrier function of a part of the host rock. The aim of the Bitumen-Nitrate-Clay interaction (BN) experiment is to study these processes. This TN reports the results of the final injection test with selenate and nitrate in interval 3, as well as the final results of the abiotic batch test. An in depth interpretation and discussion of the results is not the aim of this note.