A long-running in situ experiment in clay: 12 years of the Bitumen–Nitrate–Clay interaction experiment at Mont Terri rock laboratory
Authors: Hendrix Katrien , Bleyen Nele , Mijnendonckx Kristel , Van Gompel Veerle , Albrecht Achim , Linard Yannick , De Cannière Pierre , Surkova Maryna , Wittebroodt Charles , Small Joe , Weyand Torben , Jendras Michael , Valcke Elie 
Published: September 2023

Abstract. In various countries such as Belgium and Germany, deep
clay formations are investigated as potential host rock to dispose of
radioactive waste. Intermediate-level long-lived bituminised waste, produced
from spent fuel reprocessing, is currently foreseen to be disposed of in
such a deep repository. This type of waste typically contains large amounts
of salts embedded in an organic matrix, of which NaNO3 is the most
prevalent. NaNO3, along with radionuclides immobilised in the bitumen
matrix, will slowly dissolve and leach out into the surrounding host rock.
The transport and reactivity of nitrate in a deep clay formation, in the
presence and absence of electron donors coming from bituminised waste (i.e.
acetate, H2), has been studied extensively inside anoxic and
water-saturated chambers in the clay in the Bitumen–Nitrate–Clay interaction
(BN) experiment. This in situ experiment in Opalinus clay is located in
the Mont Terri rock laboratory (Switzerland). The effect of a nitrate plume
on redox-sensitive radionuclides is also studied with stable selenium in
selenate form as a proxy for 79Se, an important fission product for the
long-term dose to humans. The BN experiment consists of a vertical borehole containing three
packed-off intervals, in contact with the surrounding clay through
cylindrical sintered stainless steel filter screens. The intervals are
saturated with artificial Opalinus clay pore water at a pH between 7.5 and
8, containing all major ions at concentrations specific to the location of
the BN experiment but without natural organic matter. Each interval is
connected to a water circulation system with a gear pump, a flow meter and
water sampling containers. This ensures a chemically well-mixed solution,
constant water flow and easy sampling of the interval solutions under anoxic
conditions. Water samples are regularly taken to assess the chemical
composition and the microbial population in the intervals. An online UV
spectrophotometer and pH and redox electrodes are installed in the water
circuit of the intervals, to continuously monitor the nitrate and nitrite
concentrations, pH and Eh. A given solution can be injected into the
intervals to follow up redox reactions affecting nitrate and selenate in
situ, in the presence or absence of additional electron donors. From 2011 to 2019, several injections with nitrate were performed in the
intervals to investigate the biogeochemical reactivity of nitrate in the
borehole and clay. These tests showed microbial reduction of nitrate, using
electron donors from Opalinus clay (e.g. dissolved organic matter or
pyrite). Nitrite and nitrogenous gases were formed. Pulses of electron
donors (acetate or H2) boosted the microbial activity and nitrate
reduction rate. From 2019 until now, injection tests with selenate, with and
without nitrate, have been performed. These tests showed clearly that the
presence of nitrate, a stronger oxidiser, inhibited the microbial reduction
of selenate. In the absence of nitrate, the microbial population was able to
reduce selenate to selenite and more reduced Se species. The results obtained in this in situ experiment will be summarised in this work,
addressing questions concerning the effect of biogeochemical perturbations
of the clay on the migration of redox-sensitive radionuclides.