Hydro-chemical modelling of in situ behaviour of bituminized radioactive waste in Boom Clay

The hydro-chemical (CH) interaction between swelling Eurobitum bituminized radioactive waste (BW) and Boom Clay was investigated to assess the feasibility of geological disposal for the long-term management of this waste. First, the long-term behaviour of BW in contact with water was studied. A CH formulation of chemically and hydraulically coupled flow processes in porous materials containing salt crystals is discussed. The formulation incorporates the strong dependence of the osmotic efficiency of the bitumen membrane on porosity and assumes the existence of high salt concentration gradients that are maintained for a long time and that influence the density and motion of the fluid. The impacts of temporal and spatial variations of key transport parameters (i.e. osmotic efficiency (s), intrinsic permeability (k), diffusion, etc.) were investigated. Porosity was considered the basic variable. For BW porosity varies in time because of the water uptake and subsequent processes (i.e. dissolution of salt crystals, swelling of hydrating layers, compression of highly leached layers). New expressions of s and k describing the dependence of these parameters on porosity are proposed. Several cases were analysed. The numerical analysis was proven to be able to furnish a satisfactory representation of the main observed patterns of the behaviour in terms of osmotic-induced swelling, leached mass of NaNO3 and progression of the hydration front when heterogeneous porosity and crystal distributions have been assumed. Second, the long-term behaviour of real Eurobitum drums in disposal conditions, and in particular its interaction with the surrounding clay, was investigated. Results of a CH analysis are presented.Peer ReviewedPostprint (published version

The use of µCT and ESEM in the study of the osmosis-induced water uptake by eurobitum bituminized radioactive waste

Laboratory water uptake tests are performed at the Belgian Nuclear Research Centre SCKCEN to obtain insight into the hydromechanical behavior of Eurobitum bituminized radioactive waste under geological disposal conditions. Small nonradioactive and radioactive Eurobitum samples are hydrated in restricted swelling conditions (i.e., nearly constant volume conditions and constant stress conditions). Microfocus X-ray computer tomography (µCT) proves to be a very suitable technique to follow up the ingress of water in the samples. µCT analyses demonstrate that, under the studied hydration conditions, the water uptake by Eurobitum samples is a diffusion controlled process. A characterization of the partially leached samples with environmental scanning electron microscopy (ESEM) shows that the hydration of salt crystals and the subsequent dilution of the salt solution result in an increase in pore size that is limited to a few tens of µm in restricted swelling conditions. The µCT and ESEM analyses allow improvement in the understanding of water uptake by Eurobitum in restricted swelling conditions. In this article we discuss the µCT and ESEM analyses of nonradioactive Eurobitum samples that were hydrated for 2 to 4 years at a constant stress of 1, 22, 33, and 44 bar or in nearly constant volume conditions.status: publishe

Pyrite oxidation by nitrate and nitrite in sodium bicarbonate solution under anoxic and abiotic conditions

Pyrite reactivity with nitrate and nitrite was assessed in long-term batch tests to assess its possible oxidation in anoxic conditions at pH 8.5, at room temperature and atmospheric pressure. This was done in the frame of compatibility studies of nitrate-containing radioactive waste with a pyrite-containing clay host rock for geological disposal. Abiotic pyrite suspensions were prepared under inert atmosphere in 15 mM bicarbonate medium, as this simulates the inorganic carbon and pH conditions in the pore water of Boom Clay, which is a potential host rock for geological disposal in Belgium. Two forms of pyrite powder were used, formed via different genetic pathways and exhibiting a different morphology, namely pyrite obtained by crushing a large crystal cluster and pyrite extracted from Boom Clay by flotation. The reactivity of these two pyrite forms with nitrate and nitrite is reported and compared. Overall, after 2–2.5 years under abiotic conditions and inert atmosphere, no significant reaction between crushed pyrite and nitrate was detected, while a very limited reaction was observed between Boom Clay pyrite and nitrate. Between pyrite and nitrite, which is known to be more reactive than nitrate, a slow reaction took place for both forms of pyrite, with no significantly higher reactivity with the Boom Blay pyrite. The variability between the replicates of Boom Clay pyrite were also larger than for crushed pyrite. Overall, nitrate and nitrite induced a very limited, if any, oxidation reaction of the pyrite powders. These observations are important in assessing the safety of geological disposal of nitrate-containing radioactive waste

Surface-mediated redox activity in the Pyrite – Nitrate/nitrite – Hydrogen system under conditions relevant for the geological disposal of bituminized waste in Boom Clay

The reactivity of a system with pyrite powder, nitrate or nitrite, and hydrogen in a 15 mM sodium bicarbonate solution was assessed over the course of 2 years in the frame of compatibility studies of nitrate-containing bituminized radioactive waste with the host rock for final disposal. A series of batch tests was performed with pyrite suspensions in bicarbonate solution to which nitrate or nitrite was added before filling the headspace of the recipient with a constant, and non-renewed, volume of 100% pure hydrogen gas (initial PH2 1.5 bara). Under anoxic conditions and at room temperature, hydrogen reacted readily with nitrate and nitrite in the presence of pyrite powder. Ammonia was formed while hydrogen was consumed. Based on the XPS analyses of the pyrite surface and the absence of dissolved pyrite oxidation products, the pyrite surface was not oxidized. Moreover, no reaction between hydrogen and nitrate or nitrite was observed in the absence of pyrite. This reaction was thus clearly mediated by the Boom Clay pyrite surface. The reducing atmosphere kept the pyrite surface intact and protected it from precipitation of carbonates from the medium, thus effectively preventing pyrite surface deactivation, previously observed under anoxic conditions in the absence of H2. Overall, only 5% of the 0.1 M of nitrate that was added to the tests, was reduced over the course of 2 years, without complete consumption of H2. Nitrite was added in a lower concentration of 0.05 M, but was more reactive: about 50% of nitrite was reduced, producing stoichiometric amounts of ammonia, and nearly depleting the H2 in the gas phase. The possible consequences of these processes for the final repository performance are also discussed

Hydro-chemical modelling of in situ behaviour of bituminized radioactive waste in Boom Clay

The hydro-chemical (CH) interaction between swelling Eurobitum bituminized radioactive waste (BW) and Boom Clay was investigated to assess the feasibility of geological disposal for the long-term management of this waste. First, the long-term behaviour of BW in contact with water was studied. A CH formulation of chemically and hydraulically coupled flow processes in porous materials containing salt crystals is discussed. The formulation incorporates the strong dependence of the osmotic efficiency of the bitumen membrane on porosity and assumes the existence of high salt concentration gradients that are maintained for a long time and that influence the density and motion of the fluid. The impacts of temporal and spatial variations of key transport parameters (i.e. osmotic efficiency (s), intrinsic permeability (k), diffusion, etc.) were investigated. Porosity was considered the basic variable. For BW porosity varies in time because of the water uptake and subsequent processes (i.e. dissolution of salt crystals, swelling of hydrating layers, compression of highly leached layers). New expressions of s and k describing the dependence of these parameters on porosity are proposed. Several cases were analysed. The numerical analysis was proven to be able to furnish a satisfactory representation of the main observed patterns of the behaviour in terms of osmotic-induced swelling, leached mass of NaNO3 and progression of the hydration front when heterogeneous porosity and crystal distributions have been assumed. Second, the long-term behaviour of real Eurobitum drums in disposal conditions, and in particular its interaction with the surrounding clay, was investigated. Results of a CH analysis are presented.Peer Reviewe

Ex and In Situ Reactivity and Sorption of Sélénium in Opalinus Clay in the Presence of a Sélénium Reducing Microbial Community

International audience79Se is a critical radionuclide conceming the safety of deep geological disposai of certain radioactive wastes in clay-rich formations. To study the fate of sélénium oxyanions in clayey rocks in the presence of a sélénium reducing microbial community, in situ tests were performed in the Opalinus Clay at the Mont Terri Rock Laboratory (Switzerland). Furthermore, biotic and abioticbatch tests were performed to assess Se(VI) and Se(IV) reactivity in the presence of Opalinus Clay and/or stainless steel, in order to support the interprétation of the in situ tests. Geochemical modeling was applied to simulate Se(VI) réduction, Se(IV) sorption and solubility, and diffusion processes, This study shows that microbial activity is required to transform Se(VI) into more reduced and sorbing Se species in the Opalinus Clay, while in abiotic conditions, Se(VI) remains unreactive. On the other hand, Se(IV) can be reduced by microorganisms but can also sorb in the presence of clay without microorganisms. In situ microbial réduction of Se oxyanions can occur with électron donors provided by the clay itself. If microorganisms would be active in the clay surrounding a disposai facility, microbial réduction of leached Se could thus contribute to the overall rétention of Se in clayey host rocks

A phenomenological model for hydration heat evolution of a cemented waste form

This paper presents an extension of an existing phenomenological model for predicting the evolution of hydration heat of a cemented waste form. The cemented waste form is essentially a mixture of blended cement and homogeneous low- and intermediate-level liquid (simulant) radioactive waste (sludge). The motivation stems from the fact that the existing empirical or phenomenological models are not designed to handle the interaction of a blended cement system with the waste sludge. The main objective of the proposed model is to minimize the number of isothermal or semi-adiabatic experiments (and thus the cost) required to design a promising recipe for conditioning the waste form. The paper demonstrates the capability of the model to successfully predict the evolution of hydration heat for various cemented waste form recipes