The disposal of long-lived intermediate- and high-level radioactive waste is a major environmental concern. Deep geological disposal is widely regarded as the safest long-term solution. In Belgium, Boom Clay (BC) has been selected as the reference host formation due to its low permeability, self-sealing properties, and strong capacity to retain radionuclides. One specific waste type, Eurobitum--an intermediate-level bituminized waste--is stored in steel canisters within the repository. Over time, groundwater is expected to infiltrate the system and come into contact with these canisters. The bituminized waste swells upon water contact, releasing large amounts of (Na, Ca)NO3 and generating a saline plume that diffuses into the BC. This plume, rich in sodium ions (Na+), can significantly impact the clay's physicochemical and hydro-mechanical behaviour. In this study, undrained triaxial tests under isotropic consolidation were conducted on intact BC samples pre-equilibrated with (Na, Ca)NO3 solutions of varying concentrations. Solutions corresponded to sodium occupancies of 60% (1.0 mol/L) and 90% (2.0 mol/L), along with a reference Boom Clay synthetic water solution (0.015 mol/L NaHCO3). Results showed an increase in shear strength and friction angle with sodium concentration, attributed to clay particle aggregation, shrinkage, and diffuse double layer contraction induced by salinity.

Cellulosic materials make up a significant fraction of the current radioactive waste. During storage and disposal, both radiolytic and hydrolytic degradation of such materials can be expected, the latter triggered by the highly alkaline cementitious environment of the waste matrix and disposal facility. The combination of both degradation processes will cause a significant production of radionuclide-complexing agents, which can enhance the migration of certain radionuclides towards the biosphere. Knowledge regarding the degradation mechanisms is therefore required to predict the long-term production rate of these organics. In this study, the physicochemical properties of cellulosic tissues during radiolytic and/or alkaline degradation under disposal conditions were monitored. Our results indicate that the long-term alkaline degradation of cellulose is controlled by two underlying mechanisms, taking place in both the amorphous and the crystalline regions. The first one is a combination of peeling and stopping reactions taking place at the easily available reducing end groups in the amorphous regions of cellulose. This process controls the overall degradation rate until the easily accessible reducing end groups become depleted. Afterwards, the degradation slows down significantly and is rather controlled by a continuous stepwise dissolution and amorphization of the outer layer of crystalline cellulose, resulting in the liberation of reducing end groups, where secondary peeling reactions can take place. These new insights lead to a better-founded choice of the conceptual model for predicting the long-term cellulose degradation in radioactive waste.

Cellulosic materials make up a significant fraction of the current low- and intermediate-level radioactive waste. During storage and disposal, radiolytic degradation of such materials is inevitable and can occur under both oxic and anoxic conditions. In addition, the highly alkaline cementitious environment of a disposal system promotes the alkaline degradation of cellulosic materials, producing radionuclide-complexing agents, such as isosaccharinic acid (ISA). As radiolytic degradation changes the physicochemical properties of cellulose, it could also affect its alkaline degradation and thus the production of ISA during disposal. Hence, in the present work, we investigated the alkaline degradation of pre-irradiated cellulosic tissues, which are representative of real radioactive waste. Pre-irradiation occurred by exposing tissues to gamma-irradiation under oxic or anoxic conditions at absorbed doses up to 1.4 MGy and at two different dose rates. These irradiated tissues were then submerged in artificially prepared cement water (initial pH of 13.3) and monitored over 2.5 years. The results show a significantly faster production and release of dissolved organic carbon and ISA with an increasing absorbed dose during pre-irradiation, and even more so when oxygen is present during irradiation. The irradiation dose rate did not affect the subsequent alkaline degradation rate. Taken together, this work demonstrates that irradiation of cellulosic materials in radioactive waste during storage and disposal will accelerate their alkaline degradation under disposal conditions. Consequently, radionuclide-complexing agents such as ISA will form at rates far exceeding those anticipated from alkaline degradation alone. These findings are therefore pivotal for improving long-term predictions of the ISA production in radioactive waste.

The poorly indurated Boom Clay (BC) has been studied as a potential host formation for the geological disposal of radioactive waste including the intermediate-level long-lived bituminized radioactive waste called Eurobitum. In case of leaching of Eurobitum, fluids with altered chemical compositions would be expected to infiltrate locally into the BC. To evaluate the effect of salinity on the BC hydromechanical behaviour, a series of one-dimensional constant-volume swelling tests were conducted on intact BC samples that were first percolated with mixed sodium-calcium-nitrate solutions ((Na, Ca)NO3) with varying sodium concentrations, resulting in varying Na+ occupancies in the clay. Results showed that the swelling pressure of BC decreased with an increase in the solute concentration and sodium occupancy. Additionally, higher Na+ concentrations led to a faster stabilization of the swelling pressure. The hydraulic conductivity of BC increased with increasing solute concentration and sodium occupancy. Moreover, the sodium occupancy and concentration affected the swelling pressure and hydraulic conductivity in a similar way. This phenomenon could be elucidated by two primary mechanisms: (i) the influence of solute concentration on the diffuse double layer, and (ii) the effect of cation type and hydrated radius within the interbasal space of montmorillonite sheets, especially concerning sodium occupancy.

This paper presents a simple polynomial model predicting the alkali concentration in concrete pore solution as a function of the cement composition, the total alkali content and the water-to-cement ratio. The model was designed to assess the risk of alkali-silica reaction (ASR) in concrete rubble from the decommissioning of nuclear power plants. The influence of various factors on alkali concentration in the cement pore solution, such as the origin of the binder (ordinary Portland cements from different sources), the water-to-binder ratio, and the amount of ground granulated blast furnace slag was investigated. The polynomial model was derived and validated using experimental data. Further validation was based on literature data which confirmed its predictive accuracy. The model provides valuable insights into the alkali concentration of concrete pore solution and its potential impact on ASR.

Abstract The collaboration between HADES and the Mont Terri rock laboratory started in 1995, when granite was considered the most suitable host rock for radioactive waste disposal in Switzerland. When an alternative host formation was looked for by NAGRA, the choice of clay as host rock became rapidly clear. It was then decided to build an underground research laboratory in the Opalinus Clay taking advantage of the excavation of a motorway tunnel crossing the Mont Terri anticline. Exchanges of knowledge and experience on the behaviour of argillaceous formations were focused on the feasibility of constructing a safe geological repository for high-level radioactive waste: from excavation and gallery lining techniques to geochemistry, diffusion of radionuclides and coupled thermo-hydro-mechanical processes. The research programme at Mont Terri and the growing numbers of partners from four up to nine countries worldwide led to successive extensions of the galleries. The scientific and technical advances made possible by the collaboration between both laboratories dealt with the development of common methods and joint experimental and modelling efforts. This review article summarizes the main scientific lessons learned during these exchanges, stressing the added values of the knowledge transfer between partners and the overall cross-fertilization between HADES and Mont Terri.

Lignocellulosic materials can be found in a significant fraction of the current low-and intermediate-level radioactive waste. During storage and disposal, radiolytic degradation of such materials can be expected, under oxic or anoxic conditions. This degradation may lead to a significant gas production and changes in the physico-chemical properties of the lignocellulosic materials, which can affect the formation of the known radionuclide-complexing agent isosaccharinic acid (ISA) as well as other (possibly complexing) degradation products during disposal. Hence, in the present work the radiolytic degradation of cellulosic tissues - realistically found in radioactive waste - was investigated under various storage and disposal conditions. For this, cellulosic tissues were exposed to & gamma;-irradiation in gas-tight containers under oxic or anoxic conditions, at an absorbed dose ranging up to 1.4 MGy and at two different dose rates. Our results show that mainly H2, CO and CO2 are produced during irradiation of tissues, though also small amounts of CH4 are formed. The presence of oxygen does not affect the generation of H2, but results in a significant increase in the yields of CO, CO2 and CH4. Furthermore, radiation-induced chain scission is observed, causing a decreasing polymerization degree with increasing absorbed dose. Amorphization of the cellulose microstructure occurs significantly at high doses of gamma rays (& GE; 0.8 MGy). An increase in the concentration of reducing functional groups is observed with increasing absorbed doses as well. For irradiation under anoxic conditions, this increase is correlated with the observed chain scission. In contrast, additional oxidation processes occur when irradiating cellulosic tissues in the presence of oxygen, resulting in a partially oxidized polymer backbone without causing considerably more chain scission or amorphization. These radiolytic changes to the cellulose structure, both under anoxic and oxic conditions, may enhance its hydrolytic degradation under the hyper-alkaline conditions of long-term final disposal, resulting in a faster production of radionuclide-complexing agents.

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.

Water uptake and salt leaching of two simulated French Bituminized Waste Products (BWP) have been investigated under nearly constant volume conditions. The resulting pressure development was monitored during 5 to 6 years for two simulated BWP samples, varying one from the other by their inorganic load and composition. Pressure development induced by water uptake is mainly the result of two processes: (1) an osmotic phenomenon due to the presence of soluble and hygroscopic salts (NaNO3 and Na2SO4) embedded in the bitumen matrix and (2) recrystallization of anhydrous Na2SO4 into its decahydrate form, leading to an important volumetric expansion. After a certain hydration period, the pressure exerted by the hydrating BWP stabilizes when the pressure generating phenomena are fully counteracted by the leaching of soluble salts via out-diffusion, reconsolidation of pores in highly leached parts of the BWP, and/or some creep of the BWP into the technical voids of the water uptake cells. For one of the French BWP, this already occurred after 1 year of hydration. Differences are found in the pressure evolution and increase rate of the two studied BWP, though a much larger difference is observed when comparing the results of the French BWP to a Belgian BWP, i.e. Eurobitum. The faster pressure development observed for the French BWPs can be attributed to the differences in the soluble salt content, the inorganic load, the content of recrystallizing salts, but also to the presence of insoluble salts such as BaSO4, which seems to facilitate the water uptake rate in the French BWP. The faster hydration in French BWP results in a larger fraction of salts becoming available for osmosis and recrystallization within a relatively short time frame, thereby explaining the faster pressure build-up. On the other hand, BaSO4 does not seem to affect the leaching of soluble salts from BWP directly.

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 P-H2 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 H-2. 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 H-2. 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 H-2 in the gas phase. The possible consequences of these processes for the final repository performance are also discussed.

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.

79Se is a critical radionuclide concerning the safety of deep geological disposal of certain radioactive wastes in clay-rich formations. To study the fate of selenium oxyanions in clayey rocks in the presence of a selenium reducing microbial community, in situ tests were performed in the Opalinus Clay at the Mont Terri Rock Laboratory (Switzerland). Furthermore, biotic and abiotic batch 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 interpretation of the in situ tests. Geochemical modeling was applied to simulate Se(VI) reduction, 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 reduction of Se oxyanions can occur with electron donors provided by the clay itself. If microorganisms would be active in the clay surrounding a disposal facility, microbial reduction of leached Se could thus contribute to the overall retention of Se in clayey host rocks.

Within the EC-funded CHANCE project several non-destructive techniques are being considered for the assay of waste bearing drums. Such techniques include calorimetry, gamma-ray spectrometry and neutron coincidence counting. The aim is to quantify uncertainties on the inventory of radionuclides, and how these are potentially reduced by combining the signatures from different techniques in the data analysis. In this framework, neutron coincidence measurements were carried out with two slab counters based on 3 He detectors coupled to shift register electronics. Such a system consists of two identical slabs with 6 detectors each, and is transportable, rather compact and flexible in terms of sizes and geometries that can be measured. With this system three 200 L drums containing certified reference nuclear material and different filling materials were measured. The certified nuclear material was in the form of 21 pellets of mixed oxide of U and Pu with a total mass of about 10.5 g; in addition, a single pellet of about 10.05 g was also available. The pellets could be placed in predefined positions within the drum in a reproducible way. The geometry and composition of the three drums was well characterized and consisted of Ethafoam, a mixture of Ethafoam, stainless steel and PVC, and mortar with an inner core of extruded polystyrene. The measurement setup was arranged such that the drum was placed between the two slab counters. The positions of the slab counters relative to the drum were accurately measured before each measurement, and a dedicated system was used to minimize the uncertainty on the detector positioning. The measurement data were first analysed by applying the point model of Hage and the mass of nuclear material in the drum was determined from the rate of totals and reals and the radionuclide composition. Due to the fact that not all the point model conditions were met, we found that the point model overestimates the mass up to about 50%. In addition, a Monte Carlo model of the measurement geometry was developed using the MCNP code. The model was used to determine a calibration factor between the reals rate and the mass of the sample. Measurements with a calibrated 252 Cf source were used to verify the model. With a Monte Carlo based approach the mass of the mixed oxide pellets is within a few percent from the nominal values, except for strongly asymmetrical configurations where the deviation is up to about 20%. The results reveal the importance of an accurate background correction and of accounting for surrounding materials of the building such as walls, floor and ceiling in the Monte Carlo model.

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Supplementary cementitious materials (SCMs) have been widely used not only in civil engineering concrete components, but also in (nuclear) waste treatment engineering because of their beneficial effects on microstructure, engineering properties and durability of concrete. A comprehensive experimental campaign has been undertaken to investigate the effects of SCMs, which includes silica fume (SF) and blast furnace slag (BFS) in combination with ordinary Portland cement (OPC) on the behaviour of hydration heat evolution during early ages of mortars. The samples with different water/cement (w/c) ratios (0.5, 0.7, 0.9) and replacement ratios of SF (10%, 20%, 30%) and BFS (30%, 50%, 70%) were subjected to isothermal calorimetry tests at various temperatures (20, 30, 40 and 50 degrees C) in order to assess the effects of SCMs on the rate of hydration heat, cumulative heat release, activation energy and setting times of blended mortars. Knowledge obtained from these blended systems was then applied for cementation of a heavy metal containing waste sludge simulant, where the potential for thermal cracking and delayed ettringite formation due to hydration heat generation is of great concern. Results show that both BFS and SF increase the hydration rate but reduce cumulative heat release compared to pure OPC mortar. The ternary system (OPC:BFS:SF) exhibits different hydration characteristics compared to the binary system (OPC:BFS(or SF)) and there is a slight interaction between BFS and SF. The presence of sludge in the matrix significantly accelerates the hydration process and reduces the apparent activation energy. The role of temperature is more important for mortars containing BFS rather than SF, and less pronounced for the system containing sludge. Estimating the setting times based on the isothermal calorimetry data is more accurate for the final setting time rather than for initial setting time and an overestimation of 10% might occur or even more for the system containing sludge, which is still acceptable taking into account the measurement uncertainty.

A coupled hydro-chemical-mechanical constitutive law for the Belgian Eurobitum bituminized waste is being developed by the International Centre for Numerical Methods and Engineering (Polytechnical University of Cataluna, Spain) to contribute to the study of the compatibility of Eurobitum with Boom Clay as a geological disposal environment. A large experimental programme is ongoing at SCK•CEN to support the development of a constitutive law for Eurobitum. Water uptake tests are being performed under different conditions to obtain insights in the parameters that influence the water uptake behaviour of Eurobitum. Furthermore, Environmental Scanning Electron Microscopy and microfocus X-ray Computer Tomography are used to characterize hydrated samples in order to improve the understanding of the water uptake processes. The salt content, the distribution of the salt crystals, and the membrane efficiency in the Eurobitum samples affect the swelling and pressure increase rate. High membrane efficiencies and a large amount of hygroscopic salts inside the Eurobitum samples result in very high pressures when almost no swelling is allowed. The pressure in small inactive samples with 28 wt.% NaNO 3 has risen to ~ 19 MPa after ~ 3 years of hydration in nearly constant volume conditions. Slower pressure increase rates are being measured for samples with 6, 12, 18, and 33 wt.% NaNO 3 .

Sodium azide, applied as microbial inhibitor, has caused unwanted by-products in environmental samples during anaerobic, long-term (2–3 year) experiments. When ignored, this can lead to the misinterpretation of observed phenomena. Sodium azide was indeed found to react with several components of environmental samples. Azide reacted with dissolved organic matter present in clay pore water, causing a lowered reducing capacity of the system. It also reacted with pyrite, forming thiosulfate and with ferrous iron, lowering its concentration in solution. When nitrite was added to the environmental samples, nitrous oxide and nitrogen gas were formed through reaction with azide. Azide could act both as an oxidant and reductant. Some of these reactions showed slow kinetics but nonetheless gave rise to significant amounts of by-products during these long-term experiments. When working with environmental samples, sodium azide can affect the chemical composition significantly, especially for long incubation periods. Other sterilization techniques such as filter sterilization are therefore advised.

Plasma incineration might be a promising technique for the conditioning of various radioactive waste streams. Assessing the long-term durability of the plasma slag is essential to predict its performance during long-term disposal. In this paper, the stability of six plasma treated surrogate cemented concentrates or resins in a high pH environment is investigated. The slags were crushed (2 different granulometries) and immobilized in a cement matrix, after which samples were submitted to long-term durability tests (stability under water at 20 °C; stability in a high relative humidity environment at 38 °C) and to an accelerated Alkali-Silica-Reaction (ASR) test (1 M NaOH at 80 °C). The first results show that the expansion and strength loss of the cement-slag mixtures remain limited in the test conditions, although differences between the different materials and granulometries could be perceived. No visual damage was observed. Some tests are still ongoing and will last 2 years.