L. S. Wang

The use of electromigration as a qualitative technique to study the migration behaviour and speciation of uranium in the Boom Clay

Summary Under the geochemical conditions prevailing in situ in the Boom Clay Formation (pH, Eh, …), calculations predict that U(OH)4 is the dominant uranium species present in the interstitial water and the concentration is solubility limited. However the boundary of the domain where the non solubility limited UO2(CO3)34− species dominates is very close. It is therefore of prime interest to know the correct speciation of uranium during the migration process. Electromigration was used as technique with the advantage that it can provide information on the speciation because the movement of the species towards the electrodes depends on its charge and speciation. Electromigration experiments have been performed with preconditioned 233UO2(CO3)34− sources, starting from the hypothesis that this species should migrate without retardation towards the anode. Despite relatively long electromigration times, sufficient to displace strong retarded tracers, no displacement of the migration profile towards the anode was observed. All 233U remained near the source position, but the electropherograms clearly showed the presence of species moving towards the cathode. This indicates the presence of neutral or positively charged uranium species. These electropherograms are interpreted as a change in uranium valence state: reduction of UO2(CO3)34− and precipitation of U(IV) oxy-hydroxides near the source position. The solubility limited species, U(OH)4(aq), are carried with the pore water towards the cathode. The electromigration experiments indicate, in support of the speciation calculations, that the dominant migrating U-species is probably the solubility limited U(OH)4.

A New Radionuclide Sorption Database for Benchmark Cement Accounting for Geochemical Evolution of Cement

This chapter presents the data selection strategy and the selected sorption values on cement for 25 elements (Ag, Am, Be, C, Ca, Cl, Cs, H, I, Mo, Nb, Ni, Np, Pa, Pb, Pd, Pu, Ra, Sr, Se, Sn, Tc, Th, U, Zr) that need to be considered in safety assessment calculations for the future near-surface disposal facility at Dessel, Belgium. Mainly on the basis of literature data, best estimate sorption values in addition to upper and lower bound values were determined for a so-called benchmark cement—the unperturbed cement without effects of organics, high chloride content or other chemical components that might adversely impact radionuclide sorption. Effects of perturbing components are discussed separately. The geochemical evolution of the cementitious engineered barriers was also addressed to clarify the conditions under which sorption values are applicable. A substantial part of the scientific basis supporting the data selection was established at several meetings of an international panel of experts who reviewed and endorsed the data selection. To this end, the sorption data were checked for reliability, appropriateness for the conditions expected for the Dessel disposal facility, data quality, time frames (i.e. states of cement degradation), heterogeneity (presence of components in conditioned wastes that could affect sorption), and completeness (in terms of mechanisms explaining the sorption processes).

Effect of reducing agents on the uranium concentration above uranium(IV) amorphous precipitate in Boom Clay pore water

Summary The solubility of U(IV) amorphous precipitates was measured in Boom Clay water in the presence of reducing agents and under the in situ partial pressure of CO2(g) at 10-2.4 atm. Boom Clay pore water is of the sodium bicarbonate type (NaHCO3 ∼ 10-2 M) and contains about 100 mg l-1 of dissolved organic carbon. The total U concentration measured after two-months equilibrating time and the 0.45 μm filtration was about 10-6 M and higher. This concentration is about 3 orders of magnitude higher than literature data for U(IV) solubility and may suggest the existence of colloids and/or U(VI) species. The U concentration was found decreasing as a function of time within the experimental period. Further experiments are currently performed to evaluate the contribution of the colloids and the role of organic matter.

Technetium behaviour in Boom Clay: a laboratory and field study

Summary This paper describes a study of technetium solubility and migration under chemical conditions representative of those prevailing in a Boom Clay environment. Laboratory and in situ measurements yielded similar aqueous concentrations of technetium, of about 1×10−8 mol dm−3, close to the concentrations measured for hydrated technetium(IV) oxide TcO2·1.6H2O in the solubility studies. From fitting the curves of the Tc concentrations as function of time, distribution coefficient (Kd) values were estimated to lie between 0.8 cm3 g−1 and 1.8 cm3 g−1. Exposure of the system at 80 °C and to γ-radiation dose rates of several hundred Gy h−1 resulted in only minor differences in behaviour.

Removal of uranium and arsenic from groundwater using six different reactive materials: assessment of removal efficiency

Permeable reactive barriers (PRBs) are increasingly being used as a cost-effective technique for in-situ treatment of contaminated groundwater. This paper discusses the results from batch tests with six different reactive materials, including zero-valent iron (ZVI), ferric oxyhydroxides, and some composite materials. All materials were tested in their ability to remove uranium and arsenic from groundwater. Results show that fine-grained ZVI was most successful in removing uranium from solution, up to a removal efficiency of 98%. The main mechanism in removing uranium presumably is by reductive precipitation. The results further showed that arsenic removal is most efficient with ferric oxyhydroxides (maximum removal efficiency is 96%). This study illustrates that PRBs using a mixture of fine-grained ZVI and materials containing ferric oxyhydroxides as reactive material may help significantly in removing uranium and arsenic from groundwater.

COUPLING TIME-DEPENDENT SORPTION VALUES OF DEGRADING CONCRETE WITH A RADIONUCLIDE MIGRATION MODEL

Safety assessment of radioactive waste disposal facilities is usually carried out by means of simplified models. Abstraction of the numerical model from the real physical environment is done in several steps. One of the most challenging issues in safety assessment concerns the long time scales involved and the evolution of engineered barriers over thousands of years. For some processes occurring in specific engineered barriers the uncertainties related to long time scales are addressed by implementing conservative assumptions in the radionuclide migration models. Other processes such as chemical concrete degradation, however, can be estimated for long time periods by the use of coupled geochemical transport models. For many near-surface disposal facilities, concrete is a very important engineered barrier because it is used in the construction of disposal modules or vaults, in production of high-integrity monoliths and their backfilling and for waste conditioning. Knowledge on the durability of such concrete components and its relation to radionuclide sorption is important for a defensible safety assessment. Chemical degradation typically occurs as the result of decalcification, dissolution and leaching of cement components and carbonation. These reactions induce a gradual change in the solid phase composition and the concrete pore-water composition, from “fresh” concrete porewater with a pH above 13 to a pH lower than 10 for “evolved” porewater associated with fully degraded concrete. The focus of this work is to analyse the behaviour of the disposal facility in terms of radionuclides sorption values depending on the geochemical evolution of engineered barriers. The time-dependency of the concrete mineralogy and porewater is coupled with sorption values that are characteristic for the four concrete degradation states: (i) State I with a pH larger than 12.5, controlled by the dissolution of alkali-oxides, (ii) State II with a pH at 12.5 controlled by the dissolution of portlandite, (iii) State III with a pH between 12.5 and 10 when all portlandite is dissolved and the pore water composition is determined by different cement phases including calcium-silicate hydrates (C-S-H phases), and (iv) State IV with a pH lower than 10 with calcite and aggregate minerals present. Above mentioned pH values are valid for a system with a temperature of 25°C. Sorption values are obtained from a literature review. The time-dependency of the sorption values Rd is implemented in a one-dimensional radionuclide migration model used for release calculations from the planned near-surface disposal facility at Dessel, Belgium. Calculated releases will be discussed for radionuclides typical of low- and intermediate level short-lived (LILW-SL) waste.Copyright

Uranium release from boom clay in bicarbonate media

Summary The release of natural uranium from Boom Clay was studied to better understand the mechanisms governing the solid-liquid partitioning of uranium. Batch leaching experiments suggested that the portion of natural uranium released from clay is associated with colloids at a low bicarbonate concentration prevailing in Boom Clay. At increased bicarbonate concentrations, uranium was present predominantly as dissolved species indicating a formation of uranium carbonate complexes. The in situ aqueous uranium concentration, i.e., the concentration in the pore waters collected by piezometers was found to be 2 to 3 orders of magnitudes lower than the one measured by the batch techniques. These results illustrated that the batch techniques may cause a remobilization of uranium containing colloids from clay surfaces into solution when clay is suspended, agitated, and mechanically perturbed. These colloids are attributed to artefacts and are not considered to exist in situ because of the high compaction of Boom Clay. Due to the presence of colloids, a laboratory derived solid-liquid partitioning coefficient is not equivalent to and cannot simply be converted to the distribution coefficient Kd currently used in performance assessment calculations.

Sorption Values for Thorium, Uranium, Plutonium, Neptunium, and Protactinium

The actinide elements—thorium, uranium, plutonium, neptunium, and protactinium—are important constituents of radioactive waste from nuclear fuel. Uranium also occurs in a variety of wastes, e.g. from mining. These elements all hydrolyse extensively in aqueous solutions and correspondingly strong sorption onto concrete, hydrated cement paste and individual CSH phases is observed. Sorption is also more or less constant over all states of cement evolution. The sorption mechanisms are, however, not well understood to date. In case of Th, relatively fast kinetics of uptake and reversibility are observed, indicating that uptake occurs through surface processes and probably does not involve incorporation into the structures of hydrated cement phases. In contrast to Th, which exists only in the tetravalent oxidation state in aqueous environments, uranium, plutonium, neptunium, and protactinium can exist also in lower and/or higher oxidation states. In analogy to their aqueous chemistry, very similar sorption behaviour is expected for all tetravalent forms. Sorption of U(VI) on cementitious materials is similar to Th(IV)/U(IV) in magnitude, but appears to involve different mechanisms (probably solid-solution formation). In lack of specific information, the same is assumed for Pu(VI). Relatively little is known for the pentavalent forms. Strong sorption is observed for Pa(V), but the underlying mechanism is not known. For Np(V), relatively weak sorption is assumed in lack of specific data.

Solubility of an uranium(IV) amorphous phase under geochemical conditions representative for the direct disposal of spent nuclear fuel in Boom Clay

The solubility of hydrous UO2 was studied in Boom Clay porewater under in situ conditions (reducing conditions, pH ~ 8, log f CO2 -2.4). Dithionite, iron powder and sulphide were used to ensure reducing conditions and to minimise the risk of uranium(IV) oxidation during the experiments. The average total uranium concentration was 3.9 × 10-6 mol·l-1. The determined solubility is generally higher than the values reported in the literature but the effect of the natural organic matter present in the Boom Clay interstitial water is not known.

Sorption Values for Zirconium

Zirconium (Zr) is used in nuclear reactors (in the form of zircaloys) because of its low thermal neutron capture cross section and high resistance to corrosion. In aqueous solutions, zirconium exists exclusively in the tetravalent oxidation state. Zr(IV) hydrolyses strongly with Zr(OH)4(aq), and Zr(OH)6 2 − being the only relevant species in cementitious environments (and low zirconium concentration). Very high sorption of zirconium has been observed in all cementitious materials investigated to date (hydrated cement pastes, CSH phases). Sorption appears to increase with the degree of cement degradation, in particular from a C/S ratio of 1.3 to 1.0. While the actual sorption mechanisms for zirconium are not known, the available evidence suggests that mainly CSH phases are responsible for zirconium sorption on cementitious materials.