UO 2 as New Filling Material for Cesium Retention in High-Level Nuclear Waste Repositories

Solid phase

Crystalline uranium dioxide with a particle size between 50 and 100 μm was obtained from a UO₂ pellet supplied by ENUSA (Empresa Nacional del Uranio S.A.). The composition of the bulk of the solid was UO_2.01, determined by X-ray diffraction analysis. The surface area of the solid was determined by the Brunauer–Emmett–Teller (BET) method, the result obtained was 0.019±0.002 m²/g.

Before the experiments and to dissolve fines and oxidized fractions, UO₂ was submerged in a 1×10⁻³ mol/dm³ HCO₃− solution. After this step, the solid was filtered and dried under N₂ atmosphere.

Methodology

Sorption experiments were carried out inside a glovebox filled with N₂ (g), avoiding the presence of O₂ (g) and CO₂ (g), which could either oxidize UO₂ or modify the UO₂ solubility by the formation of carbonate complexes in solution, respectively. Temperature was 25°C±1°C.

A known amount of UO₂ (∼0.05 g) was added to a known volume (20 cm³) of cesium solution in stoppered polystyrene tubes. The initial cesium concentration ranged from 7.0×10⁻⁴ to 1.4×10⁻⁸ mol/dm³. Considering that the maximum sorption of cesium on other uranium solids occurred at pH>pH_zpc, and that the pH_zpc of the UO₂ was determined to be 7.7±0.4 (Clarens et al., 2003), in this work the pH was kept at 9.2±0.3 by adding HClO₄ or NaOH when necessary. This pH is considered high enough to neglect the effect of pH in the sorption, but still a pH that could be found in the groundwater of a deep geologic repository [for example: Grimsel groundwater pH=9.7 (Missana et al., 2003)].

The tubes were continuously stirred by using an end-over-end stirrer. At the end of the experiment, aqueous samples were filtered through 0.20-μm MICROPORE pore size filters and acidified with concentrated HNO₃. Cesium and Uranium concentrations were determined using ICP-MS (Agilent 7,500 cx). Uranium concentration was analyzed only to detect an unexpected dissolution of UO₂. The average concentration of uranium in all the samples was (2.73±2.87)×10⁻⁷ mol/dm³, meaning that only 0.003% of the UO₂ was dissolved. The pH of the solutions was monitored with a CRISON pH Meter GLP22 before the addition of the UO₂ solid to the solutions and at the end of the experiments, when the equilibrium was reached.

The concentration of cesium sorbed onto the solid in moles of Cs per m² of dry solid, {Cs}_s, was calculated by subtracting the final concentration, [Cs] _eq in mol/dm³, to the initial concentration of cesium added to the solution, [Cs]₀ in mol/dm³, and normalizing with the volume (V) to the surface area (SA) ratio: \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*}\{ Cs \} _s = ( [ Cs ] _0 - [ Cs ] _ { eq } ) \cdot \frac { V } { SA } \tag { 1 } \end{align*} \end{document}

The main objectives of this work were to determine the UO₂ sorption capacity for Cs as well as to improve the knowledge of the mechanism of the sorption process to evaluate its suitability for cesium retention in a high-level nuclear waste repository.

To reach these objectives, three different sets of experiments were carried out: (1) sorption kinetics (with a constant cesium concentration, 10⁻⁵ mol/dm³); (2) sorption isotherms (with [Cs]₀ ranging from 1.5×10⁻⁷ to 7.7×10⁻⁴ mol/dm³); and (3) the effect of ionic strength (in the same range of cesium concentrations in the sorption isotherm experiments).

Cesium sorption as a function of time

The variation of Cs sorption as a function of time is shown in Figure 1. As it can be seen, sorption is very fast under the experimental conditions with equilibrium values reached in 2 h. The percentage of sorption at equilibrium calculated as 100{Cs}_s/[Cs]₀ is 21%.

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FIG. 1.

Variation of cesium sorbed onto uranium dioxide as a function of contact time. Experiments carried out with 0.05 g of UO₂, [Cs]₀=6.1×10⁻⁵ mol/dm³ in the presence of 0.1 mol/dm³ NaClO₄ at pH=9. Very fast sorption can be observed.

Since the kinetics of the sorption is very fast, modelization of the experimental data cannot provide any additional information, and therefore, no models were used.

Isotherms of cesium sorption: influence of ionic strength

The concentration of cesium sorbed on UO₂(s) as a function of cesium remaining in solution at equilibrium is shown in Figure 2, at the two ionic media studied in this work: 0.01 and 0.1 mol/dm³ fixed by using NaClO₄. In the case of ionic strength 0.01 mol/dm³, the shape looks linear, but in the case of ionic strength 0.1 mol/dm³, the shape tends to a maximum of cesium sorbed into the solid, which is the typical trend of a monolayer sorption. In the case of ionic strength 0.01 mol/dm³, only the linear model was fitted. \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*}\Gamma = K_d \ [ Cs ] \tag{2}\end{align*} \end{document}

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FIG. 2.

Sorption isotherm of cesium sorbed onto UO₂. Experiments carried out with 0.05 g of uranium dioxide, at 0.1 and 0.01 mol/dm³ NaClO₄. See the difference between the ionic strengths.

The parameter gamma, Γ, is defined as the cesium concentration sorbed on the UO₂ at equilibrium (mol/m²), and the K_d is the linear constant of sorption (dm³/m²). The K_d has a value of 6.61±0.08 dm³/m² and the fitting has a correlation coefficient of 0.999.

Regarding the samples with 0.1 mol/dm³ ionic strength, the experimental data were fitted to three different models: Lineal, Langmuir, and Freundlich.

Since the isotherm is not linear, the linear model is the one with the worst fitting (Fig. 3). The K_d value is 2.24±0.28 and the correlation factor is 0.926.

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FIG. 3.

Sorption isotherm of cesium sorbed onto UO₂. Experiments carried out with 0.05 g of uranium dioxide, at 0.1 mol/dm³ NaClO₄. Fitting of the Linear, Langmuir, and Freundlich models. Note that the Langmuir model is the one that best fits.

The noncompetitive Langmuir isotherm model has the following expression: \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*}\Gamma = { \Gamma_ { \max } } \frac { { \rm K } _ { \rm L } \cdot [ { \rm Cs } ] } { 1 + { \rm K } _ { \rm L } \cdot [ { \rm Cs } ] } \tag { 3 } \end{align*} \end{document}

K_L is the Langmuir constant (dm³/mol), Γ_max is the maximum cesium sorption (mol/m²), and [Cs] is the concentration (mol/dm³) of cesium at equilibrium.

The Langmuir isotherm is the one that best fits the experimental data (Fig. 3), which corroborates monolayer coverage on the UO₂ surface. The Γ_max has a value of (2.00±0.12)×10⁻³ mol/m² and the K_L is (3.27±0.44)×10³ dm³/mol. The correlation factor is 0.996.

The characteristics of the Langmuir isotherm may be expressed by the separation factor, R_L, defined as: \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*}R_L = \frac { 1 } { 1 + K_L \{ { \rm Cs } \} 0 } \tag { 4 } \end{align*} \end{document}

If the value of R_L is 0, the sorption is irreversible; a value between 0 and 1 indicates that the sorption is favorable; R_L=1 indicates that the sorption is linear and, finally, if the value is higher than 1, the sorption is unfavorable. The R_L values were between 1.00 and 0.23. For cesium concentrations above 5×10⁻⁶ mol/dm³, the isotherm loses its linearity and R_L values are lower than 1.

The Freundlich isotherm takes into account the interactions between the adsorbed molecules and the exponential decrease in the sorption energy with the increase in occupied sorption sites.

The Freundlich isotherm is an empirical model that has the following form: \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*}\Gamma = K_f \cdot [ Cs ] ^{1 / n} \tag{5}\end{align*} \end{document}

The Freundlich constant K_f (mol sorbed/mol in equilibrium) is considered the adsorption coefficient, the quantity of sorbate sorbed per unit of sorbate concentration in equilibrium. The heterogeneity factor 1/n can be used to measure the deviation from linearity of the sorption isotherm. The linear model would have an n of 1. If the sorption process occurs due to chemical factors, then, the n is lower than 1. If the adsorption is produced by a favorable physical process, then, the n is higher than 1.

The fitting of the Freundlich isotherm can be seen in Figure 3. The correlation factor is 0.979. K_f has a value of (1.20±0.62)×10⁻¹ mol_s/mol_eq and n is 1.64±0.18. With this heterogeneity factor, the Freundlich model suggests that the sorption of cesium onto UO₂ is due to a favorable physical process.

A decrease in the sorption of Cs into UO₂ is observed when the ionic strength increases (Fig. 2). This dependence is common of the outer-sphere complexes based in electrostatic bonds, which corroborates a cesium sorption on UO₂ governed by electrostatic interactions between cesium and the surface of the solid.

Retention of cesium by UO₂(s) used as inert material in spent nuclear fuel canister

The data obtained allow the calculation of the weight of depleted UO₂ that would be necessary to sorb cesium released from the spent nuclear fuel. In the calculations shown below, only cesium is considered. In future calculations, with more information on other radionuclides sorption on UO₂, competition processes must not be discarded, producing a decrease in the surface sites available for cesium sorption. In these calculations, a continuous flow of water was supposed as a transport system for the Cs released, therefore, the Cs that has escaped the fuel matrix does not contact the fuel matrix again and the mass of UO₂ of the fuel itself cannot be subtracted from the mass of depleted UO₂ necessary to adsorb the Cs.

The total cesium released from the spent nuclear fuel might be calculated considering the cesium inventory in the fuel. Cesium inventory in a pressurized water reactor (PWR) and UO₂ spent nuclear fuel with a burn-up of 50 MWd/kgU were calculated by using KORIGEN code (Wiese and Schwenk-Ferrero, 2007). The total cesium release obtained from the total fuel matrix was 0.15% for both ¹³³Cs and ¹³⁷Cs, which corresponds to 1.52 g of ¹³³Cs plus 1.48 g of ¹³⁷Cs per kg of UO₂.

For final disposal of spent fuel, a canister with four PWR assemblies is being designed (Puig et al., 2008), each assembly containing 523.4 kg of UO₂. The total quantity of cesium released from the resulting 2093.6 kg of UO₂ fuel will be 3.18 kg (23.9 mol) of ¹³³Cs and 3.10 kg (22.6 mol) of ¹³⁷Cs.

Mass of depleted UO₂ necessary to adsorb all the cesium released from the fuel, w_D-UO2 (kg), can be calculated by: \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*}w_ { D - UO2 } = \frac { mol_ { Cs } } { Q_ { max } \cdot S } \tag { 6 } \end{align*} \end{document}

Where mol_Cs are the total moles of cesium released from the fuel (46.1 mol); Q_max is the maximum sorption capacity, obtained from the sorption isotherms (2×10⁻³ mol/m² at 0.1 mol/dm³ ionic strength was chosen to be conservative. At 0.01, the sorption capacity was much higher.); and S is the surface area of the depleted UO₂ (0.0019 m²/g if a UO₂ with the same surface area is considered).

The value obtained is w_D-UO2=1223.7 kg. Considering the void space in a PWR canister, 1.343 m³ (1), and the UO₂ density, 10,970 kg/m³, the weight calculated could be easily introduced in the void space of the canister, which indicates that the canister could contain enough weight of depleted UO₂ to incorporate the cesium released from the fuel. It was proved that depleted UO₂ would be a good filling material to retain Cs released from the spent nuclear fuel.