Retention von Radionukliden (Nb(V), Zr(IV), Ra(II), Sn(II/IV)) an zementartigen Materialien: Quantitative und mechanistische Interpretation
Year: 2026
Type: book
Abstract: Safety concepts for underground and near-surface nuclear waste repositories rely on a multibarrier system to limit radionuclide release. Cementitious materials play a central role in both waste stabilization and construction. Cement undergoes degradation through dissolution and precipitation of different phases. In the context of near-surface repositories, cement carbonation by atmospheric CO2 can also impact the long-term stability and retention properties of this material. The final stage of cement degradation is dominated by calcite (CaCO3) as aggregate material. In repositories for low and intermediate level waste (L/ILW), solubility and sorption phenomena of radionuclides (RN) such as niobium-94 (from the neutron activation of alloyed reactor components), zirconium-93 (from neutron activation of zircaloy and a fission product of 238U), radium-226 (from the decay chain of 238U) and tin-126 (a fission product of 235U or 239Pu), are considered as key processes to accurately assess the retention / mobilization of these RN. Understanding the behavior of these radionuclides in complex systems consisting of cementitious phases, pore water, waste matrix, and other components, is of great relevance to assess their longterm behavior. This includes complexing ligands such as isosaccharinic acid (ISA, main degradation product of cellulose), or high chloride content as groundwater component or from evaporator concentrates. These components may alter RN speciation or surface properties of cement, leading to changes in RN retention. This work focuses on these four radionuclides and investigates their sorption and solubility under conditions relevant in cement-based repositories. The impact of ISA and chloride on RN retention is assessed using systematic series of sorption experiments. Experiments were conducted in carbonated systems (pH ≈ 8.3) for Nb(V), Zr(IV) and Ra(II), and in anoxic conditions with C-SH phases (pH ≈ 9-13) for Sn(II/IV), to account for conditions relevant to different cement degradation stages and repository concepts. The characterization of cement pastes formed by carbonation or alteration revealed a complete phase transformation to (mostly) calcite, as well as a significantly increased specific surface area compared to unreacted cement. These solid phases and the associated equilibrated aqueous phases form the basis of subsequent solubility and sorption studies. Very low Nb(V) concentrations (≤ 10-8 M) were measured in oversaturation solubility experiments under weakly alkaline, Ca-containing systems. These values are well below the solubility of Nb2O5(s), hinting towards the formation of a new solid phase. XRD and Raman confirmed the predominance of low-crystallinity, small particle-size pyrochlore-type secondary phases (e.g., Ca2Nb2O7), for which no thermodynamic data is available. The retention of Nb(V) in calcite systems showed distribution ratios (Rd) ≈ 103 L·kg-1 at t = 3 days. At longer times, Rd values increased to > 2·104 L·kg-1, suggesting a fast adsorption followed by slower incorporation into the calcite structure. Carbonated cement systems exhibited stronger uptake (Rd ≈ 105-106 L·kg-1), consistently with literature on fresh cement and C-S-H phases. The impact of ISA and chloride on Nb(V) retention is relatively minor under near-neutral pH conditions. The more pronounced effect of ISA previously reported in hyperalkaline conditions (pH ≈ 13.6) is possibly explained by the formation of stronger Nb(V)-ISA complexes in this pHregime. Sorption experiments with Zr(IV) targeted carbonated cement materials and fresh cement pastes. Rd values determined in carbonated cement systems were ≈ 5·105–106 L·kg-1. Similarly, the uptake of Zr(IV) by cement exhibited Rd values of ≈ 105 L·kg-1. This strong uptake is consistent with the strong hydrolysis of Zr(IV) and the behavior reported for other hard Lewis’ acids and strongly hydrolyzing ions, e.g., Th(IV), Sn(IV) or Nb(V). Although similar estimated values were previously reported in the literature, this is the first experimental confirmation of the strong uptake of Zr(IV) under these conditions. The uptake of Ra(II) by calcite exhibited low-to-moderate Rd values (4–6 L·kg-1). Distribution ratios were monitored for up to 237 days and a constant retention was observed already after 3 days. This can be explained by the rapid surface adsorption or exchange with Ca2+ ions in the surface layer. Unlike Nb(V), there was no evidence of the possible incorporation of Ra(II) into the calcite structure. Consistent with these observations, theoretical calculations indicate that deeper incorporation of Ra2+ into the calcite structure is energetically very unfavorable. Neither ISA nor NaCl influenced Ra(II) retention, which is explained by the weak interaction of both anions with Ra(II). Tin was studied under anoxic conditions in the oxidation states +II and +IV. Solubility in the pH range ≈ 9–12.8 confirmed agreement between the Sn(II) data and NEA-TDB calculations, whereas a worse agreement was obtained for Sn(IV). Thermodynamic calculations based on standard databases predict the full predominance of Sn(IV) within the stability field of water, in disagreement with experimental observations in this work. This discrepancy aligns with recent developments by Rai, who claimed a gross error in selected ΔfG0(Sn4+) in reference databases [1]. Sorption experiments on C-S-H phases result in strong uptake even under reducing conditions. As confirmed by XAFS, this observation can be explained by the oxidative adsorption of Sn(II) by C-S-H. Adjusting Sn(IV) thermodynamic constants is required to match available sorption data under anoxic and reducing conditions with the developed CASH+ model. This shifts also the Sn(IV)/Sn(II) redox borderline towards less reducing conditions, in line with experimental observations obtained in this study. These results inherently imply that Sn(IV) and Sn(II) could both be relevant in the reducing, alkaline conditions expected in L/ILW repositories. This work provides insight into key retention processes (solubility, sorption) for relevant radionuclides (94Nb, 93Zr, 226Ra and 126Sn) in the context of cement-bases repository systems. Particular attention is paid to the degradation stage IV of cement, as well as to the influence of ISA and high chloride concentrations, and the redox behavior of tin under repository relevant conditions.
Source: KITopen
Author: Huber, Nils
Language: German
Subfield: Inorganic Chemistry
Field: Chemistry
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W7220710163
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