No abstract

No abstract

The solubility and redox behavior of hydrous Pu(IV) oxide was comprehensively investigated by an experimental multi-method approach as a function of different redox conditions in 0.1 M NaCl solutions, allowing a detailed characterization of Pu(IV) and Pu(III) solubility and solid phase stability in these systems. Samples were prepared at ~3≤pHm≤~6 (pHm=–log${{\text{m}}_{{{\text{H}}^{\text{ + }}}}})$and ~8≤pHm≤~13 atT=(22±2)°C under Ar atmosphere. No redox buffer was used in one set of samples, whereas mildly and strongly reducing redox conditions were buffered in two series with hydroquinone or SnCl2, respectively, resulting in (pe+pHm)=(9.5±1) and (2±1). XRD, XANES and EXAFS confirmed the predominance of Pu(IV) and the nanocrystalline character of the original, aged PuO2(ncr,hyd) solid phase used as a starting material. Rietveld analysis of the XRD data indicated an average crystal (domain) size of (4±1) nm with a mean cell parameter of (5.405±0.005) Å. The solubility constant of this solid phase was determined as log$^ * K{^\circ _{{\text{s}},0}}$=–(58.1±0.3) combining solubility data in acidic conditions and redox speciation by solvent extraction and CE–SF–ICP–MS. This value is in excellent agreement with the current thermodynamic selection in the NEA-TDB. Synchrotron-basedin-situXRD, XANES and EXAFS indicate that PuO2(ncr,hyd) is the solid phase controlling the solubility of Pu in hydroquinone buffered samples. Under these redox conditions and ~8≤pHm≤~13, the solubility of Pu is very low (~10−10.5m) and pH-independent. This is consistent with the solubility equilibrium PuO2(am,hyd)+2H2O(l)⇔ Pu(OH)4(aq). Althoughin-situXRD unequivocally shows the predominance of PuO2in Sn(II)-buffered systems, XANES analyses indicate a significant contribution of Pu(III) (30±5%) in the solid phases controlling the solubility of Pu at (pe+pHm)=(2±1). For this system, EXAFS shows a systematic shortening of Pu–O and Pu–Pu distances compared to the starting Pu material and hydroquinone-buffered systems. The solubility of Pu remains very low (~10−10.5m) at pHm>9, but shows a very large scattering (~10−9–10−10.5m) at pHm=8. Experimental observations collected in Sn(II) buffered systems can be explained by the co-existence of both PuO2(ncr,hyd) and Pu(OH)3(am) solid phases, but also by assuming the formation of a sub-stoichiometric PuO2−x(s) phase. This extensive study provides robust upper limits for Pu solubility in alkaline, mildly to strongly reducing conditions relevant in the context of nuclear waste disposal. The potential role of Pu(III) in the solid phases controlling the solubility of Pu under these conditions is analysed and discussed in view of the current NEA-TDB thermodynamic selection, which supports the predominance of PuO2(am,hyd) and constrains the formation of Pu(OH)3(am) at pHm>8 outside the stability field of water.

The uptake of 63Ni(II), 152Eu(III) and 242Pu(III/IV) by hardened cement paste (HCP, CEM I) in the degradation stage II (pH ≈ 12.5 [Ca] ≈ 0.02 M) was investigated in the absence and presence of α-hydroxyisobutyric, 3-hydroxybutyric and glutaric acids. These organic ligands were previously identified as proxies for the degradation products of UP2W (a polyacrylonitrile-based material used as filter aid in nuclear power plants) under repository conditions. Sorption experiments were conducted with various ligand concentrations (10−4 M ≤ [L]tot ≤ 0.1 M) and solid-to-liquid ratios (0.5 g⋅dm–3 ≤ S:L ≤ 20 g⋅dm–3). Redox conditions in the Pu systems were buffered with either hydroquinone (HQ, pe + pH ≈ 10) or Sn(II) (pe + pH ≈ 2). Strong sorption is observed for 152Eu(III) and 242Pu(III/IV) in the absence of proxy ligands, with distribution coefficients (log Rd ≈ 2.2–4, with Rd in m3⋅kg–1) in line with data reported in the literature. No differences are observed for sorption experiments with Pu in HQ and Sn(II) systems. Lower Rd values are determined for 63Ni(II) (log Rd,Ni63 ≈ 0-1), consistently with previous studies. In combination with log Rd,Ni determined on the basis of the concentration of stable Ni(II) in pristine HCP and in cement porewater, values of the partition coefficient (α) close to 1 are determined. This suggests that the uptake of 63Ni is possibly driven by isotopic exchange with the complete species inventory of stable Ni present in pristine HCP, including Ni in solid phases and associated with surfaces, e.g., of C-S-H phases. The presence of proxy ligands has a negligible effect on the uptake of 152Eu(III) up to [L]tot = 0.1 M. A slight decrease in the distribution ratios for 63Ni(II) and 242Pu(III/IV) is observed at [L]tot > 10−2 M, although the effect is less evident in the case of plutonium due to the dispersion of the data and the increase of the detection limits with increasing ligand concentrations. Compared to strongly complexing ligands like isosaccharinic acid or gluconate, the investigated proxy ligands show a minor capacity for radionuclide mobilization in cementitious systems, even at high concentrations.

No abstract