High H2O2 exposure leads to an irreversible change in redox reactivity of UO2 surfaces.

In a geological repository for spent nuclear fuel, studtite (UO2)­O2(H2O)4 may form on the fuel surface when in contact with groundwater under certain conditions. Studtite has a very low solubility and could thereby reduce the reactivity of spent nuclear fuel toward radiolytic oxidants. This would inhibit the dissolution of the fuel matrix and thereby also the spreading of radionuclides. It is therefore important to investigate the stability of studtite under conditions that may influence its stability. In this work we have studied the kinetics of studtite dissolution in aqueous suspensions containing no added HCO3 – and with 10 mM HCO3 –. The same type of experiment was performed also with solutions containing 0.2 mM H2O2. The solubility of studtite in the suspensions containing no added HCO3 – is very low as expected, while the solubility in solutions containing 10 mM HCO3 – is significantly higher. This is attributed to the formation of uranyl–carbonate and uranyl–peroxo–carbonate complexes. In 0.2 mM H2O2 and 10 mM HCO3 – the observed solubility of U­(VI) seems unaffected by the presence of H2O2. Again, this can be rationalized by the formation of uranyl–peroxo–carbonate complexes. It is interesting to note that H2O2 appears to be catalytically decomposed in solutions containing uranyl–carbonate complexes. In addition, γ-radiation-induced dissolution of studtite in HCO3 – deficient solutions and in 10 mM HCO3 – was studied. The dissolution rate was found to be extremely high in HCO3 – under γ-irradiation. This is attributed to a combination of radiolytic degradation of H2O2 and the formation of uranyl–peroxo–carbonate complexes keeping the concentration of free H2O2 at a very low level and thereby driving the dissolution process.

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.

Hydrogen peroxide can be catalytically decomposed to O2 and H2O on metal oxide surfaces in contact with aqueous solutions containing H2O2. The initial step in this process has been proposed to be the formation of surface‐bound hydroxyl radicals which has recently been verified using tris as a radical scavenger. Here, we make use of the unique fluorescent product 7‐hydroxycoumarin formed in the reaction between hydroxyl radicals and coumarin to probe the formation of surface‐bound hydroxyl radicals. The experiments clearly show that 7‐hydroxycoumarin is formed upon catalytic decomposition of H2O2 in aqueous suspensions containing ZrO2‐particles and coumarin, thereby confirming the formation of surface‐bound hydroxyl radicals in this process. The results are quantitatively compared to results on the same system using tris as a probe for hydroxyl radicals. The effects of the two probes on the system under study are compared and it is concluded that coumarin has a significantly lower impact on the system.