An experimental study was conducted to investigate the effects of altering fresh (U,Pu)O2 MOX of high alpha-specific activity under alkaline conditions (pH = 12), in the presence of 2 mmol.L-1 silicate ions and in anoxic environment. This test was conducted in conjunction with analogous experiments using (U,Ce)O2 simulant materials of homogeneous and heterogeneous microstructure. The results obtained indicated that the oxidative dissolution induced by alpha-radiolysis of water in contact with (U,Pu)O2 MOX was suppressed under these alteration conditions. The uranium concentration determined at equilibrium was found to be fully comparable to those obtained with the (U,Ce)O2 model materials. This observation suggested that the alteration mechanism occurring at the solid/ solution interface was independent of the alpha-activity of the material and highlighted the analogy of the two types of materials in the tested conditions. Post-alteration characterizations of the material surface using Raman spectroscopy, TEM/EDS and SEM were conducted. The results obtained supported the assumption that adsorption of silicate ions onto reactive sites in the material was responsible for the inhibition of uranium oxidation. This study revealed the important role played by silicate ions in reducing uranium release in alkaline conditions and under alpha radiolysis of water. Considering that uranium is a tracer of the MOX matrix alteration, these results had positive implications regarding the long-term management of spent fuel in deep disposal especially in case of contact with cementitious water.

Pu-enriched agglomerates present in MIMAS MOX fuels constitute a non-negligeable contribution to the Instant Release Fraction (IRF) in the scenario of direct fuel disposal in geological repository. An original experimental methodology has been developed with three initial objectives: (i) separate the Pu enriched agglomerates from the coating phase and the UO2 matrix, in order to handle and leach them selectively (ii) determine the instant release of fission products of these isolated agglomerates (iii) break up the Pu agglomerates through acoustic cavitation to reveal the labile inventory trapped within the closed porosity, by comparing leaching data from intact and disaggregated agglomerates. This paper presents the results of this new methodology: (i) the success of the separation method was demonstrated by combining FXL, SEM and Raman characterizations (ii) leaching results suggest that the thermal treatment may have induced both a modification of Cs speciation and a partial volatilization of its labile inventory (iii) mechanical effects of ultrasound were used efficiently to open the closed porosity and disclose the trapped inventory, leading to submicronic-size objects.

Studtite [(UO2)(O2)(H2O)2]& sdot;2(H2O) is a secondary phase precipitating during the alteration of uranium-bearing materials. The mechanistic link between the formation of uranyl and peroxide bonds in the solid and the nature of the oxidizing species produced by water radiolysis remains to be elucidated. In order to improve our understanding of these mechanisms, an original experimental methodology in the presence of 18O isotopes and Raman spectroscopy has been developed. It appears that there is a direct chemical relationship between the peroxo ligands inside studtite and the peroxide entities of H2O2 molecule into the solution. The link between H2O2 formation mechanism in solution by the radiolysis of water, the nature of the radiation and the isotopy of the peroxo ligands inside studtite has been described thanks to a coherent set of experimental data. For the uranyl UO22+ ions, the characterization of its isotopy allows to specify the mechanism of oxidation at the UO2 /water interface. The isotopic configurations observed for the uranyl ion inside studtite, can be explained by assuming an oxidation mechanism of UO2 involving both a simple transfer of electrons by interaction with H2O2 and the incorporation of oxygen atoms from the solution into the fluorite structure via OH degrees radicals.

Pseudomorphic transformations are related to chemical conversions of materials while conserving their shape and structural features. Structuring ceramic shapes this way can be used to tailor the physico-chemical properties of materials that can benefit particular applications. In the context of spent nuclear fuel storage interacting with radiolysis products, the sonochemical behavior of powdered UO2 was investigated in dilute aqueous solutions saturated with Ar/(20 %)O2 (20 °C). Optimized parameter settings enabled the complete conversion of UO2 micrometric platelets into uranyl peroxide precipitates, referred to as (meta-)studtite [(UO2(O2)(H2O)2)xH2O] with x = 2 or 4. While the most acidic conditions yielded elongated crystal shapes in agreement with a dissolution/reprecipitation mechanism, softer conditions allowed the pseudomorphic transformation of the platelet shape oxide suggesting a complex formation mechanism. For specific conditions, this unprecedented morphology was accompanied with the formation of a hole in the platelet center. Investigations revealed that the formation of the drilled polymorphs is related to a perfect blend of H+, in-situ generation of H2O2 and high-frequency ultrasound, and is most probably related to the sono-capillary effect. These insights pave the way for new sonochemical approaches dedicated to the preparation of material polymorphs tailoring specific structural properties.

Homogeneous and heterogeneous U 1-x Ce x O 2 (with 0≤ x≤ 0.25) materials were prepared via wet and dry chemistry routes, respectively before being submitted to dynamic leaching experiments. The feeding solution containing 0.20 mmol.L −1 H 2 O 2 was kept under air and renewed to guarantee the stability of H 2 O 2 during the experiment. Normalized alteration rates were determined from U concentration in the leachates. For homogeneous (U,Ce)O 2 materials, the dissolution rate was divided by a factor of 3 when increasing the Ce content from 0.08 to 0.25. Surface characterizations revealed that studtite precipitated all over UO 2 pellet surface and only on the UO 2 grains of heterogeneous U 0.92 Ce 0.08 O 2 samples. The behaviour of this heterogeneous material was similar to that observed for (U,Pu)O 2 in the same conditions, which revealed the reliability of cerium as a plutonium analogue.

The evolution under alpha-decay radiation of a 241 Am doped aluminoborosilicate glass-ceramic was in-vestigated in hot cells DHA-ATALANTE facility in CEA. The cumulative La2O3 -Am2O3 solubility limit was voluntarily exceeded, leading to the formation of apatite-like silicate crystals. The crystals, with a hexagonal-shaped morphology, that is characteristic of apatite crystals, have a composition and a cell pa-rameter close to those expected for an apatite phase of stoichiometric composition Ca2(La,Am)8(SiO4)6O2. Structural and microstructural evolutions under alpha self-irradiation were followed for 8 years by regu-larly analyzing the crystals and the residual glassy matrix. The evolution of the X-Ray Diffraction (XRD) patterns is the result of a progressive radiation-induced amorphization in apatite crystals. The fully amor-phous state is reached at an alpha-decay dose of around 3 x 10 18 alpha/g. Raman analyses suggest a modi-fication of the connectivity of the SiO4 tetrahedra of apatite crystals, with a transition from isolated SiO4 units in the crystalline state to connected SiO4 units in the metamict state. The crystalline-to-amorphous transformation is accompanied by an increase in macroscopic volume (swelling), highlighted by RAMAN imaging. This macroscopic dimensional change is also associated to a decohesion of the crystals from the glassy matrix, observed by Scanning Electron Microscopy (SEM). However, optical and SEM images of the glass-ceramic surface do not reveal any significant cracks in the residual glass under alpha self-irradiation, thus showing a quite good stability of this glass-ceramic material.(c) 2023 Published by Elsevier B.V.

Homogeneous and heterogeneous U 1-x Ce x O 2 (with 0≤ x≤ 0.25) materials were prepared via wet and dry chemistry routes, respectively before being submitted to dynamic leaching experiments. The feeding solution containing 0.20 mmol.L −1 H 2 O 2 was kept under air and renewed to guarantee the stability of H 2 O 2 during the experiment. Normalized alteration rates were determined from U concentration in the leachates. For homogeneous (U,Ce)O 2 materials, the dissolution rate was divided by a factor of 3 when increasing the Ce content from 0.08 to 0.25. Surface characterizations revealed that studtite precipitated all over UO 2 pellet surface and only on the UO 2 grains of heterogeneous U 0.92 Ce 0.08 O 2 samples. The behaviour of this heterogeneous material was similar to that observed for (U,Pu)O 2 in the same conditions, which revealed the reliability of cerium as a plutonium analogue.

This article focuses on the Am incorporation and location in the UMo-glass ceramic designed to manage fission product solutions enriched in 241Am generated by the reprocessing of UMo spent fuel. This type of glass ceramic contains crystalline phases, which are phosphates, molybdates and zircons, embedded in a durable borosilicate glass matrix.Two types of UMo-glass ceramics were considered in this study. One type of samples was doped with different amounts of Am and/or Nd to assess the comparative role of Nd versus Am in terms of incorporation, location and microstructure. A second type of sample was doped with 244Cm, in order to specify the behavior of 244Cm compared with that of Nd and Am, with a view to using this short-lived isotope for aging experiments.These matrices were characterized by gamma spectrometry and isothermal calorimetry to check the overall homogeneity of Am and Cm, then by Raman spectroscopy, SEM, EPMA and XRD to determine the microstructure and the actinide/Nd distribution in the different phases. The microstructure of the samples was heterogeneous as expected, with the presence of phosphate, molybdate and zircon crystalline phases. A great enrichment of Am, Cm and Nd was observed in the phosphate and molybdate phases compared to the embedding glassy matrix and the zircon crystals. A partition coefficient of around x 6 - 7 for Am and Nd was calculated in these two crystalline phases compared to the mean target doping value. Finally, Nd behaved like trivalent actinides in the UMo matrix and could therefore be considered as a relevant surrogate.

The (111) surface of uraninite obtained by monolith cleavage was reacted in H2O2 solutions of increasing concentrations ([H2O2]) from 0.005 to 0.5 M. The reacted interface was characterized in situ by mu Raman spectroscopy, or, for 0.5 M H2O2, Grazing-Incidence X-Ray Diffraction (GI-XRD). At low [H2O2], schoepite and studtite coexisted on the reacted surface. With increasing [H2O2], studtite predominated, and a pat-tern of black strips cross-cutting at 60 degrees suggested control of the solid neoformation by the (111) surface structure. In situ GI-XRD for 0.5 M H2O2 revealed the presence of studtite only, with an initial anisotropy in particle orientation (c * axes parallel to the uraninite surface), and a subsequent reorientation of grow-ing particles. Upon draining out the solution, the formation of schoepite was observed after a few hours of emersion. This result reveals that apparently benign post-experimental manipulations can lead to sig-nificant changes in the nature of observed secondary precipitates.(c) 2022 Elsevier B.V. All rights reserved.

International Simple Glass altered for years in silica saturated conditions develops a passivating gel, which re-tains some mobile elements such as B and Ca in its inner region. It has been suggested that the retention of these elements impacts the dissolution rate of the glass. However, the elements' retention mechanism, speciation, and diffusion properties are still unknown. This study finds that simultaneous presence of B and Ca in the solution can dramatically slow down glass alteration. However, splitting the B and Ca into two different solutions with other glass elements did not give similar protection against glass alteration. These experiments along with other ex-periments conducted in basic or acid pH, in which isotopically tagged B can diffuse in gels revealed that (i) water diffusion is not dramatically affected by the retention of B and Ca in passivating gels, and (ii) slow diffusion and high retention of B is observed only in gels developed in basic pH during the tracing experiments. This last mechanism therefore appears as one of the many mechanisms controlling the glass dissolution rate in the residual regime.

The alteration of a four-oxide alumino-borosilicate glass close to International Simple Glass (ISG) was studied in Si saturation conditions at pH 3, pH 7, and pH 9 using solution and post mortem solid analyses. In particular, the study examined the impact of structural disorder on the glass alteration generated by swift heavy ion irradiation. It is shown that the pre-irradiated glass altered three to four times more than the non-irradiated glass, depending on the pH. This increased alteration can be attributed to two factors: an increased hydrolysis speed of the Si–O–B bonds, which control the glass dissolution under these Si saturation conditions, and a less passivating alteration layer. The latter effect suggests that the glass structural disorder is retained by the first gel formed, which means very low Si atom mobility during alteration. However, gel maturation tends to mitigate the detrimental effect of radiation damages. These results will contribute to the development of a predictive residual rate model.

Following the NPP accident, several hundred tons of heat-generating corium and fuel debris have been cooled permanently by millions of m(3) of flowing. Knowledge on the interaction with water is crucial for any decommissioning planning. Starting from knowledge on the evolutions of the accident in the three reactor cores and associated fuel debris formations and some additional isotopic and physio-chemical information on debris fragments collected in Fukushima soils, we review the temporal evolution of the chemistry and leached radionuclide contents of the cooling water, comparing measured concentration ratios of the actinides and fission products in the water to reported results of laboratory leaching studies with either spent nuclear fuel or simulated fuel debris. As for spent fuel leaching, the fractions of inventories of Cs-134,Cs-137 in the cooling water are orders of magnitude larger than that of the actinides. After more than 10 years of fuel debris/water contact, Cs-137 release rates have decreased by about a factor of 100. The total release of actinides from the fuel debris is orders of magnitude lower than that of Cs-134,Cs-137 or of Sr-90. This high stability makes direct disposal of fuel debris in suitable containers after decommissioning a viable option.

The oxidative dissolution of a (U,Pu)O2 MOX fuel under α radiolysis of water was studied under chemical conditions of increasing complexity in the context of the geological disposal of radioactive waste. The results show noticeable effects of a synthetic Callovian-Oxfordian claystone water and of Fe(II) on the oxidative dissolution. The presence of metallic iron in the system leads to an inhibition of the oxidative dissolution by the consumption of oxidizing species produced by water radiolysis at the MOX/water interface. The environmental effects observed with groundwater and reducing Fe(II)-species are very similar to those already observed on UO2-based fuels.

A Na-alumino-borosilicate glass and its Ca-doped counterpart were altered in vapor phase (98% relative humidity) and aqueous medium at 90 °C. Both the alteration media were enriched in 17O. Characterization of the altered samples pointed out some differences between glass alteration in aqueous medium and vapor phase in terms of the alteration kinetics, the effect of Ca-doping on glass chemical durability, the behavior of elements in the gel layer, and the structure of the gel layer. Some of the key results are the recondensation of boron in the gel layer formed in the vapor phase and the utility of Rotation Echo DOuble Resonance Nuclear Magnetic Resonance spectroscopy to qualitatively distinguish between signals from the pristine glass and hydrated gel layer within a sample that was not altered to the core. The results gave rise to inferences about glass alteration mechanisms in both the alteration media and the differences between them.

The formation of studtite was studied during the oxidative dissolution of a MIMAS MOX fuel disk in aerated water enriched in O-18 under a gamma radiation source, coupling Raman spectroscopy and solution analyses. The use of isotopic labeling enabled the reactions responsible for the precipitation of studtite to be followed. At the beginning of the experiment, different O-18 enrichments in the uranyl and peroxide bonds of the studtite were observed. While the uranyl bond was primarily enriched in O-18, the peroxide bond contained large amounts of O-16. This result suggests an oxygen contribution coming from different radiolytic species for each bond: H2O2 and radicals. The comparison with a UO2 sample leached in similar conditions ruled out a role of Pu alpha self-irradiation in this different behavior. Nevertheless, the influence of the MOX MIMAS heterogeneous microstructure and chemistry was observed with the preferential dissolution of the UO2 grains, the Pu-rich areas being much more stable with regard to the dissolution. In addition, the studtite first precipitated preferentially on the Pu-poor areas of the sample before covering the entire surface, including the plutonium-enriched aggregates, at the end of the experiment.

The hydration of SON68 glass (non-radioactive surrogate for R7T7 glass, the French industrial glass for high-level radioactive waste management) was studied at between 30 and 70°C at a relative humidity of 95% for a period of 6 months. The hydration thicknesses were shown to increase with temperature and did not exceed a few nanometres at 30 and 50°C, or a few tens of nanometres at 70°C. The temperature dependence of hydration rates followed Arrhenius law and enabled the determination of an activation energy. This is 62 ± 13 kJ.mol−1, which is compatible with a hydration mechanism involving a hydrolysis of the silicate bonds. All these results are discussed, taking into account data available in the literature.

In the prospect of deep geological disposal, the long-term behavior of high-level nuclear glasses has to be investigated regarding alpha radiation induced by long-life minor actinides. The present study focuses on the effects of alpha radiation on the long-term chemical reactivity of R7T7-type glasses, by separately considering the alpha dose rate and the alpha decay dose. Old SON68 glasses doped with 238/239PuO2 or 244CmO2 were studied to simulate high alpha dose rates corresponding to an early water ingress and a high level of alpha decay doses corresponding to long-term disposal conditions. A part of the 238/239Pu-doped glass block was annealed to fully recover the irradiation-induced damage accumulated since the glass was fabricated and to dissociate the effect of the alpha dose rate from that of the alpha decay dose. The glasses were then leached under static conditions at 90 °C for several years. The results showed that the residual alteration rate is not affected by the alpha dose rate over a wide range of dose rate values expected under disposal conditions: this glass remained relatively insensitive to the alpha radiolysis phenomena at the glass–water interface. However, over the duration of the experiments, the residual alteration rate of the damaged 238/239Pu-doped glass was enhanced compared to that of the annealed glass. This result is in agreement with those obtained on the 244Cm-doped glass and with reported values in the literature on simplified externally irradiated glasses, indicating that the ballistic effects of the recoil nuclei are responsible for this increase in the residual alteration rate.