Self-healing capacity of nuclear glass observed by NMR spectroscopy

Safe management of high level nuclear waste is a worldwide significant issue for which vitrification has been selected by many countries. There exists a crucial need for improving our understanding of the ageing of the glass under irradiation. While external irradiation by ions provides a rapid simulation of damage induced by alpha decays, short lived actinide doping is more representative of the reality. Here, we report radiological NMR experiments to compare the damage in International Simplified Glass (ISG) when irradiated by these two methods. In the 0.1 mole percent 244Cm doped glass, accumulation of high alpha decay only shows small modifications of the local structure, in sharp contrast to heavy ion irradiation. These results reveal the ability of the alpha particle to partially repair the damage generated by the heavy recoil nuclei highlighting the radiation resistance of nuclear glass and the difficulty to accurately simulate its behaviour by single ion beam irradiations

Spent fuel UO2 matrix alteration in aqueous media under oxidizing conditions

International audienceFor underwater interim storage of spent UOX fuel assemblies it is necessary to consider the incidental and unusual scenario starting from the occurrence of a breach in the cladding and the presence of water. On this assumption it is important to compile data on the reactivity of the spent fuel matrix under highly oxidizing conditions and subjected to a mixed * and ** radiation field. Data such as the UO2 fuel matrix alteration rate or the nature of the precipitating secondary phases are important for modeling the behavior of a failed rod in water under incidental scenario, and especially the cladding behavior during interim storage (opening of a rod by enlargement of a defect due to secondary phase precipitation, contamination level in the pool water, etc.). Various spent fuel leaching experiments conducted at different degrees of oxidizing conditions and of accessibility of species at the reaction interface are discussed. The results indicate that despite highly variable oxidizing conditions (presence of an external * irradiation source, addition of H2O2, aerated water) the spent fuel matrix alteration determined from the release of tracer elements such as Cs and Sr is always the same. Conversely, allowing for the alteration conditions (no solution renewal and no complexants), the uranium concentrations in solution do indeed depend on the redox conditions as does the nature of the secondary phases liable to precipitate

U1-xPuxO2 more or less 948; fuel precursor synthesis through advanced thermal denitration in presence of organic additive

International audienceWithin the U and Pu recycling process from nuclear spent fuels, the conversion of purified U and Pu solutiononto oxide powder is a key step at the interface between the separation / purification processes and thefabrication of uranium-plutonium oxide fuels called MOx ("Mixed Oxides"). In order to simplify theindustrial process, conversion must be able to integrate fluxes with the minimum of adjustments whileleading to oxides or mixtures of oxides adapted to a direct shaping of fuel pellets. In the framework of Pumulti-recycling, new conversion routes have to be developed to suit new needs: increase of the quantity ofsynthetized oxides, reduction of dust generation, no redox adjustments of feed actinide solutions, simplifiedeffluent management, no proliferation risks. To meet these future requirements, CEA is developing a"direct" conversion route based on advanced thermal denitration to obtain mixed actinide oxide U1-xPuxO2±δ.This synthesis method is based on the NPG (nitrate polyacrylamide gel) route developed for non-radioactivematerials [1]. Several adjustments of this synthesis were performed prior its application to the productionof U1-xPuxO2±δ (x = 0, 0.20, 0.25, 0.50 and 0.75) batches. For each sample uranium nitrate (UO2(NO3)2) wasdissolved in a solution of nitric acid containing Pu at the stoichiometry of the targeted oxide. Total U+Puconcentration was equal to 40 g.L-1, total nitrate concentration was equal to 3.0 mol.L-1. In parallel, a mixtureof mainly acrylic acid AA and N, N'-methylene bis (acrylamide) MBAM in a molar ratio of 20:1 (AA:MBAM) was prepared and then added to the actinide nitrate solution and stirred. Once homogeneous, theresulting solution was heated up to 100°C and supplemented with 25 μL of radical initiator to quickly allowcomplete polymerization into a gel. A xerogel was then formed by dehydration of the polymeric gel at150°C. Xerogel conversion at 800°C in reconstituted air produced intimate mixtures of U3O8 and PuO2.After the second calcination, performed under Ar/H2-5% at 800°C, single-phase oxides were obtained asconfirmed by XRD analyses showing the presence of a single fluorite structure (Fm-3m) for the samplesU1-xPuxO2±δ (x = 0 to 0.75) (Figure 1a), exhibiting a 3D morphology (Figure1b). For U0.80Pu0.20O2±δ, theamount synthesized allowed the fabrication of a pellet. In this objective, the powder was pressed under 500MPa in a three part die, no grinding step was used prior to the pelletization. The green density reached was70% of the theoretical density (TD). The green pellet was then sintered in a dilatometer under Ar/H2-4%with 1200 ppm of H2O and the axial shrinkage was recorded as function of temperature (Figure 2a). Themaximum shrinkage rate is observed at 1455°C. The pellet was then maintained at 1700°C for 2h. At theend of this process, the density of the pellet reached 94±2% of TD and exhibited oxygen to metal ratio of2.00. Visually the pellet did not show any cracks (Figure 2b).The main objective of this work was to explore the possibility to synthetize and sinter mixed U/Pu oxidesby “advanced thermal denitration in the presence of organic additives”. Feasibility is demonstrated here bythe successful synthesis of U1-xPuxO2±δ, whatever Pu content. The direct sintering of the obtained powderwithout milling step before shaping is possible. Therefore a significant breakthrough in the methodscurrently used for MOx fabrication is demonstrated. Indeed, this new process allows the synthesis of mixedoxides from solutions resulting from reprocessing without any redox adjustment (i.e. using U(VI)/Pu(IV)nitrate solution) and does not require any filtration and grinding steps, limiting highly contaminant dustgeneration

Impact of ionizing radiation on iodine capture by functionalized mesoporous silica

International audienc

Elaboration of microporous CeO2 thin layers having nanocrystallites network controlled by Pluronic P123: Impact of key experimental parameters

International audienceMicroporous ceria thin films having nanocrystallites network were synthesized by evaporation-induced self-assemblyprocess, using P123 amphiphilic copolymer as structure-directing agent. The impact of key experimental parameters,i.e., the sol ageing, the relative humidity (RH), the thermal treatment, on the thin layer elaboration were investigated.The results show that the organization of the nanoparticles is possible for a sol ageing time lower than 16 days and thatthe size of the mesophase within the thin layer increases with the relative humidity fixed during the film deposition.Finally, an increase of the thermal treatment temperature from 300 to 1000 °C leads to the formation of fluorite-likedioxide material coupled with a loss of nanocrystallites organization and porosity due to the increase of the size of thecrystallites. This loss of nanocrystallites organization is more drastic in the direction perpendicular to the surface andless pronounced along the surface. Thus, a compromise between the crystallization and the preservation of the porosityneeds to be found

Analysis of Pu by Virtual-standard WDS-EPMA. Results of an Interlaboratory Round-robin Test

The understanding of the behaviour of nuclear fuels under different operating conditions requires accurate wavelength-dispersive electron-probe microanalysis (WDS-EPMA) of materials that contain actinide elements. Conventional WDS-EPMA is based on the measurement of k-ratios, i.e. the ratio of the x-ray intensity emitted by the element of interest in the unknown to that emitted by the same element in a reference standard. The advantage of using such approach is that several instrumental, spectroscopic and atomic parameters that are generally poorly known cancel out from the quantification equations and are no longer required. Moreover, the effect of the approximations adopted to describe quantities such as the depth-distribution of ionizations or the cross section for inner-shell ionization is largely reduced. As a result, the accuracy of concentrations obtained by WDS-EPMA using reference standards can be as good as 2%, even when x-ray attenuation effects are important. However, for some actinide elements such as Plutonium (Pu) or Americium (Am), reference standards are difficult to obtain. In these situations, the use of standardless methods of analysis [1,2], which use calculated intensities to replace measurements on the standards, would be an ideal alternative. However, the accuracy of such methods is limited by the poor knowledge of the different instrumental, spectroscopic and atomic parameters required, as mentioned above. Significant benefits would be gained if measurements on standards could be transferred among different WDS instruments; this would allow the creation of a set of ¿virtual standards¿ which could be shared among different laboratories. The use of virtual standards requires accurate knowledge of instrumental parameters (e.g. the spectrometer efficiency and solid angle of detection) and spectroscopic parameters (e.g. the line shape), but knowledge of atomic parameters is not required. This is important for the actinide elements, which involve the measurement of M xrays, where atomic parameters such as the fluorescence yield, the Coster-Kronig and super-Coster-Kronig transition probabilities or the cross section for M-subshell ionization are affected by large uncertainties (>30%).JRC.E.2-Hot cell

Elaboration of porous silicon carbide by softtemplating molecular precursors with semifluorinatedalkanes

International audienceWe present a novel soft templating approach (STA) for the elaboration of porous silicon carbide (SiC) in oxygen- and moisture-free atmospheres. In this method, the molecular precursor, 1,3,5-trisilacyclohexane (TSCH) which is in a liquid state at room temperature, is templated by a solid network of semi-fluorinated alkanes (SFA) that self-assemble directly in the former liquid. The obtained gel phase is then converted into a polysilane by the polymerization of TSCH molecules around the solid network of SFA. In a next step, the SFA are washed out from the polysilane, and the latter material is converted into SiC by calcination under argon atmosphere at 1000 C. The resulting material possesses a granular structure with a macroporosity. This study validates STA as a sustainable and promising method to obtain porous and mesoporous SiC

Analysis of Pu by Virtual-standard WDS-EPMA. Results of an interlaboratory round-robin test for wavelength-dispersive electron-probe microanalysis

International audienc

The mechanism of the hydrothermal alteration of cerium- and plutonium-doped zirconolite

A comprehensive study on the aqueous stability of Ce- and Pu-doped zirconolite has been performed. Four series of hydrothermal experiments were carried out with Ce-doped zirconolite powders: (1) a solution series (1 M HCl, 2 M NaCl, 1 M NaOH, 1 M NH3, pure H2O), (2) a temperature series (T = 100-300 °C), (3) a surface area-to-fluid volume ratio series, and (4) a series using different reactor materials (Teflon©, Ni, and Ag). In addition, experiments on 238Pu- and 239Pu- doped zirconolite ceramics in a 1 M HCl solution have been performed. The 238Pu-doped zirconolite had already accumulated significant radiation damage and was X-ray amorphous, while the 239Pu-doped zirconolite was still well-crystalline. The results of the different experimental series can be summarized as follows: (1) After 14 days the degree of alteration is insignificant for all solutions other than 1 M HCl, which was therefore used for all other experimental series; (2) TiO2 and m-ZrO2 replaced the zirconolite grains to varying degrees in the 1 M HCl solution, i.e., zirconolite dissolution is incongruent; (3) the degree of alteration increases only slightly with increasing temperature; (4) the alteration rate is independent on the surface to volume ratio; (5) Ag dissolved from the silver reactors dramatically increases the reaction rate, while Ni from the Ni reactors reduces the solubility of Ti and Zr in the HCl solution, indicating that background electrolytes have a strong effect on the alteration rate. From the experiment with the Pu-doped samples at 200 °C in a 1 M HCl solution it was found that the amorphous 238Pu-doped zirconolite was altered to a significantly greater extent than the crystalline counterparts. The results suggest a coupled dissolution-reprecipitation mechanism, which is discussed in detail