Ion accelerators have been used by material scientists for decades to investigate radiation damage formation in nuclear materials and thus to emulate neutron-induced changes. The versatility of conditions in terms of particle energy, dose rate, fluence, etc., is a key asset of ion beams allowing for fully instrumented analytical studies. In addition, very short irradiation times and handling of non-radioactive samples dramatically curtail the global cost and duration as compared to in-reactor testing. Coupling of two or more beams, use of heated/cooled sample holders, and implementation of in situ characterization and microscopy pave the way to real time observation of microstructural and property evolution in various extreme radiation conditions more closely mimicking the nuclear environments. For these reasons, multiple ion beam facilities have been commissioned worldwide. In France, under the auspices of the Universite Paris-Saclay, the JANNuS platform for ‘Joint Accelerators for Nanosciences and Nuclear Simulation’ comprises five ion implanter and electrostatic accelerators with complementary performances. At CSNSM (CNRS & Univ Paris-Sud, Orsay), a 200 kV Transmission Electron Microscope is coupled to an accelerator and an implanter for in situ observation of microstructure modifications induced by ion beams in a material, making important contribution to the understanding of physical phenomena at the nanoscale. At CEA Paris-Saclay, the unique triple beam facility in Europe allows the simultaneous irradiation with heavy ions (like Fe, W) for nuclear recoil damage and implantation of a large array of ions including gasses for well-controlled modelling-oriented experiments. Several classes of materials are of interest for the nuclear industry ranging from metals and alloys, to oxides or glasses and carbides. This paper gives selected examples that illustrate the use of JANNuS ion beams in investigating the radiation resistance of structural materials for today’s and tomorrow’s nuclear reactors.

Investigating radiation damage in nuclear energy materials using JANNuS multiple ion beams

In order to improve the removal rate of uranium and reduce the harm of radioactive pollution, a physically crosslinked polyvinyl alcohol/phosphorylated chitosan (PPP) hydrogel electrode was designed by freezing thawing method. The results show that PPP hydrogel has a good adsorption effect on uranium, and 200 mL of uranium tailings leachate is absorbed, and the treatment efficiency reaches 100 % within 15 min. PPP hydrogel can adapt to a wide range of pH conditions and exhibit excellent adsorption efficiency in the range of 3–9. At the same time, PPP hydrogel maintains an adsorption efficiency of over 85 % for 950 mg/L uranium solution. This lays the foundation for the practical application of PPP hydrogel. In addition, PPP hydrogel also exhibits good repeatability, after 7 cycles, the material still retains 95 % of its initial performance. The synergistic effect of various functional groups such as phosphate, hydroxyl, and ammonium in the material is the main mechanism of PPP's adsorption capacity for uranium. Furthermore, electrochemical adsorption method significantly enhances the adsorption performance of PPP hydrogel. 

Polyvinyl alcohol/phytic acid/phosphorylated chitosan hydrogel electrode highly efficient electroadsorption of low concentration uranium from uranium tailings leachate.

Annealing of the Raman defect peaks in He-implanted UO2

Combustion synthesis in uranyl nitrate-acetylacetone-2-methoxyethanol solutions was used to deposit thin UO2 films on aluminum substrates to investigate the irradiation-induced restructuring processes. Thermal analysis revealed that the combustion reactions in these solutions are initiated at ∼160 °C. The heat released during the process and the subsequent brief annealing at 400 °C allow the deposition of polycrystalline films with 5-10 nm UO2 grains. The use of multiple deposition cycles enables tuning of the film thicknesses in the 35-260 nm range. Irradiation with Ar2+ ions (1.7 MeV energy and a fluence of up to 1 × 1017 ions/cm2) is utilized to generate a uniform distribution of atomic displacements within the films. X-ray fluorescence (XRF) and alpha-particle emission spectroscopy showed that the films were stable under irradiation and did not undergo sputtering degradation. X-ray photoelectron spectroscopy (XPS) showed that the stoichiometry and uranium ionic concentrations remain stable during irradiation. The high-resolution electron microscopy imaging and electron diffraction analysis demonstrated that at the early stages of irradiation (below 1 × 1016 ion/cm2) UO2 films show complete amorphization and beam-induced densification (sintering), resulting in a pore-free disordered film. Prolonged irradiation (5 × 1016 ion/cm2) is shown to trigger a crystallization process at the surface of the films that moves toward the UO2/Al interface, converting the entire amorphous material into a highly crystalline film. This work reports on an entirely different radiation-induced restructuring of the nanoscale UO2 compared to the coarse-grained counterpart. The preparation of thin UO2 films deposited on Al substrates fills an area of national need within the stockpile stewardship program of the National Nuclear Security Administration and fundamental research with actinides. The method reported in this work produces pure, robust, and uniform thin-film actinide targets for nuclear science measurements.

Irradiation-Driven Restructuring of UO2 Thin Films: Amorphization and Crystallization.

The combination of electronic and nuclear energy deposition can have an effect on the defect production and evolution that is not simply the sum of both independent contributions. In nuclear reactors, the fuel is exposed to the irradiation of several particles such as the fission fragments (FF) whose electronic to nuclear energy loss ratio evolves along their path. To understand the impact of this energy-loss coupling on the fuel microstructure, single and dual-beam ion irradiations of uranium dioxide (UO2) were carried out. The damage evolution was investigated by Raman spectroscopy analysis correlated with Transmission Electronic Microscopy observations. It was found a significant acceleration effect of the electronic energy dissipation on the formation and evolution of defects induced by the nuclear energy deposition. The magnitude of this acceleration was shown to depend on both the electronic energy loss level and the microstructure at the moment when the coupling occurs. 

/pdf/irradiation-induced-microstructural-transformations-in-uo2-ump84e88.pdf

Irradiation-induced microstructural transformations in UO2 accelerated upon electronic energy deposition

The damage induced in uranium dioxide (UO2) during ion irradiation at low energy was studied by micro-Raman spectroscopy. Polycrystalline UO2 samples were irradiated by 0.9-MeV I, 2-MeV Au at 25 °C and by 4-MeV Kr ions at −160 °C in a wide range of fluences. In situ Raman measurements show similar spectra evolution no matter the ion beam used. The T2g band centred at 445 cm−1 related to the fluorine structure reveals a broadening with the irradiation damage increase. In addition, several bands ranging from 500 to 700 cm−1, which are attributed to sub- or sur-stoichiometric structural defects, are observed at the early stages of irradiation. Their intensities grow with the irradiation fluence to similar asymptotic relative values for all the irradiation conditions. The obtained Raman kinetics are compared with data from the literature on the microstructure evolution observed by Transmission Electronic Microscopy (TEM) and on the fraction of displaced atoms determined by Rutherford Backscattering Spectroscopy in channelling mode (RBS-C).

/pdf/effect-of-ballistic-damage-in-uo2-samples-under-ion-beam-qcwa542gz8.pdf

Effect of ballistic damage in UO2 samples under ion beam irradiations studied by in situ Raman spectroscopy

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Effect of ballistic damage in UO2 samples under ion beam irradiations studied by in situ Raman spectroscopy