Materials, processing, and characterization techniques for future nuclear technologies

Resume : New fuel cycle strategies are calling for advanced nuclear fuel concepts. This especially applies for the burning of minor actinides in fast reactor cycles or in accelerator driven systems. High minor actinides contents imply the production in hot cells. In such environment a simplified production method, with lower use of maintenance intensive mechanical devices, and with less powder contamination, becomes essential. Furthermore, the fuel usage up to high burn up is a decisive advantage to reduce the cycles. For both aspects Sphere-pac fuel is promising. It consists of nuclear fuel beads which are produced by a powder-less wet route and directly filled into pins, avoiding any pressing or grinding. The interspaces between the spheres will allow the accommodation of swelling, allowing a higher burn-up. New production methods are addressed. The internal gelation was over decades the most employed technique to produce the fuel beads. This technique has recently been improved by applying microwave technology on one hand, and instantaneous mixing on the other. Both promise an important simplification of the production and potentially an improved quality of the product. Furthermore the fuel performance known from past programs is shortly addressed.

Resume : There is a steadily growing interest in nanostructured coatings, layers and thin films consisting of increasingly complex compounds for their unique structural and functional properties that can be applied to future nuclear technologies. If composition and strain engineering at the nanometer scale should be further adopted in the manufacturing processes of future materials for nuclear applications, there is an obvious need for improvements in the control of these methods and processes and the evolution of these systems under irradiation: the characterization of these effects in thin films of metals, carbides and oxides, possessing various types of nanoscale or mesoscale organization, and in the subsurface characteristics of bulk materials irradiated by ion beams is then a subject needing attention. Glancing incidence diffraction is a sound and non-destructive technique that provides a straightforward way for understanding these systems, as it provides quantitative information about their structures, their microstructures that can be related to the technologically relevant properties. Standards for characterization of these types of materials are not well established yet. In this lecture we discuss the methods and present case studies to illustrate the importance of the specific corrections related to the grazing incidence setup. These corrections are actually implemented in a conventional Rietveld refinement code, XND, freely distributed.

Resume : The influence of gas-static processing on optical, structural and electrophysical properties of Au-CdZnTe-Au structures of X-ray and gamma-ray detectors was investigated. The processing was provided in laboratory-scale set GAUS-4/2000-35 according to regime (0.32 ± 0.02 GPa, ~ 170 ºC, 2h). Influence of mentioned processing on photoluminescence, Raman scattering, electric resistance, I-V characteristics and spectrometric parameters of Au-CdZnTe-Au structures was determined. The physical mechanisms of gas-static processing related changes of structural and functional properties were analyzed. It was obtained, gas-static processing of Au-CdZnTe-Au correspondingly with mentioned regime leads to significant increase of electric resistance and intensity of photoelectric absorption peak when registering X- and gamma-radiation with the energies near 32.19 keV. Also, Raman and photoluminescence data indicates formation of surface oxides TeOx and compensation of Cd vacancies by Au atoms. The assumption that under discussed processing two rival processes related with modification of Au-CdZnTe junction due to influence of increased temperature ~ 170ºC and pressure ~ 0.3 GPa occur, was suggested. In the first case, the formation of TeO2 oxide (additional electro resistance) on the contact has place, in the second case – destruction of the films of surface oxides and absorbed gases. Probably, the first process is dominant which was proved by Raman and photoluminescence measurements.

Resume : Hard carbon thin films were synthesized on Si (100) and fused silica substrates by the Pulsed Laser Deposition technique in vacuum or methane ambient in view of studying their suitability for nuclear reactor applications requiring high mechanical resistance to shock and low friction coefficient at high temperatures. The deposited layers were investigated in terms of surface morphology by AFM and SEM, crystalline status by XRD, packing and density measurements by XRR and bonding architecture by Raman spectroscopy and XPS. The films adherence was determined by pull-out tests, the surface energy was inferred from contact angle measurements, hardness (H) end elastic modulus (E) were assessed by nanoindentation and wear resistance by nanotribology. The deposited films had a high tendency to exfoliate on some substrates, requiring special op-timizations of the deposition process in terms of laser fluence and substrate temperature for stabilization. The deposition gas pressure played a crucial role in films thickness, structure and mechanical properties. All films were smooth, amorphous and composed of a mixture of sp3-sp2 carbon, with sp3 content ranging between 50-90%. The load-displacement curves resulted from nanoindentation evidenced that the approximately equal ratios of sp3-sp2 in films content in-duced a 6 times higher hardness and 2.5 times higher elasticity as compared to structures composed mainly of sp3 bonds (>85%).

Resume : The monazite-type ceramics (LnPO4) are considered in nuclear waste management as promising materials for immobilization of radionuclides such as minor actinides (Am, Cm and Th) and Pu. [1] This is because natural monazite can contain significant amounts of radionuclides and stay intact without damage to its crystalline structure for even billions of years. In order to understand better such a self-healing phenomenon, we perform systematic ab initio studies of various properties of these materials. This includes: structure characterization [2], thermodynamic properties of pure phases and solid solutions, elastic properties and simulations of radiation damage processes. In order to make our simulation highly reliable we tested several ab initio computational approaches and found that the best feasible method for calculations of monazite-type ceramics is DFT U with PBEsol as exchange-correlation functional and Hubbard U parameter derived ab initio for each lathanide and crystalline structure [2,3]. We confront our simulation results with available experimental data. Such a combined experimental and theoretical approach allows us to get superior, atomic-scale understanding of these materials. [1] H. Schlenz, J. Heuser, A. Neumann et al. ?Monazite as a suitable waste form?, Z. Kristallogr. 228, 113 (2013). [2] A. Blanca-Romero, P. M. Kowalski, G. Beridze et al. ?Performance of DFT U method for prediction of structural and thermodynamic parameters of monazite-type ceramics?, J. Comp. Chem. 35, 1339 (2014). [3] M. Cococcioni & S. de Gironcoli ?Linear response approach to the calculation of the effective interaction parameters in the LDA U method?, Physical Review B 71, 035105 (2005).

Resume : Supercritical water (SCW) corrosion studies have been carried out on two candidate materials for structural components of the European Generation IV SCW reactor concept: austenitic steels 1.4970 and 316NG. Specimens were exposed to SCW at 550°C, 25MPa and 2ppm O2 for up to 1200h and the corrosion behavior was examined by conventional weight gain/loss measurements, scanning electron microscopy (SEM), energy-dispersive x-rays analysis (EDX) and x-ray diffraction (XRD). SEM-EDX analysis of the cross sections revealed that a corrosion layer with characteristic bilayer structure was formed at the surface. In both cases, the structure of the external oxide layer consisted mainly of Fe2O3, as measured by XRD. The corrosion rate, i.e. the weight change per unit area, was slightly higher in 1.4970 than in 316NG (0.93 vs. 0.78 mg/cm-2), in agreement with the thicker oxide bilayer measured for 1.4970 (8.5 µm vs. 6.5 µm). Significantly larger amount of inclusions was observed in the bulk of 1.4970 as compared to 316NG. Most of the particles which segregated during the exposure to SCW consisted of Ti-N or Ti-Mo-C aggregates. Furthermore, smaller Cr-rich particles were found in the bulk and at grain boundaries of 1.4970 samples. On the other hand, very few round particles (200 nm) rich in Cr-C-O were observed close to grain boundaries of 316NG samples. Such lower Cr segregation and the lower corrosion rate suggest a higher corrosion resistance of 316NG compared to 1.4970 at 550°C in SCW

Resume : ZrC possesses both ceramic (high hardness, high melting point, good thermochemical stability) and metallic characteristics (high electrical conductivity, high infrared reflectivity). It has been suggested that it might be a superior choice for nuclear fuel encapsulation than SiC, which is very brittle. There is limited data about ion radiation effects in this material, especially in thin film forms, which are required for such applications. High crystalline quality ZrC thin films were grown on Si substrates by the Pulsed Laser Deposition technique. By changing the substrate temperature and ambient gas nature and pressure during deposition, films having a wide range of crystal grain sizes, mass densities, optical reflectivity, electrical conductivity, mechanical properties and C/Zr values were obtained. The films were ion-irradiated at IN2P3-IPNL at room temperature. In order to maximize the ballistic term of the damage, 800 keV Ar ions at doses from 1×10E14 ions/cm2 to 2×10E15 ions/cm2 were used. The damage was rather homogeneous in the deposits thickness and most of the implanted ions were in the Si substrate. The maximum damages were estimated to a few dpas (displacements per atoms) for the more irradiated samples. After irradiation, the structure and properties of the films were measured again and compared with the as-deposited ones. It appeared that doses of 10E14 ions/cm2 induced rather small changes in the lattice parameter and properties, while after doses of the order of 10E15 ions/cm2, significant changes of the structure and properties were observed. However, the films retained a high hardness of around 20 GPa.

Resume : TiC films (400-500 nm thickness) were synthesized on Si substrates heated to 500°C by pulsed laser deposition using a KrF excimer laser source. The experiments were conducted in methane or nitrogen atmosphere at a pressure of 10-4 mbar. The films’ morphology was inves-tigated by scanning electron microscopy. Films thickness, profiles and surface features were extracted from confocal microscopy image processing. All films had very smooth surfaces and were very dense, as also indicated by the simulations results of X-ray reflectivity curves. The films’ crystalline status was investigated by grazing incidence X-ray diffraction. The use of a high laser fluence (~6 J/cm2) during deposition resulted in the synthesis of films with small crystallite sizes, which influenced beneficially the mechanical properties of the films. Hardness and elasticity of the films have been studied by nanoindentation. The variable pa-rameter was set to be the indentation depth, at no more than 25% of the film thickness. Lower indentation depths resulted in inconclusive results. The hardest coatings were obtained in ni-trogen atmosphere (Hit~35 GPa). Samples synthesized in methane ambient had hardness val-ues of ~ 30 GPa. The samples were quite rigid, with Young’s modulus of ~300 GPa for films deposited in nitrogen and ~250 GPa for samples produced in methane atmosphere.

Resume : Advanced titanium carbide (TiCx) based cermets with Cr binder are potentially corrosion resistant composites with wide range of applications, such as fuel cladding or the most exposed to erosion parts in nuclear engineering, due to relatively high melting point as well as good wear and erosion resistance. Therefore, it can be expected that thermal conductivity and thermal shock resistance will be improved, comparing to pure monolithic TiCx, at the cost of reduced refractoriness. In this study, the spark plasma sintering technique (SPS) was used in order to fabricate TiC0.75-Cr cermets using stoichiometric TiC1.0, Ti and Cr powders mixture. The investigations confirmed that high relative density cermets containing, according to XRD results, just sub-stoichiometric TiCx and Cr, were obtained. The effect of SPS sintering conditions and component ratio on phase composition and microstructure as well as the results of Vickers hardness and wear resistance will be also presented.

Resume : Although uranium mononitride (UN) is considered as a possible fuel material for the future Generation IV nuclear reactors, which possesses several advantages as compared to the commonly used uranium oxide fuels, presence of oxygen impurities in nitrides and carbides unavoidably leads to unwanted contamination and further degradation of nuclear fuel. To understand this problem atomistically, we performed first principles calculations of the oxygen interaction with the UN surfaces. The results of DFT supercell calculations of oxygen behavior upon the UN (001) and (110) surfaces as well as at the tilt grain boundary will be presented. Oxygen adsorption, migration, incorporation into the surface N vacancies on (001) and (110) surfaces was modeled using 2D slabs of different thicknesses and supercell sizes. The temperature dependences of the N vacancy formation energies and oxygen incorporation energies are calculated. We demonstrate that O atoms easily penetrate into UN surfaces and grain boundaries containing N vacancies due to negative incorporation energies and a small energy barrier. The Gibbs free energies of N vacancy formation and O atom incorporation therein at the two surfaces and tilt grain boundaries are compared. It is also shown that adsorbed oxygen atoms are highly mobile which, combined with easy incorporation into surface N vacancies, explains efficient oxidation of UN surfaces.