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AbstractThe current research focuses on laser melting and successive analysis of laboratory-scale uranium dioxide nuclear fuel samples in direct contact with Zircaloy-4 cladding. The goal was to characterize the melted and refrozen interfaces, in particular, observing local changes of the melting point and interdiffusion of fuel and cladding materials under inert gas (Ar), in the presence of hydrogen (Ar + 6% H2) or in air. Results obtained by laser heating UO2pellets clad in a Zircaloy ring were interpreted in light of reference tests performed on pellets in which UO2and zirconium were blended in a series of given compositions. The sample composition was analyzed by scanning electron microscopy to verify the occurrence of diffusion and segregation phenomena during the laser-heating cycles. Laser-melting experiments were performed on pellets of uranium dioxide clad in Zircaloy-4 rings to simulate the configuration of a light water reactor fuel rod. Under inert gas, the material interdiffusion resulted in consistent melting point depression (of up to 200 K below the melting point of pure UO2) at the interface between the fuel and the cladding. Experiments carried out in the presence of H2displayed a more limited effect on the melting temperature, but they resulted in a remarkable embrittlement of the whole structure, with large fragmentation of the Zircaloy cladding. This was probably due to the formation of brittle and highly volatile Zr hydrides. The observed melting point decrease was even more pronounced (up to over 400 K) under air in uranium-rich samples, due to the change in the stoichiometry of UO2in UO2+x.

The Raman features of neptunium dioxide (NpO2) and uranium–neptunium mixed oxides were investigated. Raman spectra of 16O and 17O-NpO2enriched sample were measured with excitation energy range of Ei= 1.91–2.54 eV. Raman spectra of both NpO2samples and the uranium–neptunium rich mixed oxides show an asymmetry at low energies of the Γ25′(T2g) mode. The asymmetry was evidenced to originate from an electronic band at 431 cm–1which originates from crystal field splitting. This assignment is in good agreement with theoretical models for NpO2with the hypothesis that the electron–phonon bound state is decoupled at high temperatures. The asymmetry of the Γ25′mode can be considered as a fingerprint of the NpO2lattice.

AbstractIn the present study, the suitability of Raman spectroscopy for nuclear forensics is proven by a systematic investigation of laboratory-prepared synthetic and industrial uranium ore concentrate (UOC) samples, of different origin and with wide range of impurity content. Besides the identification of the compound type, the identification of process-related impurities observed in these samples is reported for the first time. Both handheld and laboratory Raman spectrometers were used in the measurements. The ability to measure different UOCs and the sufficient sensitivity of the handheld device demonstrate its usefulness in nuclear safeguards and nuclear security applications. Even more importantly in such a context, the likely identity of process-related impurities could be verified by Raman Spectroscopy in the present work, allowing to draw conclusions on the process history of the sample.

This study is focused on the behavior of ternary SiO2-Na2O-B2O3borosilicate glasses at temperatures between 298 and 1800 K. Unpolarized Raman spectra were measured up to high temperature. SiO2-Na2O-B2O3glass samples were prepared with different values of the ratio R = [Na2O]/[B2O3], while the ratio K = [SiO2]/[B2O3] was kept constant and equal to 2.12. Spectra were measured at room temperature in samples with 0.43 ≤ R ≤ 1.68, and the effect of the modifier content was clearly observed in these glasses, only in partial agreement with previous literature results. In particular, the formation in the glass of sodium-danburite units Na2O·B2O3·2SiO2was postulated. This feature led to a new assessment of R*, the critical value of R above which every new alkali atom added to the system breaks a Fo-O-Fo (Fo = glass former) bridge causing depolymerization of the glass. A revised formula is proposed to obtain the value of R* as a function of K.

Abstract Due to its complex phase diagram and the employment of UO2 as a nuclear fuel, the binary system U–O is of great interest both scientific and technological. Numerous experimental and theoretical studies have been carried out in the last 45 years in order to determine the properties of this system, leading to a precise definition of a considerable part of the state diagram in the region ranging from pure uranium to stoichiometric UO2, and at temperatures lower than 1500 K, up to the oxide U4O9. However, due to the poor chemical stability of O–U compounds with high oxygen content at high temperature (O/U > 2, T > 2000 K), an important part of the phase diagram still lacks experimental data. In this work measurements are presented on the melting transition of the stoichiometric and hyperstoichiometric dioxide UO2+x up to x=0.21, and on the melting point of the higher oxide U3O8. These measurements were performed under buffer gas pressures varying between 10 and 250 MPa, using a method based on subsecond laser heating developed to overcome experimental difficulties encountered by previous researchers.

This work presents a study of the melting behavior of the UO2+xsystem. Self-crucible pulsed-laser melting at high pressure is realized, further improving the recently developed experimental approach. Thermograms of the heated surface of the sample, recorded by a fast pyrometer, show thermal arrests and/or points of inflection corresponding to the phase transformations. A further probe laser beam is focused onto and reflected by the sample surface, and the reflected light intensity is used to detect the exact instants at which melting/freezing transitions occur. Furthermore, a model was developed describing the complex process of laser-induced melting in a system where a solubility gap accompanies the formation of liquid, and a corresponding numerical simulation of the melting-freezing process was performed in order to depict the heat and matter transport phenomena involved. Finally, liquidusand soliduslines are proposed for the system UO2+xin the range 0≤x≤0.21.

The melting point of uranium dioxide was measured as a function of isostatic pressure in an autoclave filled with helium up to 0.25 GPa. Containerless laser surface-heating was applied to measure the melting point by thermal arrest measurements during sample cooling. Precise control of the cooling rate, aimed at a precise determination of the freezing plateau, was obtained with a two-pulse method, where two Nd:YAG high power laser pulses are mixed within the same optical fiber and then focused on the sample surface. Calibration of the power of the two pulses permits the measurement of the melting point with very low uncertainty (0.5%). The measured melting slope of UO2is 170 and 200 K⋅GPa−1, a value which is markedly higher than that predicted by the Clausius–Clapeyron equation, which expresses this slope in terms of the enthalpy of fusion and the melting expansion. Possible reasons for this discrepancy are discussed.

The ternary oxide ceramic system UO2-ZrO2-FeO is a refractory system that is of great relevance to the nuclear industry as it represents one of the main systems resulting from the interaction of the Zircaloy cladding, the UO2 fuel and the structural elements of a nuclear reactor. It is particularly the high temperature properties that require investigation; that is, when substantial overheating of the nuclear core occurs and interactions can lead to its degradation, melting and result in a severe nuclear accident. There has been much work on the UO2-ZrO2 system and also on the ternary system with FeO but there is still a need to examine 2 further aspects; firstly the effect of sub-oxidized systems, the UO2-Zr and FeO-Zr systems, and secondly the effect of Fe/Zr or Fe/U ratios on the melting point of the U-Zr-Fe oxide system. Samples of UO2-Zr and UO2-ZrO2-FeO were fabricated at ITU and then characterized by optical microscopy (OM) and X-ray diffraction to determine the ceramic's structure and verify the composition. Thereafter the samples are to be melted by laser flash heating and their liquidus and solidus temperatures determined by pyrometry. This programme is currently ongoing. The frozen samples, after testing, were then sectioned, polished and the molten zone micro-analytically examined by OM & SEM-EDS in order to determine its structure and composition and to compare with the existing phase diagrams. Examples of results from these systems will be given. Finally, a reacted Zr-FeO thermite mixture was examined, which had been used to generate high temperatures during tests of reactor melt-concrete interactions. The aim was to assess the reaction and estimate the heat generation from this novel technique. These results allow verification or improvement of the phase diagram and are of primary importance as input to models used to predict materials interactions in a severe nuclear accident.