•Study of cerium dioxide nano-crystallites under intense laser irradiation at 1064.5.•Smaller CeO2 grains absorb laser radiation more promptly than larger grains do.•The solidification temperature of ...nanometric crystallites is about 3012 K.•Lower solidification temperatures are observed in micrometric crystallites.
The absorption of a 1064.5 nm Nd:YAG laser line and the consequent heating up to melting of nano-crystalline CeO2 has been studied in compressed air. The behaviour of crystallites ranging from 3.2 nm to 11 nm has been investigated and compared with micro-crystalline CeO2.
A transparent / opaque transition occurs in CeO2 at lower heating laser power density in smaller crystallite sizes. Also the melting behaviour is strongly dependent on the crystallite size. If one manages to directly melt nano-crystals by optimising the heating conditions under a suited oxygen overpressure, the resulting melt has been observed to solidify at the highest temperature so far reported for CeO2 (3012 K), about 250 K higher than the temperature most commonly registered under oxidising conditions (2750 K). This is attributable to the large specific surface of nano-crystals, resulting, under a high oxygen potential, in their higher stability with respect to oxygen losses.
Some example applications are presented, in which the peculiar Raman fingerprint of PuO2 can be used for the detection of crystalline Pu4+ with cubic symmetry in an oxide environment in various host ...materials, like mixed oxide fuels, inert matrices and corium sub-systems. The PuO2 Raman fingerprint was previously observed to consist of one main T2g vibrational mode at 478 cm−1 and two crystal electric field transition lines at 2130 cm−1 and 2610 cm−1. This particular use of Raman spectroscopy is promising for applications in nuclear waste management, safety and safeguard.
•The Raman fingerprint of PuO2 is presented and discussed.•It can be used for the detection of PuO2 in various environments.•Several applications are identified.
A selected composition of the initial fuel of the Molten Salt Fast Reactor (MSFR) is assessed by differential scanning calorimetry (DSC) for melting point determination and by Knudsen effusion mass ...spectrometry (KEMS) for vaporization behaviour. Partial vapour pressures and thermodynamic activities of the MSFR fuel mixture are discussed indicating departures from ideal behaviour, and further interpreted by phase equilibria calculations. The boiling point of the mixture is obtained extrapolating vapour pressure experimental results. New results on the vaporization behaviour of pure uranium tetrafluoride are presented, together with the ionization potentials of UF4 by electron impact.
•First experimental results for a selected mixture for the European Molten Salt Fast Reactor.•Boiling point determined by extrapolation of experimental results.•Results compared with calculated values and ideal behaviour.•Novel measurements for the vapour pressure of pure UF4.
Oxygen stoichiometry in (U,Pu)O2-x nuclear fuels, while of prime interest, is still difficult to be determined at the micrometric scale. It has been recently evidenced that Raman microscopy is a ...promising tool to characterize (U,Pu)O2-x samples at a microscopic scale by probing the oxygen sublattice. Its use for determining the local O/M ratio was studied in this work on mixed oxide samples mostly containing 239Pu and natural uranium, in addition to minor traces of other isotopes, including decay products and 241Am. The first step was to dissociate the influence of the Pu/(U+Pu+Am) content, self-irradiation and O/M ratio on Raman spectra and especially on the main Raman band position in fluorite structure, the T2g. In this aim, freshly annealed and aged U1-yPuyO2-x samples, with 0.19 < y < 0.46 and different O/M ratios, were analyzed by XRD and Raman spectroscopy. After figuring out that self-irradiation alone had no significant impact on the T2g position, two mathematical relations were determined, linking the T2g position to the Pu/(U+Pu+Am) content and to the lattice parameter. Finally, the oxygen hypostoichiometry direct impact on (U,Pu)O2-x Raman spectra was determined for the first time. In agreement with past results observed in CeO2-x, a O/M ratio decrease induces a T2g shift towards lower frequencies in (U,Pu)O2-x. Combining the whole results, an equation allowing to determine the T2g position according to the O/M ratio and Pu/(U+Pu+Am) content was established. Raman spectrometer associated to a confocal optical microscope can then be used to determine locally (≈1 µm) either the Pu/(U+Pu+Am) content or the O/M ratio. This study proves the relevance of such method to characterize the fuel pellets in an industrial way.
The characterization of (U,Pu)O2-x ceramics can be performed by Raman spectroscopy. This analytical technique probes their anionic sublattice and is currently used to determine the Pu/(U+Pu) content, the oxidation degree and the defect points induced by self-irradiation. Thanks to the work presented in this paper, the characterization can be improved by determining the lattice parameter, corrected from the self-irradiation effect for aged samples. Therefore, the oxygen stoichiometry, can be obtained to the micron scale. Display omitted
A commercial PWR fuel sample with a local burn-up of about 240MWd/kgHM was annealed in a Knudsen cell mass spectrometer system with a heating rate of 10K/min up to 2750K at which temperature the ...sample was completely vaporized. The release of fission gases and fission products was studied as a function of temperature. In one of the runs the heating was interrupted successively at 900, 1500 and 1860K and at each step a small fragment of the sample was examined by SEM and analysed by energy dispersive electron probe microanalysis. The release behaviour of volatile, gaseous and other less volatile fission products is presented and analysed with the EFFUS program and related to the structural changes of the fuel.
Vapour pressure measurements were performed on a (Pu
0.756Am
0.244)O
2−
x
sample using Knudsen cell mass spectrometry. The total and partial vapour pressures of the gaseous species have been measured ...in the temperature range from 2000 to 2300
K. The evolution of the plutonium and americium bearing species was also determined as a function of time, in order to evaluate the congruent vapour composition. At constant temperature, the energy of ionising electrons was stepwise increased and the ionisation efficiency curves were recorded. The results were combined with the assessment of the Pu–Am–O system using the CALPHAD method. To obtain the model of this ternary system, the data on the Pu–Am and Am–O binaries have been evaluated and the optimised phase diagrams are presented. A consistent thermodynamic description of the ternary was obtained which allows the calculation of the ternary phase diagram, the oxygen potential for (Pu,
Am)O
2±
x
and the equilibrium partial vapour pressures.
This study presents an original approach of oxygen stoichiometry determination during high temperature (>2000K) measurements of vapour pressure using the Knudsen effusion mass spectrometry technique. ...The method has been developed taking into account the vapour pressure measurements of series of (U1-x,Pux)O2-δ samples with x(Pu)=0.25, 0.5, 0.75 together with pure UO2-δ and PuO2-δ end-members coupled with equilibrium calculations based on thermodynamic assessment of the U–Pu–O system. The presented method consists of two steps; in the first step the oxygen potential of the oxide phase is determined based on the measured partial vapour pressures of UO(g), UO2(g), PuO(g) and PuO2(g) gaseous species and during the second step the thus determined oxygen potential is linked with the matching oxygen stoichiometry of the sample. From the obtained results it has been demonstrated that it is possible to accurately estimate the oxygen stoichiometry of the mixed oxide fuel samples knowing the description of the oxygen potential of the corresponding end-members only.
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