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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.

Cesium and iodine, which are formed during a fission process in a nuclear reactor, are considered as major fission products responsible for the environmental burden in case of a nuclear accident. From the safety point of view, it is thus important to understand their release mechanism when overheating of the reactor core occurs. This work presents an experimental investigation of the behaviour of caesium iodide and caesium fluoride in fluoride based molten salt reactor fuel during high temperature events. It has been demonstrated that CsF will be retained in the fuel salt and thus its volatility will be significantly reduced, while CsI will not dissolve in the fluoride-based fuel matrix and will thus remain more volatile. The influence of the presence of CsI and CsF on the melting behaviour of the fuel has been investigated using calorimetry, revealing their negligible effects.

AmPO4 was prepared by a solid-state reaction method, and its crystal structure at room temperature was solved by powder X-ray diffraction combined with Rietveld refinement. The purity of the monazi...

International audience; 238Pu-doped UO2 was characterised periodically to investigate the long-term effects of α-self-irradiation on spent nuclear fuel. Samples of two compositions (1.25 and 5% wt 238Pu), were studied by means of X-ray diffraction (XRD), Raman spectroscopy, and transmission electron microscopy (TEM). The cumulated dose received by the samples was measured in displacements per atom (dpa) to allow comparison between the two samples and to the literature values. XRD characterization showed that the lattice swelling approached saturation at 0.3% of the lattice parameter of the cubic cell at 0.4 dpa damage accumulation. Microstrain decreased for the whole duration of the study, as a result of point defect recombination into dislocation loops that were observed by TEM for dpa as low as 0.072. Raman highlighted the broadening of the T2g band and the increase and broadening of the defect triplet band. Both batches of samples followed the same trend if plotted against dpa, proving that for these dopant concentrations the self-irradiation effects do not depend on the rate of radiation damage creation in the matrix.

AmPO

A study of neptunium (Np) chemistry in the complex oxo-selenium system has been performed. Hereby, two sets of precipitation experiments were conducted, investigating the influence of the initial oxidation state of selenium using SeIVO2 and H2SeVIO4 with NpV in alkali nitrate solution, keeping the ratio of Np/Se constant. Surprising results were observed. Five novel neptunium and selenium bearing compounds have been obtained by slow evaporation from aqueous solution. The novel NpIV phase K4-x[Np(SeO3)4-x(HSeO3)x]·(H2O)1.5 (1) crystallizes in green-colored, plate-shaped crystals and was obtained by adding SeO2 and ANO3 to a NpV stock solution. Single-crystal X-ray diffraction reveals one-dimensional chain structures composed of square antiprismatic NpO8 polyhedra linked via four trigonal pyramidal SeO3 and HSeO3 units. Raman spectral analysis supports the presence of both selenite and hydroselenite due to the presence of corresponding modes within the spectra. The addition of selenic acid to a NpV stock so...

Production of actinide oxide powder via dry thermal decomposition of corresponding oxalates is currently carried out on the industrial scale at temperatures exceeding 500 °C. Although it is simple, this method presents some disadvantages such as high decomposition temperature with a direct effect on the surface area, pre-organised morphology of the nanoparticles affecting the sintering behaviour, etc. We have recently proposed the decomposition of AnIV-oxalates under hot compressed water conditions as a straightforward way to produce reactive actinide oxide nanocrystals. This method could be easily applied at low temperatures (95–250 °C) in order to generate highly crystalline nano-AnO2. We present here the formation conditions of AnO2 (An = Th, U, Np, and Pu) and some associated solid solutions, their stability, and grain growth during thermal treatment. The involvement of water molecules in the mechanism of the oxalate decomposition under the hot compressed water conditions has been demonstrated by an isotopic exchange reaction during the thermal treatment of the hydrated oxalate in H2[17O] through MAS-NMR and Raman techniques.

In the present studies, the thermal behaviour of NaUO2BO3 has been investigated. This compound is a potential product of interaction between the coolant (Na), control rods (B4C), and the oxide fuel, which could form under accidental conditions in sodium-cooled fast reactors. The thermal expansion, the heat capacity, and thermal diffusivity of NaUO2BO3 have been measured. The thermal conductivity of the material is derived from these results and presented here for the first time.

A LabVIEW®-based software for the control of the fully automated multi-sequential flow injection analysis Lab-on-Valve (MSFIA-LOV) platform AutoRAD performing radiochemical analysis is described. The analytical platform interfaces an Arduino®-based device triggering multiple detectors providing a flexible and fit for purpose choice of detection systems. The different analytical devices are interfaced to the PC running LabVIEW®VI software using USB and RS232 interfaces, both for sending commands and receiving confirmation or error responses. The AUTORAD platform has been successfully applied for the chemical separation and determination of Sr, an important fission product pertinent to nuclear waste.

Aerosols were produced from a MOX (Mixed Oxide of uranium and plutonium) sample by laser heating up to 4000 K under atmospheric conditions. They were collected with a cascade impactor as function of their aerodynamic size, and studied by SEM, EDS, TEM, EELS, SAED and ICPMS. The uranium and plutonium concentrations were related to the aerosol morphology, size and formation temperature. A clear decrease of the Plutonium concentration was observed as function of the Aerodynamic Equivalent Diameter (AED). Some bigger aerosols enriched in Plutonium were found which originated from solidification of ejected droplets of the liquid phase at the surface of the sample.

The chemistry of cesium and iodine is of main importance to quantify the radioactive release in case of a nuclear reactor accident, or sabotage involving irradiated nuclear materials. We studied the interaction of CsI with different metallic fission products such as Mo and Ru. These elements can be released from nuclear fuel when exposed to oxidising conditions, as in the case of contact of overheated nuclear fuel with air (e.g. in a spent fuel cask sabotage, uncovering of a spent fuel pond, or air ingress accidents). Experiments were performed by vaporizing mixtures of the compounds in air, and analysing the produced aerosols in view of a possible gas–gas and gas–aerosol reactions between the compounds. These results were compared with the gaseous species predicted by thermochemical equilibrium calculations and experimental equilibrium vaporization tests using Knudsen Effusion Mass Spectrometry.

In this work, the dissolution of helium in La-doped UO2 samples was studied by helium infusion in an autoclave followed by thermal desorption (laser heating) coupled to mass spectrometer systems for the determination of helium release rate and the total helium quantity. Lanthanum was chosen as a dopant in UO2 to study the effect of hypo-stoichiometry on helium solubility together with the impurity effect. Comparison of the dissolved He quantity from the samples with different La content showed that the dissolved He quantity slightly increases with increasing oxygen vacancy concentration in the samples.

An approach for Raman measurements of highly radioactive samples is presented here. The innovative part of this approach lies in the fact that no single part of the Raman equipment is in direct contact with the radioactive sample, as the sample is sealed in an alpha-tight capsule. Raman analysis is effectively performed through the optical-grade quartz window closing the capsule. This allows performing micro-Raman measurements on radioactive samples with no limitations on the laser source wavelength, polarisation mode, spectrometer mode and microscope mode (provided the focal length of the microscope objective is greater than the thickness of the quartz window and with sub mg samples). Some example results are shown and discussed. In particular, some spectral features of americium-containing oxide nuclear fuel specimens are presented. Raman spectra clearly reveal in these specimens the presence of abundant oxygen defects induced in the fcc fluorite lattice by trivalent americium. In order to complete the analysis the Raman spectrum of pure americium dioxide was also measured with a lower energy excitation source compared with previous research. The current results seem to be consistent with the possible occurrence of a photolysis process induced by the Raman laser, resulting in the formation of hyperstoichiometric americium sesquioxide Am2O3 + z. Such a photolytic process is deemed to be unavoidable when visible lasers are used as excitation sources for the Raman analysis of americium dioxide. © 2015 The Authors Journal of Raman Spectroscopy Published by John Wiley & Sons, Ltd.

Actinide-based nuclear ceramics, oxides particularly, are not only used as fuel in nuclear power reactors (uranium and plutonium) but are also used/envisaged as materials for electrical power sources in space probes (plutonium or americium). These actinides are all alpha-emitters, some having rather short half-lives. As a result of their strong alpha-activity, the actinide-based materials cumulate radiation damage and radiogenic helium. The stability of such materials needs to be assessed and understood for predicting the long-term stability of not only spent fuel in storage/disposal conditions but also of electrical power sources to be used in space probes. This paper describes the specific transmission electron microscope microstructure analyses of aged 238PuO2, 238Pu-doped UO2 (to simulate aged spent nuclear fuel), and of 241AmO2 samples (candidate electrical power source) and makes the correlation of the observed defects with other properties like helium thermal desorption and lattice parameter. It is shown that these fluorite structured materials resist to high alpha-damage levels and can accommodate large quantities of helium.

Knudsen effusion mass spectrometry (KEMS) has been used to investigate the vapour pressure over the molten LiF–ThF4 salt and determine the thermodynamic activity of LiF and ThF4 in the liquid solution. As part of the study, the vaporization of pure LiF and pure ThF4 was examined and the results were compared with the literature values finding a good agreement. Next, the vapour pressure of the LixTh1−xF4−3x liquid solution was investigated by measuring four samples having different compositions (XLiF ∼ 0.2, 0.4, 0.6, 0.8 mol%). In order to determine the thermodynamic activities, the vapour pressure of LiF and ThF4 species over the liquid solution, as calculated from our results, were compared with the vapour pressure over the pure LiF(l) and pure ThF4(l) systems. A strong deviation from the Raoult's law was observed, more evident in case of LiF species, in agreement with the predictions by our thermodynamic model.

A novel approach for the Raman measurement of nuclear materials is reported in this paper. It consists of the enclosure of the radioactive sample in a tight capsule that isolates the material from the atmosphere. The capsule can optionally be filled with a chosen gas pressurized up to 20 bars. The micro-Raman measurement is performed through an optical-grade quartz window. This technique permits accurate Raman measurements with no need for the spectrometer to be enclosed in an alpha-tight containment. It therefore allows the use of all options of the Raman spectrometer, like multi-wavelength laser excitation, different polarizations, and single or triple spectrometer modes. Some examples of measurements are shown and discussed. First, some spectral features of a highly radioactive americium oxide sample (AmO2) are presented. Then, we report the Raman spectra of neptunium oxide (NpO2) samples, the interpretation of which is greatly improved by employing three different excitation wavelengths, 17O doping, and a triple mode configuration to measure the anti-stokes Raman lines. This last feature also allows the estimation of the sample surface temperature. Finally, data that were measured on a sample from Chernobyl lava, where phases are identified by Raman mapping, are shown.

The formation of the high burnup structure (HBS) is possibly the most significant example of the restructuring processes affecting commercial nuclear fuel in-pile. The HBS forms at the relatively cold outer rim of the fuel pellet, where the local burnup is 2–3 times higher than the average pellet burnup, under the combined effects of irradiation and thermo-mechanical conditions determined by the power regime and the fuel rod configuration. The main features of the transformation are the subdivision of the original fuel grains into new sub-micron grains, the relocation of the fission gas into newly formed intergranular pores, and the absence of large concentrations of extended defects in the fuel matrix inside the subdivided grains. The characterization of the newly formed structure and its impact on thermo-physical or mechanical properties is a key requirement to ensure that high burnup fuel operates within the safety margins. This paper presents a synthesis of the main findings from extensive studies performed at JRC-Karlsruhe during the last 25 years to determine properties and behaviour of the HBS. In particular, microstructural features, thermal transport, fission gas behaviour, and thermo-mechanical properties of the HBS will be discussed. The main conclusion of the experimental studies is that the HBS does not compromise the safety of nuclear fuel during normal operations.

Significant amounts of spent uranium dioxide nuclear fuel are accumulating worldwide from decades of commercial nuclear power production. While such spent fuel is intended to be reprocessed or disposed in geologic repositories, out-of-reactor radiation damage from alpha decay can be detrimental to its structural stability. Here we report on an experimental study in which radiation damage in plutonium dioxide, uranium dioxide samples doped with short-lived alpha-emitters and urano-thorianite minerals have been characterized by XRD, transmission electron microscopy, thermal desorption spectrometry and hardness measurements to assess the long-term stability of spent nuclear fuel to substantial alpha-decay doses. Defect accumulation is predicted to result in swelling of the atomic structure and decrease in fracture toughness; whereas, the accumulation of helium will produce bubbles that result in much larger gaseous-induced swelling that substantially increases the stresses in the constrained spent fuel. Based on these results, the radiation-ageing of highly-aged spent nuclear fuel over more than 10,000years is predicted.

The vaporisation behaviour of (U, Pu)O 2 mixed oxides (Pu/M = 0.25, 0.50 and 0.75, with M = U + Pu) was studied by means of mass spectrometry. Hyperstoichiometric samples were heated in a Knudsen cell up to T = 2300 K. The evolution of the uranium and plutonium bearing gaseous species was studied as a function of time in order to evaluate the congruent vapour composition. Ionisation efficiency measurements were performed and the partial pressures of the gaseous species involved in the vaporisation process were determined. The vapour pressure has also been calculated using a thermochemical model for the (U + Pu + O) system. A quasi congruent composition with respect to the O/M ratio has been assumed, in agreement with the experiments. Nevertheless, the evaluation of all the experimental and calculated results shows that a total congruent composition exists for a single composition of the mixed oxide (MOX) samples with a Pu/M content slightly lower than 0.50. A good agreement is obtained between the calculated and experimental vapour pressure data, as well as the quasi congruent vaporisation compositions.

A new system has been developed to determine absolute quantities of gas (mainly noble gases) released during thermal desorption in the range from 10−12 to 10−5 mol with a precision of few percent. ...

UO 2 samples doped with 0.1 wt.% plutonium oxide have been investigated with the aim to study alpha-damage effects and the behaviour of radiogenic helium in spent nuclear fuel by single effects studies. Characterizations were performed by X-ray diffraction, scanning and transmission electron microscopy. Helium release rate as a function of temperature and total helium quantity were measured using a Knudsen effusion mass spectrometer and a quantitative gas measurement system, respectively. A new model for the simulation of helium release has been developed taking into account single gas atom and bubble motion, trapping, grain and bubble growth. The model, embedded into commercial multiphysics software, provided a good description of the experimental data and correlations for gas-atom and bubble diffusion coefficients have been proposed. Finally, the effect of radiation damage on helium diffusion was assessed by comparing the results of the present study with literature data.

The Knudsen Effusion Mass Spectrometer (KEMS) and the mechanistic MFPR (Module for Fission Product Release) code are tools which seem particularly interesting to support studies of the Instant Release Fraction (IRF) of Cs from spent nuclear fuel in a final repository. With KEMS, the thermal release of 137Cs and 136Xe were analysed by annealing up to total vaporization (2500K) of high burn-up (60 GWd/tU) Spent Nuclear Fuel (SNF) samples. Powder samples from the centre of the fuel, without high burn-up structure, were used. To determine the IRF, samples were analysed before and after being submitted to corrosion experiments in bicarbonated aqueous media.MFPR was applied to determine the localization of Cs and fission gases in the SNF at the end of irradiation; the results are compared and supported by dedicated thermodynamics calculations performed for equilibrium conditions at various temperatures and fuel oxygen potentials by the non-ideal thermodynamic MEPHISTA (Multiphase Equilibria in Fuels via Standard Thermodynamic Analysis) database. A possible mechanism for Cs release during thermal annealing is proposed, taking into account inter-granular release and Cs oxide vaporization, atomic diffusion, ternary oxide phase formation and bubble release.Differences in KEMS release profiles before and after submitting the samples to aqueous corrosion are attributed to the IRF and to changes in the vaporisation mechanism because of differences in the oxygen potential (pO2). The IRF of Cs estimated from the KEMS spectra, consisting on the part located at the grain boundaries and in inter-granular bubbles, is not significantly different from that corresponding to the experimental results found using classical static leaching experiments.New experimental campaigns are being designed to confirm our interpretation proposed after this first run.

Samples of irradiated MOX of 44 GWd/tHM mean burn-up were prepared by core drilling at three different radial positions of a fuel pellet. They were subsequently heated in a Knudsen effusion mass spectrometry system up to complete vaporization of the sample (~2600 K) and the release of fission gas (krypton and xenon) and helium was measured. Scanning electron microscopy was used in parallel to investigate the evolution of the microstructure of a sample heated in the same condition up to given key temperatures as determined from the gas release profiles. A clear difference for fission gas release and microstructure was observed as a function of the radial position of the samples and therefore of irradiation temperature. A good correlation between the microstructure evolution and the gas release could be established as a function of the temperature of irradiation and (laboratory) heating., JRC.E.3-Materials research

Experimental tests have been performed to characterize the aerosols representative of radiological dispersion devices (RDDs, a.k.a. "dirty bombs") by applying to chosen surrogate compound rapid high temperature transients, vaporizing the sample and forming aerosols mainly by rapid cooling of the vapour. The materials, which were tested in their non-radioactive form, had been chosen from the radioactive sources widely used in industries and nuclear medicine applications, Co, CsCl, Ir and SrTiO3. Our analyses permitted the characterization of the inhalable fraction of the aerosols released, and the study of the influence of cladding materials on the aerosol release and on its characteristics.

Florent LEBRETON, Denis HORLAIT, Damien PRIEUR, Philippe MARTIN, Jean-Yves COLLE, Arne JANSSEN, Thierry WISS, Andreas SCHEINOST, Thibaud DELAHAYE CEA-DEN-DTEC, Service d’Etudes des Combustibles et materiaux a base d’Actinides, SECA, (Marcoule, France) CEA-DEN-DTEC, Service de Developpement des Technologies du Cycle, SDTC, (Marcoule, France) European Commission, Joint Research Centre, Institute for Transuranium Elements (Karlsruhe, Germany) Helmholtz Zentrum Dresden Rossendorf (HZDR), Institute of Resource Ecology (Dresden, Germany) CEA-DEN-DRCP, Service d’Etude du Recyclage des combustibles et d’Analyses, SERA, (Marcoule, France) Am isotopes are amongst the largest contributors to the radiotoxicity and heat load of spent nuclear fuels, especially in scenarios including plutonium recycling in MOX fuels. Transmutation in heterogeneous mode is a promising option to lower Am inventory in the ultimate nuclear waste, thus decreasing the ecological impact of deep-geological repository sites. It consists in the irradiation Ambearing blankets (AmBB, i.e., U1-xAmxO2±δ with x values close to 10-15 at.%) in the periphery of a fast neutron reactor core. In this context, studies are being performed to assess the feasibility and safety of AmBB transmutation, notably focusing on innovative fabrication processes [1–3], AmBB behavior under irradiation [4] and U1-xAmxO2±δ properties [5]. Am being a strong α emitter (activity of 130 MBq), U1-xAmxO2±δ behavior under α self-irradiation is also monitored [6–8].

Vapour pressure measurements at high temperatures have been performed on plutonium oxide samples using a Knudsen cell coupled with a mass spectrometer. Different experimental conditions were applied for the derivation of the solid–gas phase relations for three composition in the Pu–O phase diagram: the (Pu2O3–PuO2−x) two-phase domain, the congruent vapourisation composition and near stoichiometric PuO2.00. The partial pressures of the gaseous species PuO2, PuO, and Pu were assessed from ionisation efficiency curves recorded at constant temperature. The vapour pressure results for different phase fields were discussed together with all the available literature data.

The thermal stability and high temperature behaviour of candidate fuels and targets for transmutation of minor actinides has been investigated. Zirconia-based solid solution, MgO-based CERCER and molybdenum-based CERMET fuels containing Am and/or Pu in various concentrations were heated up to 2700 K in a Knudsen cell coupled with a quadrupole mass spectrometer, to measure their vapour pressure and vapour composition. The results reveal that the vaporisation of the actinides from the samples is not only determined by the thermodynamics of the system but is also related to the dynamic evolution of multi-component mixtures with complex composition or microstructure.

Knudsen cell high temperature mass spectrometric measurements have been performed on neptunium dioxide. Several samples have been analysed for vaporisation in vacuum and oxidative conditions and different behaviours have been observed. The results for the vapour pressure of pure neptunium dioxide samples are discussed together with the data on americium dioxide and plutonium–americium mixed oxide samples that contained very small amounts of neptunium, in which neptunium is produced by alpha-decay from americium. In the view of extreme dilution of neptunium in the sample, Raoult’s law has been applied. The ionisation efficiency curves for Np+, NpO+, and NpO 2 + were recorded for constant temperature. The dissociative ionisation contribution was evaluated with respect to the composition of the gaseous phase which under Knudsen conditions is in equilibrium with the condensed phase.

The vaporisation behaviour of americium dioxide in vacuum at high temperatures up to 2400 K has been studied. A Knudsen cell coupled with a mass spectrometer was used to perform vapour pressure measurements. The ionisation efficiency curves of Am+, AmO+ and AmO 2 + were simultaneously recorded. Appearance potentials of the key molecular species were determined by varying the energy of the ionising electrons at constant temperature. The partial and total vapour pressures above the americium oxide samples measured as a function of the temperature. The results on the vapour pressure of the pure americium dioxide samples were discussed together with the available literature data on plutonium dioxide containing small amounts of americium. Additional measurements have been performed on a mixed dioxide sample of plutonium containing 6.1 wt.% americium.

A particular low temperature behaviour of the 131Xe isotope was observed during release studies of fission gases from MOX fuel samples irradiated at 44.5 GWd/tHM. A reproducible release peak, representing 2.7% of the total release of the only 131Xe, was observed at ∼1000 K, the rest of the release curve being essentially identical for all the other xenon isotopes. The integral isotopic composition of the different xenon isotopes is in very good agreement with the inventory calculated using ORIGEN-2. The presence of this particular release is explained by the relation between the thermal diffusion and decay properties of the various iodine radioisotopes decaying all into xenon.

A commercial PWR fuel sample with a local burn-up of about 240 MWd/kgHM was annealed in a Knudsen cell mass spectrometer system with a heating rate of 10 K/min up to 2750 K 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 1860 K 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.

A new method of high-temperature mass spectrometry (MS) with laser-induced vaporization (LIV) has been developed. The initial problem of LIV MS, consisting of an inadequate correlation between the temperature of the surface and the MS signal, was successfully overcome.The method was developed on graphite, of which fast time-resolved MS measurements (ca. 20 ms) were performed over a large mass interval; the influence of geometrical parameters and of the laser pulse length on MS measurements was studied. Carbon sublimation relative partial pressures of C1, C2, C3, and C5 were measured up to 3810 K. This corresponds to a total pressure of about 0.8 bar estimated independently by the integral mass flux using the Hertz–Knudsen equation. The vaporization of UO2 was studied at temperatures above ≈ 2500 K, where conventional Knudsen-cell mass spectrometry cannot be applied. The vaporization enthalpy obtained for the main species in UO2 vapor was in good agreement with that of conventional mass spectrometry.

Understanding the long-term behaviour of the UO2 spent fuel in terms of helium build-up and oxidation is a very important issue for safety aspects of storage and disposal. Although helium behaviour in stoichiometric UO2 has been studied by many authors, it has not been well established and there is a lack of experimental studies in non-stoichiometric UO2. In this study, an infusion technique was chosen to introduce helium in stoichiometric and hyperstoichiometric single and polycrystalline UO2 and polycrystalline U3O8 samples. Characterization of the samples before and after infusion and after thermal desorption measurements were performed by thermogravimetry, X-ray diffraction, laser flash technique and scanning electron microscopy. It was observed that the increase of stoichiometry in UO2 affects barely the dissolved helium quantity (under the same infusion conditions as in stoichiometric), while it has a more pronounced influence on the helium diffusion. These two effects are much more pronounced for U3O8. (C) 2013 Elsevier Ltd. All rights reserved.

International audience; Due to the chemically unstable nature of uranium dioxide its vapor composition at very high temperatures is, presently, not sufficiently studied though more experimental knowledge is needed for risk assessment of nuclear reactors. We used laser vaporization coupled to mass spectrometry of the produced vapor to study urania vapor composition at temperatures in the vicinity of its melting point and higher. The very good agreement between measured melting and freezing temperatures and between partial pressures measured on the temperature increase and decrease indicated that the change in stoichiometry during laser heating was very limited. The evolutions with temperature (in the range 2800–3400 K) of the partial pressures of the main vapor species (UO2, UO3, and UO2+) were compared with theoretically predicted evolutions for equilibrium noncongruent gas-liquid and gas-solid phase coexistences and showed very good agreement. The measured main relative partial pressure ratios around 3300 K all agree with calculated values for total equilibrium between condensed and vapor phases. It is the first time the three main partial pressure ratios above stoichiometric liquid urania have been measured at the same temperature under conditions close to equilibrium noncongruent gas-liquid phase coexistence

International audience; Reliable experimental data on graphite vaporization and especially on carbon vapor composition exist only up to 2500–3000 K. Data measured at higher temperatures are questionable due to several experimental limitations, such as the difficult temperature determination and the not straightforward correlation of measured temperatures and intensities of signals in mass spectra. That is why a new method of high-temperature mass spectrometry with laser vaporization was developed, in order to extend the accessible temperature range while overcoming these limitations and to shed more light on the still poorly known behavior of carbon at high temperatures. Thus, carbon sublimation relative partial pressures of the species C1, C2, C3, C4, and C5 were measured up to 4100 K. Moreover, the values of the relative vaporization coefficients of C1, C2, C3, C4, and C5, estimated by comparison of the experimentally obtained partial pressures with the predicted equilibrium ones, are proposed.

A very high temperature furnace (up to 3000 degrees C) for the Knudsen cell mass spectrometry (KCMS) based on a laser heating technique has been developed. It is demonstrated that this system overcomes some of the typical technological problems encountered by the standard methods and can be more easily handled in special environments such as gloveboxes or hot cells. This paper describes the laser heated KCMS general design. The technology of the laser furnace along with its advantages, disadvantages, and applications is presented. Mechanical designs, some technical details, and the importance of the temperature control are also discussed.

The problem of correlation between the temperature of the target surface and the mass-spectrometer signal in laser-vaporization mass spectrometry has been analyzed theoretically. An approach based on statistical mechanics has been applied in order to describe the transient vaporization into vacuum of molecules effused from the area of the target surface struck by a laser pulse of moderate power density and time duration of some tens of ms (Langmuir vaporization). In particular, an expression for the intensity of the output signal of the mass spectrometer, I ( l,t ), has been derived as a function of the detection time, t , and of the distance, l , of the ionizing chamber of the spectrometer from the target. A simple numerical method for the calculation of I ( l , t ) according to the time profile of the target temperature is also provided. By fitting experimental I ( t ) values with the theoretical expression one can retrieve thermodynamic quantities involved in the sublimation/evaporation process of the molecular species analyzed, such as enthalpy and equilibrium vapor pressure (or, alternatively, vaporization coefficient). As an illustration, this fitting was performed on experimental measurements of pyrolytic graphite sublimation in the temperature range 3200–3700 K. The analysis developed will be useful for the interpretation of experimental datasets in order to retrieve high-temperature thermodynamic data, especially on high-melting materials. Research in this domain is being launched for nuclear materials, particularly for Generation IV advanced fuels.

International audience; In the frame of the development of uranium-plutoniummixed oxide fuels for Sodium-cooled Fast Reactors (SFRs),characterizing nuclear materials by various techniques isparamount.These fast neutron reactors imply the use of a (U,Pu)O$_{2-x}$ceramic fuel with a Pu/(U+Pu) content between 19 and30 mol.%. Furthermore, the physico-chemical andmicrostructural properties of such fuels, such as chemicalhomogeneity, oxygen stoichiometry (O/(U+Pu) ratio) andcrystallographic structure, have to meet precise criteria forbeing introduced in the reactor core.