Validation of the Wiedemann-Franz Law in solid and molten tungsten above 2000 K through thermal conductivity measurements via steady state temperature differential radiometry

We measure the thermal conductivity of solid and molten tungsten using Steady State Temperature Differential Radiometry. We demonstrate that the thermal conductivity can be well described by application of Wiedemann-Franz Law to electrical resistivity data, thus suggesting the validity of Wiedemann-Franz Law to capture the electronic thermal conductivity of metals in their molten phase. We further support this conclusion using ab initio molecular dynamics simulations with a machine-learned potential. Our results show that at these high temperatures, the vibrational contribution to thermal conductivity is negligible compared to the electronic component

The UC2-x - carbon eutectic: A laser heating study

The UC2-x – carbon eutectic has been studied by laser heating and fast multi-wavelength pyrometry under inert atmosphere. The study has been carried out on three compositions, two of which close to the phase boundary of the UC2-x – C miscibility gap (with C/U compositions 2 and 2.1), and one, more crucial, with a large excess of carbon (C/U = 2.82). The first two compositions were synthesised by arc-melting. This synthesis method could not be applied to the last composition, which was therefore completed directly by laser irradiation. The U-C-O composition of the samples was checked by using a combustion method in an ELTRA® analyser. The eutectic temperature, established to be at 2737 K ± 20 K, was used as a radiance reference together with the cubic – tetragonal (alpha-beta) solid state transition, fixed at 2050 K ± 20 K. The normal spectral emissivity of the carbon-richer compounds increases up to 0.7, whereas the value 0.53 was established for pure hypostoichiometric uranium dicarbide at the limit of the eutectic region. This increase is analysed in the light of the demixing of excess carbon, and used for the determination of the liquidus temperature (3220 ± 50 K for UC2.8). Due to fast solid state diffusion, also fostered by the cubic – tetragonal transition, no obvious signs of a lamellar eutectic structure could be observed after quenching to room temperature. The eutectic surface C/UC2-x composition could be qualitatively, but consistently, followed during the cooling process with the help of the recorded radiance spectra. Whereas the external liquid surface is almost entirely constituted by uranium dicarbide, it gets rapidly enriched in demixed carbon upon freezing. Demixed carbon seems to quickly migrate towards the inner bulk during further cooling. At the alpha-beta transition, uranium dicarbide covers again the almost entire external surface.JRC.E.3-Materials researc

Co-development of experimental and simulation methods for the laser flash heating and melting technique: The thermoelastic effects of UO2

The thermoelastic effects on the behaviour of subsecond laser heating experiments on UO2 to temperatures approaching 3000 K are explored using a thermoelastic model coupling heat transport and large deformation hyperelasticity, with consideration of thermoelastic effects. A series of steady state experiments at temperatures up to 2600 K are conducted and used to calibrate heat loss models. A series of subsecond transient laser flash melting experiments are then conducted and simulated. Simulation results reproduce the observations well without tuning of the transient model. The importance of considering thermoelastic effects in modelling the behaviour of solid materials with temperature and stress gradients typical of laser flash heating experimentation is discussed.JRC.G.I.3-Nuclear Fuel Safet

Laser-heating and Radiance Spectrometry for the Study of Nuclear Materials in Conditions Simulating a Nuclear Power Plant Accident

We present experiments in which real nuclear fuel, cladding and containment materials are laser-heated to temperatures beyond 3000 K, while their behavior is studied by radiance spectroscopy and thermal analysis. These experiments simulate on a laboratory scale the typical formation of a lava-phase (corium) following a nuclear reactor core meltdown.JRC.G.I.3-Nuclear Fuel Safet

Laser Melting and Annealing of Uranium Carbides

In the context of the material research aimed at supporting the development of nuclear plants of the fourth Generation, renewed interest has recently arisen in carbide fuels. A profound understanding of the behaviour of nuclear materials in extreme conditions is of prime importance for the analysis of the operation limits of nuclear fuels, and prediction of possible nuclear reactor accidents. In this context, the main goal of the present paper is to demonstrate the feasibility of laser induced melting experiments on stoichiometric uranium carbides; UC, UC1.5 and UC2. Measurements were performed, at temperatures around 3000 K, under a few bars of inert gas in order to minimise vaporisation and oxidation effects, which may occur at these temperatures. Moreover, a recently developed investigation method has been employed, based on in situ analysis of the sample surface reflectivity evolution during melting. Current results, 2781 K for the melting point of UC, 2665 K for the solidus and 2681 K for the liquidus of U2C3, 2754 K for the solidus and 2770 K for the liquidus of UC2, are in fair agreement with early publications where the melting behaviour of uranium carbides was investigated by traditional furnace melting methods. Further information has been obtained in the current research about the non-congruent (solidus¿liquidus) melting of certain carbides, which suggest that a solidus¿liquidus scheme is followed by higher ratio carbides, possibly even for UC2.JRC.E.3-Materials researc

On the melting behaviour of uranium/plutonium mixed dioxides with high-Pu content: A laser heating study

The melting behaviour of mixed uranium-plutonium dioxides (MOX) has been investigated by laser heating under controlled atmosphere in the PuO2-rich composition range (with amount-of-substance fraction x(PuO2) 75 %). The observed solidus/liquidus points are in agreement with the newly measured melting point of pure plutonium dioxide (3017 K). They suggest the existence of a minimum freezing temperature at a composition x(PuO2) between 50 % and 80 %, in contrast with earlier research carried out with traditional furnace heating methods. The current results have been obtained under optimised experimental conditions aimed at maintaining the integrity and original composition of the samples throughout the laser heating cycles. With this goal in mind, experiments have been carried out under controlled gas pressure (pressurised air or argon) and short time duration (< 0.25 s). A critical discussion of the present results highlights the fact that the formation of the gas phase has to be taken into account in the study of the high-Pu MOX behaviour at high temperature. The experimental results obtained thus correspond to slightly hypo-stoichiometric (U, Pu)O2-x compositions in equilibrium with the gas phase. Key words: Laser heating, Melting point, Nuclear fuel safety, Uranium-Plutonium MOX, Generation Four.JRC.E.3-Materials researc

Identifying Thermodynamic Mechanisms Affecting Reactor Pressure Vessel Integrity During Severe Nuclear Accidents Simulated by Laser Heating at the Laboratory Scale

n this work, laser heating is used to experimentally investigate the high-temperature behavior of the U-Fe-Zr-O system using arc-melted samples with various nominal compositions. Three-phase transitions are observed in the vicinity of ~1100, ~1700, and ~2200 K. Principal component analysis of the phase transition temperatures in the course of laser-heating thermal cycling indicates that the phase transition around ~1100 K is driven by the interaction of stainless steel (SS) with metallic U, the phase transition around ~1700 K by the melting of stainless steel, and the phase transition above ~2000 K by the eutectic melting of UO2. The results also reveal two hitherto overlooked interactions in the U-Fe-Zr-O system, which could have severe consequences for the contain-ment of corium inside the reactor pressure vessel (RPV). First, the phase transition temperatures of the samples varied extensively as a result of the laser-driven rapid thermal cycling. Variations of up to 390 K were observed in the phase transition temperatures, suggesting that depending on the initial conditions of corium formation, the corium-driven ablation of the RPV wall could commence significantly earlier than the current state-of-the-art severe accident codes would predict. Additionally, evidence of a large exothermic reaction between zirconium and molten steel was observed upon SS melting. Such phenomenon may also be driven by material segregation during fast heating and cooling. If such a mechanism is activated during a severe nuclear accident, it can have an important impact on the overall thermal balance of the RPV

The melting behaviour of oxide nuclear fuels: effects of the oxygen potential studied by laser heating

The present work highlights the importance, in nuclear fuel safety and performance, of the very high oxygen potentials of actinide oxides (ThO2, UO2, NpO2, PuO2) at temperatures close to melting (around 3000 K), i.e. a tendency to oxidize their environment and chemically react with it. Laser heating coupled with fast pyrometry constitutes an effective approach of studying the melting behaviour of these materials under a controlled atmosphere. Novel results reported in this work show that earlier data are confirmed, with the current technique, only for compounds with a relatively low oxygen potential, such as ThO2 and UO2 and their mixed compositions.JRC.E.3-Materials researc