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

Thermal Diffusivity of UO2 up to the Melting Point

The thermal diffusivity of uranium dioxide was measured from 500 to 3060 K with two different set-ups, both based on the laser-flash technique. Above 1600 K the measurements were performed with an advanced laser-flash technique, which was slightly improved in comparison with a former work. In the temperature range 500 to 2000 K the thermal diffusivity is decreasing, then relatively constant up to 2700 K, and tends to increase by approaching the melting point. The measurements of the thermal diffusivity in the vicinity of the melting point are possible under certain conditions, and are discussed in this paper.JRC.G.I.3-Nuclear Fuel Safet

Thermal Diffusivity of UO2 up to the Melting Point

The thermal diffusivity of uranium dioxide was measured from 500 to 3060 K with two different set-ups, both based on the laser-flash technique. Above 1600 K the measurements were performed with an advanced laser-flash technique, which was slightly improved in comparison with a former work. In the temperature range 500 to 2000 K the thermal diffusivity is decreasing, then relatively constant up to 2700 K, and tends to increase by approaching the melting point. The measurements of the thermal diffusivity in the vicinity of the melting point are possible under certain conditions, and are discussed in this paper.JRC.G.I.3-Nuclear Fuel Safet

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

Upper plenum temperature calculations: comparison of TRANSURANUS with a 2–D model under steady–state conditions

The model for the evaluation of the upper plenum temperature (pressure) of the TRANSURANUS code is further developed to predict with better accuracy the role played by this parameter under steady–state and accident conditions (LOCA, RIA), rather than providing this parameter on input. At present, for the gas contained in the upper plenum volume, the code has a “low temperature” and a “high temperature” model. The former assumes that the plenum temperature coincides with the coolant temperature while the latter adopts a weighted value of the cladding inner temperature and the fuel central temperature evaluated in the uppermost zone of the fuel stack. To refine the accuracy of the code predictions, the adopted strategy was to improve the description of the plenum volume sub–system by means of 2–D models.. For this purpose, both a transient heat transfer model and a FEM model by using the commercial software COMSOL Multiphysics were developed. In addition, ENEA implementated in TRANSURANUS the FRAPCON–3 plenum temperature model for comparison with the two other models. The paper presents the comparison of the code results for a PWR fuel pin under steady–state conditions. In addition, thanks to the capability of the code to account for the change in the plenum gas composition, preliminary calculations on the effect of fission gas release on the plenum temperature are also presented. Finally, we discuss the plenum spring gamma heating and the clad–to–coolant heat transfer coefficient under transient conditions in the light of the FK–1 test (NSRR) and the results published in the open literature.JRC.E.3-Materials researc

The High-temperature Heat Capacity of the (Li,Na)F Liquid Solution

The enthalpy increments of four liquid solutions Li0:8Na0:2F; Li0:6Na0:4F, Li0:4Na0:6F, and Li0:2Na0:8F were measured in the temperature range (1230 to 1470) K by drop calorimetry. The liquid phases of the LiF and NaF end-members were also measured, in the temperature range (1230 to 1470) K and (1320 to 1540) K, respectively. From the obtained results the molar heat capacity was derived and the data for pure LiF and NaF were correlated with the literature. In order to overcome problems related with the corrosive nature of alkali metal fluoride vapours at high temperature, a novel encapsulation technique is presented in this study. Small nickel crucibles were developed for this purpose which are sealed by laser welding.JRC.E.3-Materials researc

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

A new method for the characterization of temperature dependent thermo-physical properties

The proposed method is based on the laser flash technique. Radially distributed thermograms are calculated via a finite element model and used in an inverse method by optimizing either specific heat or thermal conductivity of a material. These properties are evaluated as a function of radius and respective temperature. Two approximations are introduced inferring the dependence of each property as a function of radius – a polynomial (PNOM) approximation and an iterative gradient (IG) approximation. The method was tested using synthetic thermograms and both approximations were capable of yielding excellent results. The IG approximation was more universal and less sensitive to initial fitting parameters. The PNOM approximation was less computationally expensive but was prone to artefacts (such as un-physical minima or maxima) and more dependent on initial fitting parameters. Both approximations were successfully used on experimental data from UO2 and isostatically pressed graphite. Thermal conductivity was within 5% of the reference empirical correlation for UO2 and within 7% of the reference curve for graphite.JRC.G.I.5-Advanced Nuclear Knowledg