Claudio Ronchi

Thermal Conductivity of Uranium Dioxide up to 2900 K from Simultaneous Measurement of the Heat Capacity and Thermal Diffusivity.

The thermal diffusivity and heat capacity of uranium dioxide have been measured from 500 to 2900 K with an advanced laser-flash technique. These two quantities were determined simultaneously by means of an accurate numerical fitting of the experimental thermograms. At high temperatures the precision of the method used is much better than that associated with conventional laser-flash measurements. It was found that the heat capacity continues to increase even at temperatures above the expected lambda transition (2670 K). The inverse of the thermal diffusivity increases linearly with temperature up to 2600 K, whilst at higher temperatures the slope markedly decreases. A new expression for the thermal conductivity as a function of temperature is proposed, which is corroborated by some theoretical considerations on the underlying heat transport mechanisms.

Molecular dynamics simulation of premelting and melting phase transitions in stoichiometric uranium dioxide

Results of molecular dynamics (MD) simulation of UO2 in a wide temperature range are presented and discussed. A new approach to the calibration of a partly ionic Busing-Ida-type model is proposed. A potential parameter set is obtained reproducing the experimental density of solid UO2 in a wide range of temperatures. A conventional simulation of the high-temperature stoichiometric UO2 on large MD cells, based on a novel fast method of computation of Coulomb forces, reveals characteristic features of a premelting lambda transition at a temperature near to that experimentally observed (T(lambda)=2670 K). A strong deviation from the Arrhenius behavior of the oxygen self-diffusion coefficient was found in the vicinity of the transition point. Predictions for liquid UO2, based on the same potential parameter set, are in good agreement with existing experimental data and theoretical calculations.

Advances in the use of laser-flash techniques for thermal diffusivity measurement

A laser-flash apparatus has been constructed for the measurement of thermal diffusivity. The apparatus is specially designed to operate under conditions imposed by the requirement to measure the thermal diffusivity of highly radioactive reactor-irradiated nuclear fuels. Among the various requirements, relating to the measurement of irradiated samples, were the ability to characterize sample platelets of irregular contours and different sizes, to make these measurements in a sufficiently short experimental time frame, and to maintain good experimental accuracy while keeping pulse laser energies at low levels. This article shows that improvement of key components above the current standards—in particular of the laser-beam homogeneity and of the transient temperature detector—makes it possible to create more flexible and controllable experimental conditions, enabling reliable measurements to be carried out in a broad range of modes. The method used to analyze the collected temperature pulse data is based on ...

The molten state of graphite: An experimental study

A subsecond laser heating technique has been successfully applied for graphite melting under controlled isobaric conditions. During the applied pulses, relatively large amounts of graphite were melted and subsequently solidified under good stability conditions of the liquid mass. The solid–liquid-vapor triple point was determined. From metallographic analysis of the quenched liquid, the expansion upon melting could be estimated. A mathematical model was then applied to analyze the measured thermograms and the thermal conductivity of liquid carbon was deduced. Both experimental observations and calculation results indicate a nonmetallic nature of liquid carbon in the pressure range of 110–2500 bar. Finally, an analysis of the melting line Tm(p) based on Simon’s empirical equation of state confirms the self-consistency of all results obtained.

An efficient method for computation of long-ranged Coulomb forces in computer simulation of ionic fluids

Angular averaging of Ewald sums eliminating the nonphysical cubic symmetry of electrostatic field in the uniform ionic system under conditions of computer simulation with periodic boundaries is proposed. The resulting effective potential is central, has simple analytical form and its range is correspondent to the main box size. The approach provides a fast method for computation of electrostatic contribution to the energy of ionic fluids and other dense, uniform Coulomb systems in Monte Carlo or molecular dynamics computer simulation.

Melting point of MgO

Despite large commercial production of MgO-based ceramics for a wide gamut of applications, the melting point of magnesia remained uncertain for almost one century. This article shows that a number of problems must be solved to attain experimental conditions where the solid–liquid phase transition of magnesia can be unambiguously detected, and the temperature be measured with sufficient accuracy. The method adopted in the reported work is based on controlled laser pulse heating. The solidification point was measured by the thermal arrest occurring during cooldown from the melt. The measurement of temperature, a most delicate problem for pyrometry applications in semitransparent materials, was obtained by using combined brightness and spectral pyrometers. The experimental and analytical methods are described in some detail. The resulting melting point of MgO is 3250±20 K, which is approximately 150 K higher than the value currently recommended.

Laser-pulse melting of nuclear refractory ceramics

An accurate method for melting-point measurement of refractory nuclear ceramics was developed, based on laser-pulse heating and thermal arrest detection. The temperature measurement is performed by a combined use of a brightness pyrometer and a high-speed spectrometer working in the range of 500 to 900 nm. This method provides both the true temperature and the spectral emissivity function of the examined materials. Pure sintered MgO and a Mg:Am mixed oxide were first measured. The resulting melting point of the former (2350±20 K) is significantly higher than that commonly recommended and decreases with the addition of americium. Furthermore, UO2 irradiated to 37,000 MWd/t and submitted to a reactor loss-of-coolant test was investigated: the melting point decreases from 3120 K, in the as-fabricated state, to 2950 K. Both fresh Zr:U mixed oxides and “corium” lava from a reactor meltdown experiment were also investigated.

Fission-fragment spikes in uranium dioxide

The article deals with some fundamental aspects of the fission energy dispersion in nuclear reactor fuel. The analysis starts from the transmission electron microscope observation of tracks produced by energetic fission fragments in thin foils of UO2 single crystals. On the one hand, these tracks appear on the free surface as explosive material displacements. On the other, the passage of fission fragments in the bulk does not leave visible, continuous traces. Furthermore, irradiated U4O9, which consists of a UO2 lattice hosting a temperature sensitive superstructure of interstitial oxygen, persists after experiencing the near-field action of fission fragment thermal spikes. These seemingly inconsistent phenomena have been explained by showing that a large fraction of the fission fragment electronic losses is converted in strong shock waves whose passage in the solid is too fast for producing atomic displacements, but which can release high energies by unloading on free surfaces. The consequent matter disp...

Thermophysical Properties Affecting Safety and Performance of Nuclear Fuel

Improvement of the nuclear fuel exploitation has been one of the main objectives of reactor technology during the last decades. Today, in view of a sustainable nuclear energy production, development of advanced reactors re-proposes the choice of innovative fuel cycle concepts, in a context of greater expectations and more stringent requirements. From the experience gained in the past, a fuel research and development strategy can be devised, by which selected physical properties are taken as in-pile fuel performance indices.Uranium dioxide, by far the most important fuel used in power plants, proved from the very beginning to have good design-related properties as well as an excellent resistance to radiation damage. Therefore, increasingly higher performance was demanded concerning lifetime in current power reactors, maximum burn-up and safe operation. Yet, fuel test campaigns carried out in the last years have shown that at very high burn-ups, conditions are attained where radical restructuring processes take place in the UO2 lattice, irrespective of the irradiation regime of the fuel rods. This has led to an intense research activity on the effects of radiation damage on the thermophysical properties of the fuel. Energy and matter transport processes were found to be strongly affected by reactor irradiation, the in-pile performance of the fuel being governed by self-healing processes that can be only in part controlled.Furthermore, in the severe reactor accidents the fuel high temperature thermodynamic properties must comply with safety requirements to be satisfied under conditions which have been not yet explored. Therefore, their description and formulation for applications in different scenarios represent one of the main goals of the future research on advanced fuels.

Thermodynamic model of solid non-stoichiometric uranium dioxide

A new equation of state for solid UO2+x is presented, based on an extended ionic model. A thermodynamic description of the imperfect and non-stoichiometric ionic solid is obtained accounting for short- and long-ranged inter-ionic forces, as well as for formation of Frenkel defects. Both Coulomb and short-range interactions between defects are encompassed in a highly non-ideal ionic system where interactions of Frenkel defects are taken into account explicitly as short-ranged interactions of quasi-dipoles. A simplified analytical form for the free energy of the perfect anharmonic crystal was obtained and then combined with additional contributions from formation and interaction of defects. By fitting a few numerical constants, the variations of thermodynamic properties of UO2+x are predicted as functions of temperature, density and stoichiometry. The model describes the pre-melting transition into the superionic state in solid stoichiometric UO2 and predicts the behaviour of the transition line in the non-stoichiometric domain.