The thermal diffusivity of reactor irradiated (U,Gd)O2 fuels has been measured, for burn-ups from 33 to 97GWdtHM−1 and for irradiation temperatures from 670 to 1580K. Measurements under thermal annealing cycles were performed in order to investigate the recovery of the thermal conductivity as a function of temperature. The analysis of the results showed a lower thermal conductivity for (U,Gd)O2 when compared to UO2, with similar effects of the burn-up and irradiation temperature. A correlation for the thermal conductivity could be proposed on the basis of that for UO2 presented in an earlier work, which describes the separate effects of soluble fission products, of fission gas frozen in dynamical solution and of radiation damage.

The effect of α-damage on the thermophysical properties of UO2 was investigated using samples doped with 238Pu. Characterisations were performed after different storage periods, including X-ray diffraction to monitor the lattice parameter evolution, Knudsen-cell helium release experiments and transmission electron microscopy examinations. The apparent heat capacity was measured by differential scanning calorimetry and the recovery stages observed were attributed to the recombination of a certain kind of point or extended defect. The thermal diffusivity, measured by the laser-flash technique during annealing cycles, displayed similar recovery stages. The measurements show that the degradation of the diffusivity with increasing α-dose is not linear, and that saturation occurs at relatively low doses. A correlation quantifying this degradation is proposed. Comparison with the thermal diffusivity of very low burn-up rector irradiated samples, where the main source of degradation is radiation damage, shows that the annealing stages are similar.

Diffusion coefficients of helium in solid UO2+x at high temperature and various stoichiometric compositions are evaluated by using molecular dynamics simulations based on a partly-ionic model in conjunction with a polaron ‘Free Hopping Approximation’ proposed earlier. The results are compared with existing experimental and simulation data. A strong dependence of the apparent diffusion activation energy on stoichiometry is found and the mechanisms of He migration in non-stoichiometric solid UO2+x are discussed.

Formation and stability of different types of clusters in hyperstoichiometric UO2+x, including Willis’s 2:2:2 interstitial dimers as well as cuboctahedral tetra- and pentamers are investigated under static and dynamic conditions based on a partly-ionic model. A ‘Free Hopping Approximation’ for small polarons is proposed and implemented in a molecular dynamic simulation computer code. Lattice parameter and other equilibrium properties of UO2+x are calculated and compared with existing experimental data in a wide range of temperature and stoichiometry.

An original technique for the measurement of high-temperature phase transitions was implemented based on a laser-heating method, enabling chemically unstable, refractory materials to be melted under controlled conditions. This technique includes two independent but correlated methods: In the first, fast multichannel pyrometry is employed to measure thermograms and spectral emissivity; in the second, a low-power probe laser beam is used for the detection of reflectivity changes induced by phase transitions on the sample surface. The experiments are carried out under medium ( approximately 10(2) kPa) or high ( approximately 10(2) MPa) inert-gas pressures in order to kinetically suppress evaporation in volatile or chemically instable samples. Two models for the simulation of the laser-heating pulses are as well introduced. Some results are presented about the successful application of this technique to the study of the melting behavior of oxides such as UO(2+x), ZrO(2), and their mixed oxides. The method can be extended to a broad class of refractory materials.

The article gives an account of measurements of the thermal conductivity of liquid UO2. The sample was heated up to above the melting point by a laser pulse of a controlled shape, and the produced thermogram of temperature history was measured by a fast and accurate pyrometer with a time resolution of 10 μs. The experiment shows that the rate of temperature increase during the ascending part of the pulse changes moderately across the melting point. Due to the high power input, this effect cannot be explained in terms of the sole intake of latent heat of fusion. By solving the related heat transfer equation with a 2D-axisymmetric numerical model, it is demonstrated that this feature depends principally on heat conduction in the sample, and proves that the thermal conductivities of solid and liquid are not very different. A theoretical sensitivity study assessing the influence of the liquid thermal conductivity on the pulse temperature evolution showed that the conductivity of the liquid can be deduced from the fitting of the thermograms with a numerical precision of the order of 1%. The analysis reveals that the thermal conductivity is weakly correlated with the effective heat losses during the pulse and to the melting enthalpy, so that the uncertainty in its evaluation by fitting the experimental thermograms with model predictions is satisfactory. The value of the thermal conductivity of liquid UO2 near the melting point resulted to be 2.6±0.35 W m−1 K−1, where the magnitude of the uncertainty is much lower than the scatter of the previously published, discordant measurements.

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.

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.

In order to describe the complex reactions taking place during a severe accident in a PWR, the knowledge of the Zr–O system is critical. However, thermodynamic data are lacking concerning this system for oxygen rich compositions. The aim of this work is to obtain new experimental data on the liquidus/solidus transitions in the O-rich side of the Zr–O binary system. This work provides liquidus and eutectic transition temperatures for oxygen atomic fractions from x(O) = 0.28 to x(O) = 0.667. These data are used to validate the Zr–O thermodynamic model in the TAF-ID thermodynamic database. The current study confirms the eutectic transition temperatures (T = 2084 ± 51 °C) and its domain of existence. However, our measurements for x(O) > 0.6 suggest the existence of a congruent melting lying between x(O) = 0.65 and pure ZrO2, with an estimated value lower than the current calculation outcome from the TAF-ID.

Results of oxidation experiments on high-burn-up UO2 are presented where fission-product vaporisation and release rates have been measured by on-line mass spectrometry as a function of time/temperature during thermal annealing treatments in a Knudsen cell under controlled oxygen atmosphere. Fractional release curves of fission gas and other less volatile fission products in the temperature range 800–2000 K were obtained from BWR fuel samples of 65 GWd t−1 burn-up and oxidized to U3O8 at low temperature. The diffusion enthalpy of gaseous fission products and helium in different structures of U3O8 was determined.

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.

Vaporisation processes in uranium oxide fuel were investigated under controlled oxidation conditions. A Knudsen cell with a mass spectrometer was adapted to feed an adjustable oxygen flux up to temperatures of 1900K producing an oxygen pressure in the cell ranging up to 1000Pa. Experiments were carried out on fresh and irradiated fuels, both pre-oxidised to U3O8 or oxidised online in the Knudsen cell, to measure effects associated with fuel sublimation in the form of different uranium-bearing oxides. Mass spectrometry, electromotive force measurements and thermo-gravimetric analysis were used to determine the vapour equilibrium conditions at fixed free oxygen potentials.

The solid–liquid transition in stoichiometric and hyperstoichiometric UO2 was investigated by means of advanced techniques. Laser heating enabling fast melting and freezing processes was used under container-less conditions and buffer gas pressures up to 250 MPa, making non-congruent evaporation ineffective. Pulse thermograms of UO2+x with 0⩽x⩽0.21 were recorded with fast pyrometers and interpreted with computer simulations. In addition, a novel method for identification of phase transitions was implemented, based on the detection of surface reflectivity variations. The melting line of UO2.00 was for the first time determined at pressures between 10 and 250 MPa, and the melting temperature of the stoichiometric oxide was measured to be 3147±20 K, in fair agreement to previous measurements reported in the literature. The liquidus and solidus lines of UO2+x differ from the currently recommended data, which substantially underrate the effect of oxygen on melting.

Applications of a new method for computation of Coulomb forces in Monte Carlo or molecular dynamics simulation of a wide class of disordered systems including plasmas, ionic fluids and amorphous solids is discussed. This method, based on angular averaging of Ewald sums over all orientations of the reciprocal lattice under conditions of computer simulation, eliminates periodicity artifacts imposed by conventional Ewald scheme and provides much faster computation of electrostatic energy in computer simulations of disordered condensed systems.

The first ionization potential of the PuO2 molecule was for a long time considered to be 4-5 eV higher than that of UO2. This feature could hardly be explained by the most advanced "ab initio" calculations, which, on the other hand, provide satisfactory results for other actinide oxides. From recent experiments, performed with different techniques, a lower ionization potential of approximately 7 eV was measured, in better agreement with the theoretical predictions. Our recent experiments, where thermally produced ions were measured, make it possible to formulate an accurate relation between the ionization potential of PuO2 and that of PuO: I0(PuO2) = I0(PuO) + 0.42 +/- 0.005 eV. The present uncertainty of I0(PuO) leads to the final assessment, 6.2 < or = I0(PuO2) < or = 6.6 eV, whereby the upper limit is more in line with the aforementioned recent measurements. Considering the still existing uncertainties, one can conclude that these results remove major doubts on the validity of the current theoretical predictions. However, the very small ionization cross section of PuO2 by low-energy electron collisions, which led to the previous spurious assessment of the ion appearance potential, has still an unexplained cause.