High-precision molecular dynamics simulation of UO2-PuO2: Anion self-diffusion in UO2

Our series of articles is devoted to high-precision molecular dynamics simulation of mixed actinide-oxide (MOX) fuel in the approximation of rigid ions and pair interactions (RIPI) using high-performance graphics processors (GPU). In this article we study self-diffusion mechanisms of oxygen anions in uranium dioxide (UO2) with the ten recent and widely used sets of interatomic pair potentials (SPP) under periodic (PBC) and isolated (IBC) boundary conditions. Wide range of measured diffusion coefficients (from 10^-3 cm^2/s at melting point down to 10^-12 cm^2/s at 1400 K) made possible a direct comparison (without extrapolation) of the simulation results with the experimental data, which have been known only at low temperatures (T < 1500 K). A highly detailed (with the temperature step of 1 K) calculation of the diffusion coefficient allowed us to plot temperature dependences of the diffusion activation energy and its derivative, both of which show a wide (~1000 K) superionic transition region confirming the broad lambda-peaks of heat capacity obtained by us earlier. It is shown that regardless of SPP the anion self-diffusion in model crystals without surface or artificially embedded defects goes on via exchange mechanism, rather than interstitial or vacancy mechanisms suggested by the previous works. The activation energy of exchange diffusion turned out to coincide with the anti-Frenkel defect formation energy calculated by the lattice statics.Comment: 18 pages, 11 figures, 5 table

Experimental Measurement of Pre-Melting and Melting of Thorium Dioxide

Thorium dioxide samples have been submitted to homogeneous four-beam laser pulses in an autoclave filled with different gases. Phase transitions were analysed through the produced thermal arrest stages, which were detection by a precise multichannel pyrometer. The spectral emissivity of the samples, measured on line during heating up to 4000 K, is also reported in the paper. Thoria exhibits a melting point at 3651 K, and a pre-melting transition at 3090 K. These temperatures are weakly influenced by chemical reduction of the sample. Whilst virtually stoichiometric thoria melts congruently, in the hypostoichiometric oxide, the liquidus and solidus temperatures could be clearly observed at O/Th = 1.98. The analysis of the measured data and, in particular, of the heat capacity shows that the features of the pre-melting transition can be described by a Debye-Hueckel oxygen defect model, where co-operative effects lead, in the oxygen sub-lattice, to a critical oder-disorder transition, whose thermodynamic nature depends on the parameters of the defect mean-field coulomb interaction.JRC.E-Institute for Transuranium Elements (Karlsruhe

Fission Gas Release and Volume Diffusion Enthalpy in UO2 Irradiated at Low and High Burnup.

Abstract not availableJRC.E-Institute for Transuranium Elements (Karlsruhe

Experimental Studies on Behaviour and Properties of Nitride Fuels

see attachmentJRC.E.3-Materials researc

Volatile Molecule PuO3 Observed from Subliming Plutonium Dioxide.

Abstract not availableJRC.E-Institute for Transuranium Elements (Karlsruhe

Detection of trace radioisotopes in soils, sediments and vegetations by glow discharge mass spectrometry

The possibility for the determination of some radioisotopes of cesium, strontium, plutonium, uranium and thorium by glow discharge mass spectrometry (GDMS) in soils, sediments and vegetations is investigated. The preparation of samples is described as a combination of the use of a conductive host matrix and a secondary cathode in order to decrease the dilution effect of the blending material for the trace level determination and to gain a stable discharge. Effects of interferences arising from the nature of the conductive host matrix and of the secondary cathode on the sensitivity of the method are discussed. The determination of Cs-137 and Sr-90 has been attempted and the results obtained were in agreement with those from other analytical techniques. Accuracy, internal and external precisions have been also evaluated. GDMS is shown to be a helpful technique for the determination of radioisotopes in environmental samples. Radioisotopes can be determined according to the matrix of the sample (e.g. grass), also in presence of isobaric interferences. However, limitations still exist on the application of GDMS

Mass Spectroscopic Measurements of Fission Product Effusion from Irradiated Light Water Reactor Fuel

Irradiated LWR fuel from the BR3 reactor was thermally annealed up to 2500 K in a Knudsen-cell, and the effusing vapors were measured by mass spectrometry. The experiments provide data on the stoichiometry evolution of the fuel during release as well as a reliable method to evaluate the diffusion coefficients of volatile and less volatile products. The analysis of the data starts from diffusion on Xenon which clearly shows three typical release stages respectively controlled by radiation damage annealing, self-diffusion and matrix vaporization. The experimental measurements are also in agreement with the predictions of intragranular trapping models. Barium and Caesium showed faster release than Xenon, the former being likely diffusion atomically to the grain boundaries where no evidence of formation of stable zirconates was found. These results were compared with those obtained by a burnup-simulated fuel, where Barium was initially present in a perovskite phase, producing essentially different release patterns.JRC.E-Institute for Transuranium Elements (Karlsruhe

Helium and Xenon Behaviour in Irradiated Am-Containing MgAl2O4 (Reactor Experiment EFTTRA-T4).

Abstract not availableJRC.E-Institute for Transuranium Elements (Karlsruhe