27 research outputs found
3D reconstruction of two-phase random heterogeneous material from 2D sections: An approach via genetic algorithms
No full textThis paper introduces a method to reconstruct the three-dimensional (3D) microstructure of two-phase materials, e.g., porous materials such as highly irradiated nuclear fuel, from two-dimensional (2D) sections via a multi-objective optimization genetic algorithm. The optimization is based on the comparison between the reference and reconstructed 2D sections on specific target properties, i.e., 2D pore number, and mean value and standard deviation of the pore-size distribution. This represents a multi-objective fitness function subject to weaker hypotheses compared to state-of-the-art methods based on n-points correlations, allowing for a broader range of application. The effectiveness of the proposed method is demonstrated on synthetic data and compared with state-of-the-art methods adopting a fitness based on 2D correlations. The method here developed can be used as a cost-effective tool to reconstruct the pore structure in highly irradiated materials using 2D experimental data
Modeling high burnup structure in oxide fuels for application to fuel performance codes. part I: High burnup structure formation
No full textWe propose a model describing the HBS formation and the progressive intra-granular xenon depletion in UO2. The HBS formation is modeled employing the Kolmogorov-Johnson-Mehl-Avrami (KJMA) formalism for phase transformations, which has been fitted to experimental data on the restructured volumetric fraction as a function of the local effective burnup. To this end, we employed available experimental data and novel data extracted in this work. The HBS formation model is coupled to a description of the intra-granular fission gas behavior, allowing to estimate the evolution of the retained xenon in order to consistently compute fission gas retention and its effect on the fuel matrix swelling. The satisfactory agreement of the model predictions to experimental data and state-of-the-art models’ results, in terms of both xenon depletion and fuel matrix swelling as a function of the local burnup, paves the way to the inclusion of the model in fuel performance codes
A semi-empirical model for the formation and depletion of the high burnup structure in UO2
No full textIn the rim zone of UO2 nuclear fuel pellets, the combination of high burnup and low temperature drives a microstructural change, leading to the formation of the high burnup structure (HBS). In this work, we propose a semi-empirical model to describe the formation of the HBS, which embraces the polygonisation/recrystallization process and the depletion of intra-granular fission gas, describing them as inherently related. For this purpose, we performed grain-size measurements on samples at radial positions in which the restructuring was incomplete. Based on these new experimental data, we infer an exponential reduction of the average grain size with local effective burnup, paired with a simultaneous depletion of intra-granular fission gas driven by diffusion. The comparison with currently used models indicates the applicability of the herein developed model within integral fuel performance codes
Laser melting of nano-crystalline uranium dioxide
No full textThe present work is focused on the investigation of possible melting temperature depression of nanocrystalline UO2. The behaviour of nano-crystalline refractory oxide of this kind has been studied above 1500 K for the first time here. The technique used is based on sub-second laser heating and fast
pyrometry.
Nano-crystalline UO2 with grains of (30 20) nm underwent fast oxidation due to the high specific surface. The measured melting point depression (up to over 300 K below 3120 K, the melting temperature of micrometer-sized UO2) was thus mostly attributable to a non-negligible oxygen-to-metal molar
ratio increase and, possibly, only to a much lesser extent to a real grain-size effect
Modeling high burnup structure in oxide fuels for application to fuel performance codes. Part II: Porosity evolution
No full textWe propose a model describing the high burnup structure inter-granular porosity evolution under irradiation. The evolution of the porosity collecting the gas diffusing from the grains is modeled by exploiting a second-order Fokker-Planck expansion of the cluster-dynamics master equations governing the problem, considering nucleation of pores, gas absorption due to the diffusional flow from the grains, size-dependent re-solution of gas from pores due to interaction with fission fragments, vacancy absorption, and pore coalescence. Model predictions on xenon local retention, matrix fuel swelling, and porosity evolution are compared to experimental data and to models available in fuel performance codes
High-temperature and melting behaviour of nanocrystalline refractory compounds: an experimental approach applied to thorium dioxide
No full textThe behaviour from 1500 K up to melting of nanocrystalline (nc) thorium dioxide, the refractory binary oxide with the highest melting point (3651 K), was explored here for the first time using fast laser heating, multi-wavelength pyrometry and Raman spectroscopy for the analysis of samples quenched to room temperature. Nc-ThO2 was melted at temperatures hundreds of K below the melting temperature assessed for bulk thorium dioxide. A particular behaviour has been observed in the formed liquid and its co-existence with a partially restructured solid, possibly due to the metastable nature of the liquid itself. Raman spectroscopy was used to characterize the thermal-induced structural evolution of nc-ThO2. Assessment of a semi-empirical relation between the
Raman active T2g mode peak characteristics (peak width and frequency) and crystallites size provided a powerful, fast and non-destructive tool to determine local crystallites growth within the nc-ThO2 samples before and after melting. This semi-quantitative analysis, partly based on a phonon-confinement model, constitutes an advantageous, more flexible, complementary approach to electron microscopy and powder x-ray diffraction (PXRD) for the crystallite size determination. The adopted experimental approach (laser heating coupled with Raman spectroscopy) is therefore proven to be a promising methodology for the high temperature investigation of nanostructured refractory oxides
Chromia-doped UO2 fuel: An engineering model for chromium solubility and fission gas diffusivity
No full textIncreasing the average grain size of fuel pellets by doping them with chromium oxide is one strategy to improve oxide nuclear fuels performance. The promoted fission gas retention is thought to improve the performance of the fuel at high burnup. In this work, we review models for the solubility of chromium in UO2, and the evolution of the chromium phases in the fuel matrix during irradiation. These models are implemented in SCIANTIX, an open- source mesoscale code describing inert gas behaviour in nuclear fuel. We adjusted the chromium solubility model keeping each parameter within its range of compatibility with experimental data, targeting a better representation of available electron probe microanalysis data of chromium content in fuel after irradiation. As for fission gas behaviour, we considered a physics-based description of the chromium impact on the fission gas diffusivity in fuel grains. The expression for the fission gas diffusivity in standard non-doped uranium oxide has been extended by introducing the impact of the concentration of defects introduced by interstitial oxygen excess representing the effect of chromium content in the fuel itself. A preliminary integral assessment of the proposed models has been carried out against the available experimental data
Characterization of thyroid 'follicular neoplasms' in fine-needle aspiration cytological specimens using a panel of immunohistochemical markers: A proposal for clinical application
No full textThree-dimensional reconstruction from experimental two-dimensional images: Application to irradiated metallic fuel
No full textThis work applies reconstruction methods based on a genetic algorithm to derive 3D material properties, namely porosity and percolation fraction, in irradiated U-Pu-Zr fuel with minor actinides. We provide two-dimensional experimental data regarding the radial distribution of fission gas bubbles in the fuel and apply the algorithm successfully developed in a companion paper to reconstruct the fuel pore structure in 3D which is unknown a priori. The algorithm returned a set of best structures that constituted the best candidate solutions representing the pore phase. From these, it was possible to extract statistics on the 3D percolation fraction of the reference medium and infer a mean value, the related uncertainty, and an upper and lower bound of the percolation fraction. The algorithm proved able to infer this 3D property from 2D information of the metallic fuel with confidence intervals, thus establishing a path to infer 3D properties directly from 2D experimental images. The knowledge of such a relationship can be used to extrapolate the percolation threshold with confidence interval, which is a crucial property in defining microstructure-based fission gas release models of metallic fuels
Critical assessment of the pore size distribution in the rim region of high burnup UO2 fuels
No full textA new methodology is introduced to analyse porosity data in the high burnup structure. Image analysis is coupled with the adaptive kernel density estimator to obtain a detailed characterisation of the pore size distribution, without a-priori assumption on the functional form of the distribution. Subsequently, stereological analysis is carried out. The method shows advantages compared to the classical approach based on the histogram in terms of detail in the description and accuracy within the experimental limits. Results are compared to the approximation of a log-normal distribution. In the investigated local burnup range (80-200 GWd/tHM), the agreement of the two approaches is satisfactory. From the obtained total pore density and mean pore diameter as a function of local burnup, pore coarsening is observed starting from 100 GWd/tHM, in agreement with a previous investigation.JRC.G.III.8 - Waste Managemen
