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High temperature investigation of nanostructured actinide oxides
Get PDFLAUREA MAGISTRALEAltri studi hanno dimostrato che la zona del combustibile nucleare UO2 ricristallizzata con
grani nanometrici sotto alto irraggiamento (high burn-up structure) ha migliori proprietà
di ritenzione dei gas di fissione e meccaniche, rispetto al combustibile meno irraggiato.
Composti sintetici di attinidi nanostrutturati potrebbero dunque essere impiegati come
combustibili avanzati, riproducendo tali benefiche proprietà. Tuttavia, i nanomateriali
possono manifestare peggiori proprietà termofisiche, come una inferiore temperatura di
fusione. Essa è fondamentale per la sicurezza nei reattori, quindi la comprensione di una
sua variazione in funzione della dimensione dei grani è cruciale per determinare pro e
contro di tali potenziali combustibili. Questo lavoro è focalizzato sullo studio del possibile
abbassamento della temperatura di fusione per UO2 e ThO2 nanocristallini (nc), studiati
qui per la prima volta sopra i 1500 K usando un riscaldamento laser rapido e pirometria.
L’apparato include un laser sonda: le variazioni nel segnale riflesso indotte da vibrazioni
della superficie liquida rivelano l’inizio della fusione. Sono state applicate anche altre
tecniche di caratterizzazione. In particolare, la Spettroscopia Raman è stata usata come
metodo non distruttivo per un’analisi locale dell’evoluzione termica della dimensione dei
grani per nc-ThO2.
Nc-UO2 con grani di (30 ± 20) nm subisce una rapida ossidazione per l’elevata
superficie specifica. L’abbassamento misurato (fino a 300 K sotto 3120 K, temperatura di
fusione di UO2 bulk) è dunque dovuto a effetti sia di dimensione sia di stechiometria. Per
stimare il solo contributo dei primi, è stato studiato il ThO2, chimicamente più stabile,
analizzando nc-ThO2 con grani di 3-5 nm, per quantificare il massimo abbassamento
possibile. Benchè non sia stato possibile determinare l’inizio della fusione, la presenza di
liquido è stata misurata a 400-600 K sotto la temperatura di fusione del ThO2 bulk (3651
K). Questa è la prima evidenza sperimentale di un notevole effetto legato alla dimensione
dei grani in questo tipo di ossidi refrattari.
Ulteriori esperimenti sono in corso per confermare questi primi originali risultati.Other studies have shown that the zone of UO2 nuclear fuel recrystallised under high
irradiation in the nanometer-size range (high burn-up structure) has better fission gas
retention and mechanical properties than the less irradiated fuel. This suggests that some
synthetic actinide nanomaterials could be used as advanced nuclear fuels to reproduce
these benefits. However, nanomaterials can show worse thermophysical properties crucial
for the fuel safety, such as a lower melting temperature (Tm). The grain size dependence
of Tm defines the performance limits of such potential advanced fuels. The present work is
mainly focused on the investigation of possible melting point depression of nanocrystalline
(nc) UO2 and ThO2, studying their behaviour above 1500 K for the first time.
The technique used is based on sub-second laser heating and fast pyrometry. The
apparatus included a probe laser. The changes in its reflected signal induced by vibrations
of the molten surface have been used to detect the melting onset. Other material
characterization techniques have also been applied. In particular, Raman Spectroscopy
permitted a first non-destructive analysis of the local grain size evolution with temperature
for nc-ThO2.
Nc-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 Tm
of micrometer sized UO2) was thus due both to size effect and oxygen hyperstoichiometry.
Chemically more stable ThO2 was then studied in order to estimate the size effect only.
Nc-ThO2 with grains of 3-5 nm was firstly studied to quantify the maximum possible
melting variation. Although it was not possible to determine the exact melting onset
temperature, the presence of liquid was detected 400-600 K below the Tm of micrometer
sized ThO2 (3651 K). This experimental evidence is paramount in that it shows for
the first time the occurrence of a large size effect on the melting point of this kind of
refractory oxides.
Further experiments are ongoing to confirm these first original results
Untersuchung von hohen Abbrands UO<sub>2</sub> Brennstoffen in Leichtwasserreaktoren
No full textThis work presents tailored post-irradiation examinations for the study of mechanical properties and porosity in high burnup UO2 fuels. The data are used to improve fuel Young´s modulus correlation in fuel performance codes. First study on ageing of fuel microhardness during interim storage are performed. The measurements of fuel porosity, obtained using an improved methodology for data analysis, are complemented with preliminary semi-empirical modelling of fission gas behaviourDiese Arbeit befasst sich mit speziell entwickelten Methoden für die Nachbestrahlungsuntersuchung von UO2 Brennstoffen hohen Abbrands. Untersucht werden die mechanischen Eigenschaften und die Porosität der Brennstoffe. Die gewonnenen Daten werden verwendet um die Werte des Elastizitätsmoduls in Computerprogrammen zur Berechnung der Brennstoffleistungsfähigkeit zu verbessern. Erste Untersuchungen über die Alterung der Mikrohärte von Brennstoffen während der Zwischenlagerung werden durchgeführt. Die Messungen der Brennstoffporosität, die über eine verbesserte Methode zur Datenauswertung gewonnen werden, werden ergänzt durch eine vorläufige halbempirische Modellierung des Spaltgasverhalten
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Experimental investigation of axial gas flow in simulated LWR fuel rods
No full textLAUREA MAGISTRALEQuesta ricerca esplora il comportamento del flusso di gas assiale all'interno delle barre di combustibile nucleare nei Reattori ad Acqua Leggera (LWRs), con l'obiettivo di valutare l'impatto delle fratture nei pellet di combustibile simulando la condizione di fine vita del combustibile stesso. Implementando una metodologia sperimentale che varia la pressione di prova della barra di combustibile simulata e lo stato di occlusione dello spazio tra pellet e rivestimento, abbinata all'analisi delle immagini per caratterizzare la struttura del pellet, lo studio cerca di comprendere i fenomeni del flusso di gas assiale in caso di rottura della guaina. I risultati rivelano che il tasso di depressurizzazione subisce variazioni minime rispetto alla pressione di prova iniziale e alla struttura del pellet, ma è fortemente correlato alla dimensione dell'intercapedine tra guaina e pellet. Queste conclusioni quantitative sussistono fino a un certo grado di frammentazione del pellet stesso. Queste intuizioni quantitative rappresentano un input per la progettazione e la verifica delle barre di combustibile in scenari accidentali e aprono la strada a modelli dedicati al flusso di gas assiale nei codici di prestazione del combustibile.This work explores the behavior of axial gas flow within nuclear fuel rods in Light Water Reactors (LWRs), aiming to assess the impact of cracks in fuel pellets by simulating the end-of-life condition of the fuel itself. By implementing an experimental methodology that varies the testing pressure of the simulated fuel rod and the occlusion state of the gap between pellet and cladding, paired with image analysis to characterize the pellet structure, the study seeks to understand the phenomena of axial gas flow in case of cladding breach. The results reveal that the rate of depressurization undergoes minimal variations relative to the initial test pressure and the pellet structure but is strongly correlated to the gap size. These quantitative conclusions hold up to a certain fragmentation of the pellet itself. These quantitative insights represent input for fuel rod design and verification in accidental scenarios and pave the way to dedicated models for axial gas flow in fuel performance codes
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
An improved methodology to determine pore size distribution and pore density in the high burnup structure
No full textIn the periphery of UO2 fuel pellets, a restructuring of the fuel occurs, leading to the formation of the high burnup structure (HBS). Main characteristics of the HBS are the refined grain structure following polygonisation/recrystallization and the steep increase of porosity. The newly formed micron-size intergranular pores trap most of the created fission gas. The study of the pore size-distribution and its evolution with burnup is fundamental to estimate the fission gas pressure in the pores and can influence the burst release in accidental conditions. Quantitative image analysis is usually employed to quantify the pore size-distribution. It is determined mainly in two-dimension (2D) using a histogram. Few attempts have been made in the past to derive also three-dimensional (3D) pore size-distribution employing the Schwartz-Saltykov method. In this work, we determine experimentally the pore size-distribution in the HBS and apply a new procedure to determine the distribution, both in 2D and 3D. A detailed characterisation of the pore size distribution and pore mean diameter is carried out in the burnup range 80-200 GWd/tHM. The evolution of the total pore density and mean pore diameter as a function of the local effective burnup give evidence of pore coarsening starting from 100 GWd/tHM, confirming previous investigations. The present data set the basis for modelling of bubble growth and coarsening in the HBS with a level of complexity suitable for applications in integral fuel performance codes like TRANSURANUS.JRC.G.I.3 - Nuclear Fuel Safet
3D reconstruction from 2D sections via genetic algorithms : an application to metallic fuel
Get PDFLAUREA MAGISTRALELa porosità del combustibile nucleare è una delle principali proprietà che ne influenzano il comportamento durante l’irraggiamento in reattore. Di conseguenza, la caratterizzazione della porosità e della sua evoluzione è cruciale per lo sviluppo di combustibili innovativi, come il combustibile metallico.
Diversi fenomeni fisici peculiari del combustibile, quali la formazione di fasi, il rilascio dei gas, il rigonfiamento da gas e la conducibilità termica sono influenzati dalla presenza e dalle caratteristiche della porosità. In particolare, la soglia di percolazione—la porosità limite alla quale un materiale poroso percola—è estremamente significativa, dal momento che governa il trasporto di prodotti di fissione nel combustibile. La percolazione è una proprietà intrinsecamente 3D e può essere ricavata solo tramite una conoscenza della struttura tridimensionale della proprietà. Questa conoscenza non è sempre disponibile sperimentalmente, essendo i metodi diretti per ottenerla—come micro-tomografia a raggi X (Micro-CT) e il sezionamento e ricostruzione con fascio di ioni focalizzati (FIB)— inapplicabili a materiali densi (quali il combustibile) e grandi volumi (necessari per la natura statistica della struttura).
Vista la difficoltà di caratterizzare sperimentalmente la porosità in 3D, l’approccio canonico è quello di impiegare metodi di ricostruzione che, partendo da immagini di sezioni sperimentali 2D, estraggano informazioni statistiche sulla microstruttura 3D. Da questa, si possono ottenere informazioni per calcolare, ad esempio, la succitata soglia di percolazione.
Diversi metodi di ricostruzione sono disponibili in letteratura, ciascuno basato su peculiari ipotesi stereologiche e processi di ottimizzazione. Tuttavia, la complessità della microstruttura della microstruttura 3D del combustibile nucleare (e metallico in particolare) rende necessario il rilassamento di alcune ipotesi di ricostruzione e la scelta di opportuni metodi di ottimizzazione. In questo lavoro di tesi si propone un nuovo metodo di ricostruzione basato su un algoritmo genetico e sulla definizione di una funzione obiettivo che sia slegata da precise ipotesi sulla struttura 3D da ricostruire. Il nuovo metodo è stato verificato su microstrutture 3D virtuali di riferimento, e quindi applicato alla ricostruzione di campioni di combustibile metallico DOE-1 della campagna di irraggiamento FUTURIX-FTA.Porosity is one of the main properties that affects irradiation performance of nuclear fuel. Its measurement and study his crucial for the characterization of innovative fuel concepts, such as metallic fuels, the performance of which is still under scrutiny.
The development of porosity impacts important phenomena such as phase formation, components migration, fission gas release, gas driven swelling, and thermal conductivity. The property of percolation threshold specifically—i.e., the limit porosity at which the porous material percolates—is extremely significant, since it governs the transport of fission products within the fuel itself. Percolation is a property that can be obtained only directly from 3D information on the porous material. This kind of information is however not always available, as direct methods to gain it—such as micro computed tomography (Micro-CT) and focused ion beam sectioning and scanning (FIB)—have serious limitations with dense irradiated materials and large volumes. Therefore, a simpler way to proceed is to adopt a reconstruction procedure that from images of 2D sections of experimental samples can extract statistical information on the 3D microstructure. From this, useful information can be gained, and percolation threshold can be computed.
There are several reconstruction procedures that have been introduced in other works. However, the complexity of the microstructure of metallic fuel makes necessary to introduce a new approach. Genetic Algorithms have several good properties that make them attractive in solving complex reconstruction problems. In this work we propose a reconstruction procedure based on a Genetic Algorithm. We verify it against known 3D microstructures, then apply it to experimental samples taken from DOE-1 metallic fuel from FUTURIX-FTA irradiation campaign
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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