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Introduction to neutron transport
No full textIntitulé de la formation : Master Nuclear Energy, option Nuclear Reactor Physics and EngineeringMaste
Introduction to neutron transport
No full textIntitulé de la formation : Master Nuclear Energy, option Nuclear Reactor Physics and EngineeringMaste
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
Neutron transport in stochastic geometries. Benchmark calculations and study of a melted reactor core
Get PDFLAUREA MAGISTRALENegli ultimi anni è andato crescendo l'interesse nella risoluzione dell'equazione del trasporto in mezzi stocastici. In particolare, vengono studiate le miscele binarie stocastiche, utilizzabili in molti campi (diffusione dei contaminanti radioattivi in mezzi geologici, trasporto in mezzi turbolenti per il confinamento della fusione inerziale, attraversamento di schermi di cemento da parte di neutroni e di raggi gamma,..). Le geometrie che trattiamo in questo lavoro sono supposte essere markoviane, cosa che non rispecchia la realtà della maggior parte dei casi che queste geometrie cercano di simulare, ma risulta essere un'approssimazione necessaria. Abbiamo preso in considerazione un problema di trasporto per cui si avevano già dei risultati di riferimento (ottenuti da una geometria planare composta da strati alternati di due materiali). La nostra geometria è invece composta da poligoni creati in modo casuale su un piano ed è markoviana in due dimensioni, cosa che la rende più affine e rappresentativa delle reali geometrie stocastiche. Lo scopo dei nostri calcoli di riferimento è quello di confrontare e analizzare i risultati precedentemente ottenuti e di testare la validità dei modelli del problema che sono stati elaborati. La realizzazione della geometria markoviana è basata sul processo descritto da Switzer nel 1964 che permette di creare una geometria planare con proprietà markoviane. Per ognuna delle molte realizzazioni geometriche effettuate, abbiamo risolto il problema di trasporto neutronico con il codice Monte Carlo creato al CEA, TRIPOLI-4. Abbiamo poi mediato i risultati e costruito gli istogrammi per avere informazioni anche sulle distribuzioni delle quantità cercate.Nella seconda parte della tesi, abbiamo utilizzato la stessa procedura di realizzazione di geometrie markoviane e di risoluzione dell'equazione del trasporto per simulare un nocciolo fuso di un PWR. Anche in questo caso, non conosciamo le distribuzioni dei materiali che compongono il core, ma solo le loro proprietà. Abbiamo analizzato tre diversi casi, ognuno caratterizzato da un certo livello di mescolanza degli elementi del nocciolo e abbiamo cercato il valore del k_eff, per vedere come varia e come si distribuisce quando i materiali si mescolano in modo casuale gli uni con gli altri.The interest in resolving the equation of transport in stochastic media has continued to increase these last years. Binary stochastic mixtures are particularly studied because of their several applications (diffusion of radioactive contaminants in geological media, transport in turbolent media for the inertial confinement fusion, crossing of neutrons or gamma rays throughout concrete shields, ...). The geometries we deal with in this present thesis are assumed to be Markovian, which is never the case in usual environments, but which is a necessary approximation. We consider a two-suite neutron transport problem for which other benchmark results (based on a stochastic geometry composed by a labeled plane filled with two alternating materials) were already performed. The geometry we use is composed by random polygons built in a plane and it is Markovian in two dimensions, thus it better represents a real stochastic geometry. The goal of our benchmark calculations is to compare and analyze the previous obtained results and to test the validity of different elaborated models of the problem. The creation of the Markovian planar geometry is done in accordance with the process described by Switzer in 1964, which allows to construct a two-dimensional geometry with Markovian properties. For each one of the several geometry realizations, we solve the neutron transport problem with the Monte Carlo code TRIPOLI-4, developed at CEA. Then, we average the results and we build histograms to see also the distributions. In the second part of the thesis, we use the same procedure of Markovian geometry creation and stochastic resolution of transport equation to simulate a melted core of a PWR reactor. In this case, we do not know the distributions of the materials composing the core and we study three cases, each one characterized by a different material mixing level. We obtain and analyze the value of k_eff, to see how it varies and distributes depending on the way the materials actually mix after the fusion of the core
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
Functional Expansion Tallies using Fission Matrix Eigenmodes for Full Core Criticality Simulations
No full textInternational audienceA new Functional Expansion Tally based on the Fission Matrix eigenmodes has been defined, to compute the assembly-integrated power distribution of a large power reactor. This new tally, while giving the same results as the usual assembly-integrated one when all the modes are used (there are as many modes as assemblies in the reactor) can have better performances when a reduced number of modes is used to reconstruct the 2D power map. This is due to the fact that the modes with the larger statistical noise can be discarded, and what is lost in the bias introduced, is gained a few-fold in less statistical noise. This favorable behavior is more pronounced when a limited number of histories is simulated. The new FM-FET can be used effectively when several similar configurations of a same large reactor need to be computed. As an example we used boron variations and temperature variations on a large 2D PWR description, and the numerical results show that a gain of a factor 5-10 can be achieved on the Figure of Merit. Applications to neutronics/thermo-hydraulic coupling are planned where iterative convergence demands to compute several costly Monte-Carlo solutions which are all successively discarded but the last one
Comment converger un calcul Monte-Carlo critique ?
No full textCe document présente les recherches qui ont étés effectuées autour des problématiques de convergence des codes Monte-Carlo sur les grands systèmes. Une modélisation de la convergence en tant que processus auto-régressif sur les amplitudes des modes propres est proposé. Ce modèle permet de déduire des formules simples qui caractérisent les vitesses de convergence des calculs monte-carlo. Il est également possible de se servir de ce modèle pour expliquer de nombreuses choses telles que la sous-estimation de la variance, les vitesses de convergences observées ou encore le clustering
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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