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Tesi sul tema "Nuclear fission"
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1
Hormaza, Joel Mesa. "Estudo de propriedades nucleares em regime de deformação fora do equilíbrio". Universidade de São Paulo, 2001. http://www.teses.usp.br/teses/disponiveis/43/43134/tde-05122013-110600/.
Testo completoAbstract (sommario):
Foi desenvolvido, implementado e testado um formalismo que permite descrever propriedades nucleares em regimes de deformação fora do equilíbrio, utilizando o método Macroscópico-Microscópico em associação com o Método Semimicroscópico Combinado. Esse formalismo foi utilizado para a interpretação e elucidação de alguns problemas abertos como as estruturas na seção de choque de fotofissão do 237.ANTPOT Np, distribuições angulares de fotofissão do 239.ANTPOT. Pu, e a concentração de transições M1 no 237.ANTPOT Np e 239.ANTPOT. Pu em baixas energias. Também, foram interpretadas informações experimentais referentes aos estados isoméricos do 237.ANTPOT Pu e 239.ANTPOT. Pu. Com relação ao potencial preditivo do formalismo, foram identificados os possíveis candidatos (spin e paridade) a estados isoméricos no 241.ANTPOT Np, bem como foi calculada também pela primeira vez a distribuição de momentos de nêutrons para o 239.ANTPOT Np fora da deformação de equilíbrio.It was developed, implemented and tested a formalism suitable to the description of nuclear properties at non equilibrium deformations regime, by using the Macroscopic-Microscopic method in association with the Combined Semimicroscopic Method. This formalism was used for the interpretation and elucidation of some open problems, as the structures in the 237Np photofission cross section, photofission angular distributions of 239Pu, and the concentrations of low-energy Ml transitions in 237Np and 239Pu. Also, experimental information on 237Pu and 239Pu isomeric states was interpreted. Concerning the prediction potentialities of the formalism, were identified possible candidates (spin and parity) for the 241Pu isomeric states, as well as the 239Pu neutron momentum distributions, at non equilibrium deformation, was also calculated for the first time.
2
Belloni, Francesca. "Neutron induced fission cross section measurements aimed at nuclear technology development". Doctoral thesis, Università degli studi di Trieste, 2010. http://hdl.handle.net/10077/3720.
Testo completoAbstract (sommario):
2008/2009Neutron induced fission cross sections of 233U, 238U, 241Am, 243Am and 245Cm in the energy range between 500 keV and 20 MeV obtained at the n_TOF Neutron Time of Flight facility at CERN (Geneve) are presented. Fission fragments had been detected by a gas counter with good discrimination between nuclear fission products and background events. A comparison between the extracted cross sections, previous experimental results and evaluated libraries is reported.
In questa tesi sono presentate le sezoni d'urto indotte da neutroni di energia compresa tra 500 KeV e 20 MeV su 233U, 238U, 241Am, 243Am e 245Cm ottenute alla facility di tempo di volo n_TOF al CERN (Geneve). I frammenti di fissione sono stati rivelati da un contatore a gas con una buona capacit\`a di discriminare i prodotti di fissione dagli eventi di background. La tesi riporta un paragone tra le sezioni estratte, i dati sperimentali ottenuti in passato da altri gruppi e le principali librerie valutative.
1980
3
Elmgren, Klas. "Experimental Fission Studies at Intermediate Energies". Doctoral thesis, Uppsala : Acta Universitatis Upsaliensis : Univ.-bibl. [distributör], 2002. http://publications.uu.se/theses/91-554-5335-X/.
Testo completo
4
Cantwell, Helena Rose. "Nuclear size control and homeostasis in fission yeast". Thesis, University College London (University of London), 2018. http://discovery.ucl.ac.uk/10054466/.
Testo completoAbstract (sommario):
Despite it being recognised as a problem worthy of consideration over a century ago, we have little mechanistic understanding of how the size of a cell’s nucleus is determined. The simply shaped fission yeast Schizosaccharomyces pombe is genetically tractable and undergoes a closed mitosis, making it a useful system in which to probe mechanisms of nuclear size control. In S. pombe cells, nuclear volume scales with cell volume, and not DNA content, across a wide range of cell volumes and throughout the cell cycle, maintaining a constant nuclear volume to cell volume (N/C) ratio. This thesis explores the mechanisms by which this scaling is achieved, using physiological, genetic and biochemical approaches. N/C ratio is perturbed and resultant nuclear and cellular growth rates of individual cells are assessed. N/C ratio homeostasis is observed. Both high and low aberrant N/C ratios correct rapidly in individual cells. Analysis of the kinetics of N/C ratio recovery is carried out and mathematical models of nuclear size control are proposed. To identify molecular components and biological processes with roles in nuclear size control mechanisms, a genetic screen for deletion mutants with aberrant nuclear size and biochemical analysis of a nuclear size mutant are carried out. Ribosome biogenesis, RNA processing and nucleocytoplasmic transport are all implicated in nuclear size control.
5
Gabro, Dany. "Virtual Studies of Nuclear Fission : A comparison of n- and p- induced fission using GEF". Thesis, Uppsala universitet, Tillämpad kärnfysik, 2017. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-324014.
Testo completoAbstract (sommario):
A General Description of Fission Observables (GEF) version 2016/1.2 is a software which simulates various types of fission. The main objective of this project is to compare the proton induced fission with the neutron induced fission of the same fissioning system. The proton induced fission was recently introduced to GEF and is relatively untested. Furthermore another task is to study the energy dependence in the same compound nucleus. The project will focus on simulating and comparing 238U(p,f) with 238Np(n,f) and 239Np*(f) as well as 232Th(p,f) with 232Pa(n,f) and 233Pa*(f ). The simulations were also compared to experimental data acquired by the division of applied nuclear physics at Uppsala University. The results show that the p-induced channel behaves very similar to the Compound Nucleus (CN) channel in contrary with the (n,f) channel. However when comparing the simulated data to the experimental data, there seems to be clear differences.
6
EL-HAJJE, REFAAT Safety Science Faculty of Science UNSW. "A SIMULTANEOUS MEASUREMENT OF THE ANGULAR DISTRIBUTION, MASS AND KINETIC ENERGY OF 235U AND 232Th FISSION FRAGMENTS". Awarded by:University of New South Wales. School of Safety Science, 2000. http://handle.unsw.edu.au/1959.4/17612.
Testo completoAbstract (sommario):
Simultaneous measurements of the angular distribution, mass distribution and average total kinetic energy of fission fragments produced by the neutron-induced fission of 235U and 232Th have been made using a gridded ionisation chamber. The neutron energy range used was thermal to 1.9 MeV for 235U and 1.4 to 1.7 MeV for 232Th. The following topics were investigated: the interdependence of the fission fragment angular and mass distribution; the anomalous behaviour of fragment anisotropy for 235U(n,f) at neutron energies En below 150 keV; the possible existence of a third symmetric mass peak for 232Th(n,f); the mass fine structure in 235U(n,f) and 232Th(n,f); and the dependence of the fission fragment average total kinetic energy on the excitation energy of the fissioning nucleus. For this study, mono-energetic neutrons were produced by the and reactions. Four signals produced by the fission chamber were fed into a data acquisition system and processed by a specially modified comprehensive computer program. The results indicate that there is no interdependence between the angular and mass distributions of fragments for 235U(n,f) and for 232Th(n,f). The angular distribution of 235U fission fragments showed an anisotropy of less than one for En below 150 keV. For 232Th, the expected minimum in the anisotropy near En = 1.6 MeV was confirmed. No evidence for a third peak in the mass symmetry region of 232Th(n,f) was observed, within the yield sensitivity limitation of the chamber. Fine structure was observed in the mass yield distributions for 235U(n,f) and 232Th(n,f) at mass locations predicted by theory. The fission fragment average total kinetic energy for 235U(n,f) and 232Th(n,f) showed no significant dependence on the excitation energy of the fissioning nucleus. Possible reasons for some of these results are advanced.
7
Ananthanarayanan, Vaishnavi. "Dynein dynamics during meiotic nuclear oscillations of fission yeast". Doctoral thesis, Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2014. http://nbn-resolving.de/urn:nbn:de:bsz:14-qucosa-135620.
Testo completoAbstract (sommario):
Cytoplasmic dynein is a ubiquitous minus-end directed motor protein that is essential for a variety of cellular processes ranging from cargo transport to spindle and chromosome positioning. Specifically, in fission yeast during meiotic prophase, the fused nucleus follows the spindle pole body in oscillatory movements from one cell pole to the other. The three molecular players that are essential to this process are: (i) the motor protein dynein, which powers the movement of the nucleus, (ii) microtubules, which provide the tracts for the movement and (iii) Num1, the anchor protein of dynein at the cortex. Dyneins that are localized to the anchor protein at the cortex and simultaneously bound to the microtubule emanating from the spindle pole body, pull on that microtubule leading to the movement of the nucleus. The spindle pole body, by virtue of its movement establishes a leading and a trailing side.
Previous work by Vogel et al. has elucidated the mechanism of these oscillations as that of asymmetric distribution of dynein between the leading and trailing sides. This differential distribution is a result of the load-dependent detachment of dynein preferentially from the trailing microtubules. This self-organization model for dynein, however, requires a continuous redistribution of dynein from the trailing to the leading side. In addition, dyneins need to be bound to the anchor protein to be able to produce force on the microtubules. Anchored dyneins are responsible for many other important processes in the cell such as spindle alignment and orientation, spindle separation and rotation. So we set out to elucidate the mechanism of redistribution of dynein as well as the targeting mechanism of dynein from the cytoplasm to cortical anchoring sites where they can produce pulling force on microtubules.
By employing single-molecule observation using highly inclined laminated optical sheet (HILO) microscopy and tracking of fluorescently-tagged dyneins using a custom software, we were able to show that dyneins redistributed in the cytoplasm of fission yeast by simple diffusion. We also observed that dynein bound first to the microtubule and not directly to the anchor protein Num1. In addition, we were able to capture unbinding events of single dyneins from the microtubule to the cytoplasm. Surprisingly, dynein bound to the microtubule exhibited diffusive behaviour. The switch from diffusive to directed movement required to power nuclear oscillations occurred when dynein bound to its cortical anchor Num1. In summary, dynein employs a two-step targeting mechanism from the cytoplasm to the cortical anchoring sites, with the attachment to the microtubule acting as the intermediate step.
8
NALLO, GIUSEPPE FRANCESCO. "Modelling liquid metals for nuclear fusion and fission reactors". Doctoral thesis, Politecnico di Torino, 2021. http://hdl.handle.net/11583/2928622.
Testo completo
9
Kingdon, David Ross. "Safety characteristics of a suspended-pellet fission reactor system". Thesis, National Library of Canada = Bibliothèque nationale du Canada, 1998. http://www.collectionscanada.ca/obj/s4/f2/dsk1/tape11/PQDD_0001/NQ42856.pdf.
Testo completo
10
Patel, N. M. "Speciation and separation of fission product rhodium". Thesis, Loughborough University, 1985. https://dspace.lboro.ac.uk/2134/7406.
Testo completoAbstract (sommario):
The rhodium speciation in nitric acid has been identified, primarily by the use of 103Rh Nuclear Magnetic Resonance Spectroscopy. The results have indicated that the rhodium species in stored high level waste (HLW) will range from the hexaaquo ion, {Rh(H20)6}3+-to complexes of the general formula {Rh(H20)6-n(NO2)nl(3-n)+, depending on the nitrite ion concentration. The solvent extraction of these complexes by dinonylnaphthalene sulphonic acid and various organo-phosphine sulphides has been investigated, and an integrated scheme for recovering rhodium from HLW is proposed.
11
Rêgo, Maria Eugênia de Melo. "Gestão dos rejeitos radioativos gerados na produção de 99Mo por fissão nuclear". Universidade de São Paulo, 2013. http://www.teses.usp.br/teses/disponiveis/85/85131/tde-16122013-113937/.
Testo completoAbstract (sommario):
O Brasil planeja construir uma planta de produção do 99Mo por fissão do 235U, devido à crescente utilização deste radioisótopo no setor de medicina nuclear. Neste trabalho buscou-se estimar as características físico-químicas e radiológicas dos rejeitos radioativos que serão gerados em tal instalação, além de fornecer subsídios teóricos a serem utilizados na definição do gerenciamento desses rejeitos. Estabelecidos dois cenários de produção para este projeto e utilizando-se o código Scale®, foram calculados os inventários radioisotópicos dos fluxos de rejeitos. Considerando-se o processamento químico dos alvos de urânio, foi possível caracterizar os rejeitos a partir de suas características químicas e radiológicas. Utilizando-se o software MicroShield®, determinou-se então as concentrações de atividade desses rejeitos em até 3 meses de produção. Este trabalho ainda apresenta o cálculo das taxas de dose variando-se tanto a espessura da blindagem, a ser utilizada numa embalagem para transporte in-site, quanto a quantidade de rejeito líquido retido em tal embalagem. Os radionuclídeos responsáveis pela maior parcela da dose de radiação foram identificados de forma a facilitar a determinação do método mais apropriado para o gerenciamento dos rejeitos após sua separação e antes de seu armazenamento. Os resultados aqui apresentados constituem uma compilação inicial do que é esperado em termos de rejeitos radioativos líquidos numa planta produtora de 99Mo e podem auxiliar no desenvolvimento do plano de gerenciamento de rejeitos de tal instalação.Brazil intends to build a facility for the 99Mo production through 235U fission, once this radioisotope is largely used in nuclear medicine. This study aimed at estimating the physical, chemical and radiological characteristics of radioactive waste expected to be generated in that facility, and to provide theoretical subsides that can be used on the definition of a proper waste management system. Two production scenarios were established and the radioisotope inventories of the wastes were calculated by Scale®. From the chemical processing of the uranium targets the wastes were characterized on their chemical and radiological features. MicroShield® was used to determine the activity concentrations up to three months of 99Mo production. In addition, this work presents dose rate calculation for several sizes of shielding and different amount of wastes, collected in a proper package for in-site transportation. Radionuclides responsible for higher doses were identified in order to facilitate choosing the most appropriate method for managing the wastes after their chemical separation and before their storage. These results are part of what is expected on radioactive wastes at a 99Mo production facility and might help on the development of the waste management planning for that facility.
12
Scheffler, Kathleen. "Microtubule-dependent nuclear congression in fission yeast and a novel factor in cellular morphogenesis of fission yeast". Thesis, Paris 6, 2014. http://www.theses.fr/2014PA066510/document.
Testo completoAbstract (sommario):
(I) J'ai étudié les mécanismes contrôlant la congression des noyaux pendant la conjugaison de la levure S. pombe. A l'aide d'imagerie à long terme basée sur la microfluidique, j'ai mesuré la durée précise de la congression nucléaire et démontré que deux moteurs moléculaires des MTs, la dynéine et la kinésine-14 Klp2 contribuent à ce processus, dans des voies parallèles. La dynéine s’associe aux SPBs. Son niveau au SPB dépend de la chaine légère intermédiaire Dli1 qui pourrait potentiellement stabiliser le complexe dynéine et est requise pour la congression. Klp2 se localise sur les MTs. La localisation différentielle des deux moteurs suggère des rôles distincts pour tirer les noyaux l'un vers l'autre. Klp2 pourrait induire le glissement de MTs antiparallèles émanant des SPBs, alors que la dynéine localisée au SPB pourrait tirer sur des MTs émanant du SPB opposé.(II) J'ai caractérisé un nouveau facteur morphogénétique, l’AAA+-ATPase Knk1, qui promeut la croissance linéaire chez S. pombe. L’absence de Knk1 provoque la formation d’un coude à proximité des extrémités cellulaires. Ce défaut ne résulte pas de défauts des MTs, qui participent à la linéarité de la croissance. Knk1 se localise aux extrémités de la cellule indépendamment des MTs et des câbles d’actine. Cette localisation requiert son N-terminus et est renforcée quand le domaine ATPase C-terminal lie l’ATP. La concentration de Knk1 aux extrémités est aussi contrôlée par Sla2 et Cdc42, de manière anti-correlée, et indépendamment de l’endocytose. Enfin, Knk1 oscille périodiquement entre les deux extrémités, indépendamment des oscillations de Cdc42, suggérant l'existence d'au moins deux systèmes oscillatoires séparés(I) I studied the molecular mechanisms underlying nuclear congression during fission yeast conjugation. Using microfluidic-based long-term imaging, I defined the precise timing of nuclear congression compared to cell mating and found that two MT molecular motors, dynein and the kinesin-14 Klp2 promote nuclear congression in parallel pathways. Dynein associates with SPBs. Dynein level at SPBs is controlled by the light intermediate chain Dli1 that may promote stabilization of the dynein complex and is essential for dynein-dependent nuclear congression, while dynactin is surprisingly not required for this process. Klp2 localizes along MTs. These differential localization patterns suggest distinct roles for the two motors in pulling the nuclei together: Klp2 may slide anti-parallel MTs emanating from the SPBs, while dynein at the SPB may pull on MTs emanating from the opposite SPB.(II) I characterized a novel morphogenetic factor, the AAA+-ATPase Knk1, supporting linear growth in fission yeast. knk1Δ cells display a kink close to cell tips, a unique shape phenotype that is neither caused by defects in behavior of MTs that promote linear extension. Knk1 localizes to cell tip independently of MTs and actin cables. This localization is mediated by Knk1 N-terminus and enhanced upon ATP binding to Knk1 C-terminal ATPase domain. Knk1 tip levels are enhanced in a sla2 or cdc42, independently of Sla2 role in endocytosis. Finally, Knk1 oscillates between the two cell tips in an anti-correlated periodic manner possibly uncoupled from Cdc42 oscillations suggesting the existence of at least two separated oscillatory systems contributing to fission yeast morphogenesis
13
Scheffler, Kathleen. "Microtubule-dependent nuclear congression in fission yeast and a novel factor in cellular morphogenesis of fission yeast". Electronic Thesis or Diss., Paris 6, 2014. https://accesdistant.sorbonne-universite.fr/login?url=https://theses-intra.sorbonne-universite.fr/2014PA066510.pdf.
Testo completoAbstract (sommario):
(I) J'ai étudié les mécanismes contrôlant la congression des noyaux pendant la conjugaison de la levure S. pombe. A l'aide d'imagerie à long terme basée sur la microfluidique, j'ai mesuré la durée précise de la congression nucléaire et démontré que deux moteurs moléculaires des MTs, la dynéine et la kinésine-14 Klp2 contribuent à ce processus, dans des voies parallèles. La dynéine s’associe aux SPBs. Son niveau au SPB dépend de la chaine légère intermédiaire Dli1 qui pourrait potentiellement stabiliser le complexe dynéine et est requise pour la congression. Klp2 se localise sur les MTs. La localisation différentielle des deux moteurs suggère des rôles distincts pour tirer les noyaux l'un vers l'autre. Klp2 pourrait induire le glissement de MTs antiparallèles émanant des SPBs, alors que la dynéine localisée au SPB pourrait tirer sur des MTs émanant du SPB opposé.(II) J'ai caractérisé un nouveau facteur morphogénétique, l’AAA+-ATPase Knk1, qui promeut la croissance linéaire chez S. pombe. L’absence de Knk1 provoque la formation d’un coude à proximité des extrémités cellulaires. Ce défaut ne résulte pas de défauts des MTs, qui participent à la linéarité de la croissance. Knk1 se localise aux extrémités de la cellule indépendamment des MTs et des câbles d’actine. Cette localisation requiert son N-terminus et est renforcée quand le domaine ATPase C-terminal lie l’ATP. La concentration de Knk1 aux extrémités est aussi contrôlée par Sla2 et Cdc42, de manière anti-correlée, et indépendamment de l’endocytose. Enfin, Knk1 oscille périodiquement entre les deux extrémités, indépendamment des oscillations de Cdc42, suggérant l'existence d'au moins deux systèmes oscillatoires séparés(I) I studied the molecular mechanisms underlying nuclear congression during fission yeast conjugation. Using microfluidic-based long-term imaging, I defined the precise timing of nuclear congression compared to cell mating and found that two MT molecular motors, dynein and the kinesin-14 Klp2 promote nuclear congression in parallel pathways. Dynein associates with SPBs. Dynein level at SPBs is controlled by the light intermediate chain Dli1 that may promote stabilization of the dynein complex and is essential for dynein-dependent nuclear congression, while dynactin is surprisingly not required for this process. Klp2 localizes along MTs. These differential localization patterns suggest distinct roles for the two motors in pulling the nuclei together: Klp2 may slide anti-parallel MTs emanating from the SPBs, while dynein at the SPB may pull on MTs emanating from the opposite SPB.(II) I characterized a novel morphogenetic factor, the AAA+-ATPase Knk1, supporting linear growth in fission yeast. knk1Δ cells display a kink close to cell tips, a unique shape phenotype that is neither caused by defects in behavior of MTs that promote linear extension. Knk1 localizes to cell tip independently of MTs and actin cables. This localization is mediated by Knk1 N-terminus and enhanced upon ATP binding to Knk1 C-terminal ATPase domain. Knk1 tip levels are enhanced in a sla2 or cdc42, independently of Sla2 role in endocytosis. Finally, Knk1 oscillates between the two cell tips in an anti-correlated periodic manner possibly uncoupled from Cdc42 oscillations suggesting the existence of at least two separated oscillatory systems contributing to fission yeast morphogenesis
14
Claisse, Antoine. "Open porosity fission gas release model applied to nuclear fuels". Licentiate thesis, KTH, Reaktorfysik, 2015. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-168502.
Testo completoAbstract (sommario):
Nitride fuels have gained a new interest in the last few years as both a candidate for GEN IV reactors and as accident tolerant fuels for current light water reactors. They however are decades behind oxide fuels when it comes to qualication and development of tools to assess their performances. In this thesis, such a tool is developed. The fuel performance codeTRANSURANUS, which has very good results with oxide fuels, is extended to handle nitride fuels. The relevant thermo-mechanical properties are implemented and fuel type dependent modules are updated. Their limitations and discrepancies are discussed. A particular attention is brought to the athermal ssion gas release, and a new model based on the open fabrication porosity is developed and added to the code, as a starting point toward a mechanistic model. It works well on oxide fuels, but its eciency is harder to evaluate for nitride fuels, due to large uncertainties on many key correlations such as the thermal conductivity and the eective diusion coecient of gas atoms. Recommendations are made to solve the most important problems.QC 20150604
15
CARON, DOMINIC. "Neutronics methods for the multiphysics analysis of nuclear fission systems". Doctoral thesis, Politecnico di Torino, 2017. http://hdl.handle.net/11583/2677635.
Testo completoAbstract (sommario):
The accurate representation of the overall behaviour of advanced nuclear fission systems, including both nuclear reactors and accelerator-driven subcritical systems, typically requires to account for the interactions among the various physical phenomena that occur in the complex system comprised by a nuclear reactor. Correspondingly, the numerical codes that are intended for application to the design and the analyses of such systems are required to couple individual physics models, perhaps most notably those for neutronics and thermal-hydraulics, as these two phenomena are of great importance for safety considerations. The doctoral research activity involves the development of physical models and mathematical methods for spatial-temporal analyses of neutronics that are appropriate for use in the context of multiphysics analyses of nuclear reactor dynamics. The activity is carried out through the management, the development and the application of a multiphysics reactor analysis code, the Fast REactor NEutronics/Thermal-hydraulICs (FRENETIC) code, developed at Politecnico di Torino. First, for questions related to the practicality of the execution of spatial kinetics analyses, the quasi-static method is formulated and methods of solution of the resulting systems of equations are developed and characterised. Next, as the accuracy and the efficiency of the quasi-static approach depend on the use of proper time steps, an automatic, adaptive time step selection methodology for the quasi-static method is developed and studied. Additionally, as the effects of photon energy deposition lead to non-negligible effects on the spatial-temporal behaviour of the power in a nuclear reactor, a methodology for coupled neutron-photon dynamics appropriate for use in spatial kinetics analyses is developed and assessed. Finally, with these models and methods implemented in the FRENETIC code, the first coupled neutronic/thermal-hydraulic validation of the FRENETIC code is performed against experimental data from a formerly operating sodium-cooled fast reactor.
16
BONIFETTO, ROBERTO. "Computational thermal-hydraulic modeling for nuclear fusion and fission applications". Doctoral thesis, Politecnico di Torino, 2014. http://hdl.handle.net/11583/2572946.
Testo completoAbstract (sommario):
Both nuclear fusion and fission devices have to face the issue of the cooling of some critical components, and in particular the cooling of the superconducting coils and of the core fuel assemblies, respectively.
In order to investigate these issues during the design phase of new reactors, e.g. new tokamak experiments and the forthcoming Generation IV reactors, new dedicated models and reliable tools should be developed. In this work, two separate tools, for fusion and fission devices, respectively, are developed relying on the same quasi-3D thermal-hydraulic model, i.e. the 1D advection flow of the coolant in a diffusive (solid) bundle along the channel axis and the 2D coupling between the channels.
The already available 4C code for the analysis of thermal-hydraulic transients in the superconducting coils of fusion devices is firstly validated against experimental data, then benchmarked against other tools and finally its predictive capability are checked performing the simulations before the experiment. The 4C code is thus proved to be able to simulate transients spanning from very short to week-long timescales with the very same model. The code is then applied to the analysis of operational transients in the ITER TF coils. Whenever the minimum temperature margin requirements are not satisfied, different possible mitigation strategies are investigated.
Concerning the fission devices, a new multi-physics simulation tool (FRENETIC) is developed for the quasi-3D analysis of a lead-cooled fast reactor core with the closed hexagonal fuel element configuration, as currently proposed within the framework of the European project LEADER. The code couples a neutronic and a thermal-hydraulic analysis of the full reactor core, each performed in separate modules. A first validation of the thermal-hydraulic module of this code is presented both in the single assembly and, even if preliminarly, in the multi-assembly configuration. In the latter case, a benchmark against another qualified tool (RELAP5-3D©) is also carried out. Finally the coupled code is tested in some simple transients in order to assess at least qualitatively the effectiveness of the coupling and the representativity of the multi-physics results.
17
McParland, Brian James. "A study of the 6Li([pi]+,3He)3He reaction at 60, 80 and 100 MeV". Thesis, University of British Columbia, 1985. http://hdl.handle.net/2429/25934.
Testo completoAbstract (sommario):
An experimental study of the pion-induced fission, ⁶Li (π⁺, ³He) ³He, has been performed at TRIUMF using 60, 80 and 100 MeV pions. Angular distributions for this reaction at these energies, along with the energy dependence at fixed center-of-mass angles, are presented. Two theoretical models of this reaction predict widely differing angular and energy dependences. Prior to this experiment, the available data on the ⁶Li (π⁺, ³He) ³He reaction (and its inverse, ³He (³He, π⁺) ⁶Li) were insufficient to determine which of the two calculations better represent the reaction. The new data presented here have thoroughly tested these two models in this energy regime and have determined their suitability in their descriptions of the ⁶Li (π⁺, ³He) ³He reaction.
From these data (and from results previously published for the inverse ³He (³He, π⁺) ⁶Li reaction at an equivalent pion energy of 15.4 MeV), the differential cross-sections were fit to an orthogonal Legendre polynomial series at each energy. These fits allowed the total cross-section to be extracted as a function of pion energy between 15.4 and 100 MeV. The total cross-section, and the center-of-mass differential cross-section at a fixed center-of-mass angle, were found to exhibit an exponential decrease with pion energy over this range. The coefficients of these polynomial fits also clearly show the growing importance of higher-order partial-waves with increasing energy.
Finally, a phenomenological search for systematics in the world data of the ⁶Li (π⁺, ³He) ³He and ³He (³He, π⁺) ⁶Li reactions was made. This attempt was successful in finding a dependence of the reaction upon the spin-state of the exit channel which is similar to that previously seen in (p, π⁺) experimental data.Science, Faculty of
Physics and Astronomy, Department of
Graduate
18
Prokofiev, Alexander. "Nucleon-Induced Fission Cross Sections of Heavy Nuclei in the Intermediate Energy Region". Doctoral thesis, Uppsala : Acta Universitatis Upsaliensis : Univ.-bibl. [distributör], 2001. http://publications.uu.se/theses/91-554-5009-1/.
Testo completo
19
Leong, Lou Sai. "Fission fragment angular distribution and fission cross section validation". Phd thesis, Université Paris Sud - Paris XI, 2013. http://tel.archives-ouvertes.fr/tel-00924483.
Testo completoAbstract (sommario):
The present knowledge of angular distributions of neutron-induced fission is limited to a maximal energy of 15 MeV, with large discrepancies around 14 MeV. Only 238U and 232Th have been investigated up to 100 MeV in a single experiment. The n_TOF Collaboration performed the fission cross section measurement of several actinides (232Th, 235U, 238U, 234U, 237Np) at the n_TOF facility using an experimental set-up made of Parallel Plate Avalanche Counters (PPAC), extending the energy domain of the incident neutron above hundreds of MeV. The method based on the detection of the 2 fragments in coincidence allowed to clearly disentangle the fission reactions among other types of reactions occurring in the spallation domain. I will show the methods we used to reconstruct the full angular resolution by the tracking of fission fragments. Below 10 MeV our results are consistent with existing data. For example in the case of 232Th, below 10 MeV the results show clearly the variation occurring at the first (1 MeV) and second (7 MeV) chance fission, corresponding to transition states of given J and K (total spin and its projection on the fission axis), and a much more accurate energy dependence at the 3rd chance threshold (14 MeV) has been obtained. In the spallation domain, above 30 MeV we confirm the high anisotropy revealed in 232Th by the single existing data set. I'll discuss the implications of this finding, related to the low anisotropy exhibited in proton-induced fission. I also explore the critical experiments which is valuable checks of nuclear data. The 237Np neutron-induced fission cross section has recently been measured in a large energy range (from eV to GeV) at the n TOF facility at CERN. When compared to previous measurements, the n TOF fission cross section appears to be higher by 5-7 % beyond the fission threshold. To check the relevance of n TOF data, we simulate a criticality experiment performed at Los Alamos with a 6 kg sphere of 237Np. This sphere was surrounded by enriched uranium 235U so as to approach criticality with fast neutrons. The simulation predicts a multiplication factor keff in better agreement with the experiment (the deviation of 750 pcm is reduced to 250 pcm) when we replace the ENDF/B- VII.0 evaluation of the 237Np fission cross section by the n TOF data. We also explore the hypothesis of deficiencies of the inelastic cross section in 235U which has been invoked by some authors to explain the deviation of 750 pcm. The large distortion that should be applied to the inelastic cross sections in order to reconcile the critical experiment with its simulation is incompatible with existing measurements. Also we show that the nubar of 237Np can hardly be incriminated because of the high accuracy of the existing data. Fission rate ratios or averaged fission cross sections measured in several fast neutron fields seem to give contradictory results on the validation of the 237Np cross section but at least one of the benchmark experiments, where the active deposits have been well calibrated for the number of atoms, favors the n TOF data set. These outcomes support the hypothesis of a higher fission cross section of 237Np.
20
IBRAHIM, Fadi. "Production et étude des fragments de fission, de LOHENGRIN à ALTO". Habilitation à diriger des recherches, Université Paris-Diderot - Paris VII, 2005. http://tel.archives-ouvertes.fr/tel-00009789.
Testo completoAbstract (sommario):
L'étude des noyaux loin de la stabilité est constitutive de l'histoire de la physique nucléaire dès son origine, et connaît un essor considérable. Parmi les nucléides les plus éphémères, ceux situés aux frontières de notre connaissance sont qualifiés d' « exotiques ». Etudier ces noyaux, c'est se donner la chance d'accéder à des informations nouvelles sur la structure nucléaire et ainsi mesurer la solidité de notre conception de la matière nucléaire et sa validité lorsque cette dernière est poussée dans des états « extrêmes ». Le travail présenté dans cette habilitation à diriger des recherches concerne l'étude et la production des noyaux exotiques riches en neutrons produits par le mécanisme de la fission. L'originalité du travail effectuer consiste en partie en une étude croisée des mécanismes de production de faisceaux radioactifs riches en neutrons, tant par la méthode dite « en vol » que par la méthode ISOL (Isotope Separation On Line). Le travail porte sur des études effectuées auprès du spectromètre LOHENGRIN installé à l'ILL de Grenoble, ainsi que sur le programme PARRNe (Production de Noyaux Radioactifs Riches en Neutrons) qui constituait au début du projet en une installation dédiée à une activité de Recherche et Développement pour les futures installations de deuxième générations de faisceaux radioactifs. Les difficultés inhérentes aux techniques de production des noyaux exotiques ont laissé en particulier une grande part des noyaux riches en neutrons dans l'inconnu. Afin d'accéder à ces réserves de découvertes intactes, il est nécessaire de produire ces noyaux exotiques non seulement en plus grande quantité possible mais aussi sous forme de faisceaux accélérés. Les problèmes que soulève la mise en œuvre de tels faisceaux d'ions radioactifs suscite à l'heure actuelle un énorme effort de recherche et développement au sein de notre discipline. Depuis plusieurs années, l'Institut de Physique Nucléaire d'Orsay s'est montré à la pointe en ce domaine grâce au programme PARRNe. La réaction nucléaire choisie pour produire les noyaux exotiques dans le cadre de PARRNe est la fission de l'U238. Les noyaux produits par la fission sont dits « de masse intermédiaire » et sont très excédentaires en neutrons. La technique de « mise à disposition » de ces noyaux retenue est celle dite « en cible épaisse » ou méthode ISOL. Le programme a donc pour objectif la détermination des conditions optimales de production des noyaux riches en neutrons issus de la fission de l'U238 par la technique ISOL. Lors de la réaction, les produits de la fission apparaissent sous forme atomique dans la cible, la difficulté est ensuite de les en faire sortir pour éventuellement les ioniser et ainsi pouvoir les accélérer. La facilité avec laquelle les éléments produits lors de la fission sortent de la cible dépend de leurs propriétés physico-chimiques. L'optimisation des conditions de production passe donc par l'optimisation des conditions de la fission (l'énergie et la nature du faisceau incident, le type et la géométrie du convertisseur, la structure de la cible) et par l'optimisation des conditions d'extraction et d'ionisation des produits de fission (optimisation de l'ensemble cible-source d'ions). Pour mettre en œuvre ce programme une ligne de type ISOL, dotée d'un séparateur en masse, a été installée au TANDEM. Ce montage permet de s'approcher au mieux des conditions expérimentales qui seront rencontrées sur la future installation SPIRAL 2 au GANIL. Les résultats obtenus avec les faisceaux du tandem ont été tels qu'il a été possible d'initier un programme de physique original auprès de cet outil initialement destiné à des études de R&D. L'une des originalités du travail effectué consiste en la mise en place d'un programme de physique original auprès de PARRNe sur l'étude de l'évolution de la magicité pour les noyaux exotiques proches de la couche fermée en neutron N=50. Ces premiers résultats très prometteurs préfigurent les futures expériences auprès des accélérateurs de deuxième génération.
21
Oberer, Richard B. "Fission multiplicity detection with temporal gamma-neutron discrimination from higher-order time correlation statistics". Diss., Georgia Institute of Technology, 2002. http://hdl.handle.net/1853/17632.
Testo completo
22
Todd, Jamie R. D. (James Robert Drysdale). "Angular distributions and kinetic energies of fission products from the 238U(12C,f) reaction". Thesis, McGill University, 1991. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=59911.
Testo completoAbstract (sommario):
The angular anisotropy, $ omega$, of individual fission products produced from the $ sp{238} rm U( sp{12}$C, f) 119.9 MeV incident heavy-ion induced fission reaction, was modelled in terms of the angular momentum, l, conferred upon the intermediate nucleus. Evidence of incomplete momentum transfer led to a model treating fission products as being the result of two fission inducing processes: complete fusion resulting in compound nucleus formation, and an $ alpha$-transfer incomplete fusion process. The average angular anisotropies for each of the two processes were calculated to be, $ omega sb{ rm CF}$ = 2.57, and $ omega sb alpha$ = 1.71, respectively, which fit well to the experimental data. A new method was developed to estimate the total kinetic energy release of heavy-ion fission events leading to individual fission products from the above reaction. The average total $ langle$E$ sb{ rm k}{ rangle} approx 195$ MeV calculated using the new method is consistent with other experimental data, and with theories regarding heavy-ion induced fission.
23
Fiorito, Luca. "Nuclear data uncertainty propagation and uncertainty quantification in nuclear codes". Doctoral thesis, Universite Libre de Bruxelles, 2016. http://hdl.handle.net/2013/ULB-DIPOT:oai:dipot.ulb.ac.be:2013/238375.
Testo completoAbstract (sommario):
Uncertainties in nuclear model responses must be quantified to define safety limits, minimize costs and define operational conditions in design. Response uncertainties can also be used to provide a feedback on the quality and reliability of parameter evaluations, such as nuclear data. The uncertainties of the predictive model responses sprout from several sources, e.g. nuclear data, model approximations, numerical solvers, influence of random variables. It was proved that the largest quantifiable sources of uncertainty in nuclear models, such as neutronics and burnup calculations, are the nuclear data, which are provided as evaluated best estimates and uncertainties/covariances in data libraries. Nuclear data uncertainties and/or covariances must be propagated to the model responses with dedicated uncertainty propagation tools. However, most of the nuclear codes for neutronics and burnup models do not have these capabilities and produce best-estimate results without uncertainties. In this work, the nuclear data uncertainty propagation was concentrated on the SCK•CEN code burnup ALEPH-2 and the Monte Carlo N-Particle code MCNP.Two sensitivity analysis procedures, i.e. FSAP and ASAP, based on linear perturbation theory were implemented in ALEPH-2. These routines can propagate nuclear data uncertainties in pure decay models. ASAP and ALEPH-2 were tested and validated against the decay heat and uncertainty quantification for several fission pulses and for the MYRRHA subcritical system. The decay uncertainty is necessary to define the reliability of the decay heat removal systems and prevent overheating and mechanical failure of the reactor components. It was proved that the propagation of independent fission yield and decay data uncertainties can be carried out with ASAP also in neutron irradiation models. Because of the ASAP limitations, the Monte Carlo sampling solver NUDUNA was used to propagate cross section covariances. The applicability constraints of ASAP drove our studies towards the development of a tool that could propagate the uncertainty of any nuclear datum. In addition, the uncertainty propagation tool was supposed to operate with multiple nuclear codes and systems, including non-linear models. The Monte Carlo sampling code SANDY was developed. SANDY is independent of the predictive model, as it only interacts with the nuclear data in input. Nuclear data are sampled from multivariate probability density functions and propagated through the model according to the Monte Carlo sampling theory. Not only can SANDY propagate nuclear data uncertainties and covariances to the model responses, but it is also able to identify the impact of each uncertainty contributor by decomposing the response variance. SANDY was extensively tested against integral parameters and was used to quantify the neutron multiplication factor uncertainty of the VENUS-F reactor.Further uncertainty propagation studies were carried out for the burnup models of light water reactor benchmarks. Our studies identified fission yields as the largest source of uncertainty for the nuclide density evolution curves of several fission products. However, the current data libraries provide evaluated fission yields and uncertainties devoid of covariance matrices. The lack of fission yield covariance information does not comply with the conservation equations that apply to a fission model, and generates inconsistency in the nuclear data. In this work, we generated fission yield covariance matrices using a generalised least-square method and a set of physical constraints. The fission yield covariance matrices solve the inconsistency in the nuclear data libraries and reduce the role of the fission yields in the uncertainty quantification of burnup models responses.Doctorat en Sciences de l'ingénieur et technologie
info:eu-repo/semantics/nonPublished
24
Minoda, Akiko. "The role of fission yeast nuclear actin-related protein in mitosis". Thesis, University College London (University of London), 2005. http://discovery.ucl.ac.uk/1444847/.
Testo completoAbstract (sommario):
Nuclear actin-related proteins (Arps) have 20-30% identity to conventional actin and many are found to be in chromatin remodelling complexes. There are two families of chromatin remodelling complexes, one of which carries out covalent modification on histones such as acetylation. The other is an ATPase complex, which alters the nucleosomal spacing, and nuclear Arps are found in both complexes. These complexes are believed to be required for transcriptional activation by increasing the accessibility of the transcription machinery to the target DNA. The alp5-1134 mutant was isolated from a screen for temperature-sensitive (ts) mutants with altered polarity and shows severe mitotic defects. Cloning of alp5+ revealed that Alp5 is an essential actin-related protein, most similar to budding yeast Arp4 and human BAF53. Alp5 localises to the nucleus and immunoprecipitates with Mst1, a histone acetyltransferase. These results strongly indicate the role of Alp5 in chromatin remodelling process, as its homologues. Given the interaction between Alp5 and Mst1, its function in histone acetylation was investigated both genetically and biochemically. It was found that Alp5 is required for acetylating the N-terminus tail of histone H4 lysine residues, and functionally counteracts with the histone deacetylases Clr6, Hst4 and Sir2. At the restrictive temperature, the alp5-1134 mutant shows a mitotic delay due to the activation of a spindle assembly checkpoint, which suggests a defect in the kinetochore-spindle interaction. This study also reveals that the function of Alp5 is required for the transcriptional repression at the core centromere region. Possible roles of Alp5 in mitosis are discussed.
25
Hoffmann, S. M. A. "Fission/spallation competition in the de-excitation of '1'8'1RE". Thesis, University of Manchester, 1987. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.233085.
Testo completo
26
Martin, Jerry Lynn. "DABLE--a facility for measuring fission product transport in gas-cooled reactors". Thesis, Massachusetts Institute of Technology, 1991. http://hdl.handle.net/1721.1/13906.
Testo completo
27
Edwards, Ellen. "Measurement of Krypton Fission Product Yields from 14 MeV Neutrons on 238U". Thesis, University of California, Berkeley, 2018. http://pqdtopen.proquest.com/#viewpdf?dispub=10930784.
Testo completoAbstract (sommario):
Precisely-known fission yield distributions are used to determine a fissioning isotope and the incident neutron energies in nuclear security applications. 14 MeV neutrons from DT fusion at the National Ignition Facility (NIF) induced fission in depleted uranium (DU) contained in the target assembly hohlraum. The fission yields of Kr isotopes (85m, 87, 88, 89, and 90) were measured relative to the cumulative yield of 88Kr. The fission gas was pumped from the target chamber, collected, and analyzed in the Radiochemical Analysis of Gaseous Samples (RAGS) diagnostic. Isotopes with half-lives ranging 8 s-9 hr can be measured. Kr fission yields have been measured both from the fission of DU in the hohlraum and DU doped into the capsule ablator. Since the mass of U was not known, the relative amounts of Kr isotopes were calculated and compared to existing fission product distribution tables. It was found that measurements can be performed with high precision for isotopes with half lives longer than 4 minutes. A more precise quantification of gas transport needs to be achieved to quantify isotopes with shorter half lives to a precision of the published tables.
28
Bagher, Nori Mohammad. "Derivation of the angular momentum of primary fission fragments from isomeric yield ratio by TALYS using Python". Thesis, Uppsala universitet, Tillämpad kärnfysik, 2021. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-436869.
Testo completoAbstract (sommario):
The general fission process is well known and is applied in nuclear power plants all over the world. However many properties of fission fragments are still not well understood. The angular momentum distribution of fission fragments is an important property to gain a better understanding of the fission process, and that can be derived indirectly from isomeric yield ratios. The goal of this project has been to develop a script in Python that runs the nuclear reaction code TALYS with the Total Monte Carlo method to calculate the isomeric yield ratio. The script generates a matrix consisting of excitation energies and angular momenta that is provided to TALYS. One matrix corresponds to one calculation of the isomeric ratio. Thus, the dependency of the isomeric yield ratio on these matrices can be observed. After looking into the matrices, the dependencies of the isomeric yield ratios on the excitation energies and the angular momentum distribution are observed. In this project, the calculated isomeric yield ratios are compared with the experimental value obtained from an experiment conducted in August of 2019 at the IGISOL-JYFLTRAP facility in Jyväskylä, Finland. It is worth mentioning that, fission system is of Uranium-238 which was induced by a proton beam at an energy of 25 MeV. The dependency of the isomeric yield ratio (IYR) on the angular momentum and the excitation energy has been investigated. However, it has proved more difficult than expected, to deduce an estimation for the angular momentum distribution. Another finding of this project is that the two codes used, GEF and TALYS sometimes produce inconstant results.
29
Bao, Xun. "Identifying the nuclear envelope receptor of Mto1 in fission yeast Schizosaccharomyces pombe". Thesis, University of Edinburgh, 2017. http://hdl.handle.net/1842/28729.
Testo completoAbstract (sommario):
Microtubules are essential components of eukaryotic cytoskeleton and play a crucial role in variety of cell activities, such as cell mobility, intracellular transportation, cell division and organelle spatial organization. The initiation of microtubule nucleation is an important event to trigger the microtubule growth from different microtubule organizing centres (MTOCs) during the cell cycle in fission yeast. In interphase, the major MTOCs in the cells are nuclear envelope (NE) and microtubules existing in the cytoplasm. During mitosis, spindle pole body (SPB) that is the centrosome equivalent is the MTOC where astral and spindle microtubules initiate from. Once cells enter anaphase, the post-anaphase array (PAA) of microtubules will initiate from the equatorial MTOC (eMTOC) at the cell division site. To initiate microtubule nucleation, γ-tubulin small complex (γ-TuSC) will be recruited to the MTOC to form the “lock-washer” like γ-tubulin ring complex (γ- TuRC) as the scaffold of the microtubule. Fission yeast γ-TuSC is composed of two molecules of γ-tubulin and one each of GCP2 and GCP3 homologue, Alp4 and Alp6, respectively. Apart from Alp4 and Alp6, the homologue of human GCP4, GCP5 and GCP6, named Ghf1, Mod21 and Alp16 were identified as independent components of γ-TuRC in fission yeast. Protein Mto1 form a complex with its partner Mto2 and the complex directly interacts with γ-TuSC at all the MTOCs through the cell cycle. Mto1 is a large coiled-coil protein composed of 1115 amino acids. It has three main functional domains, including an N-terminal ~60 amino acids region termed CM1 motif that is required for the recruitment γ-TuSC to MTOCs, a central region that is required for the interaction with Mto2 at all cytoplasmic MTOCs and a ~44 amino acids region close to the C-terminus (named MASC) which is required for the binding of Mto1 to SPBs and eMTOC. The C-terminal truncation for Mto1 shows Mto1[1-549]-GFP mainly localizes on the NE and this Mto1 mutant is still functional for microtubule nucleation. It is referred as “Mto1[NE]”. In addition, truncation of 1-130 amino acids region for Mto1[1-549]-GFP creates the smallest Mto1 mutant that is able to initiate the microtubule nucleation in cytoplasm in a random manner. This Mto1[131-549]-GFP mutant fails to localize at any MTOCs, including NE. It is then referred as “Mto1[bonsai]”. To understand the mechanism that how is Mto1[NE] recruited to the NE, I performed two-step purification and mass spectrometry for both Mto1[1-9A1-549] and Mto1[131-9A1-549] strains, which form more significant puncta on both NE and cytoplasm respectively, to identify the potential receptor(s) of Mto1[1-9A1-549] on the nuclear envelope by comparing the different interactomes of Mto1[1-9A1-549] and Mto1[131-9A1-549]. Here, I show both exportin Crm1 and nucleoporin Nup146 are essential for the binding of Mto1 to the NE. I find the Localization of Mto1[1-9A1-549] GFP on the NE depends on binding of Mto1 to Crm1 via a nuclear export signal (NES)-like sequence within the N-terminus of Mto1. Further, I figure out that Spi1GTP (RanGTP in fission yeast) is involved in the formation of Crm1-Spi1GTP-Mto1 complex and required for the interaction of Mto1 to the NE. In addition, I also find that the FG repeats of Nup146 are essential for the binding of Mto1 to the nuclear envelope. It’s likely Nup146 anchors Crm1-Spi1GTP-Mto1 complex to the NE through the interaction between its FG repeats and Crm1. Last, my data are consistent with previous findings that the association of Mto1 to the NE is important for the microtubule nucleation from the NE.
30
Ledesma, Michelle N. (Michelle Nicole) 1975. "Medical room design for a fission converter-based boron neutron capture therapy facility". Thesis, Massachusetts Institute of Technology, 1998. http://hdl.handle.net/1721.1/50533.
Testo completo
31
Kiger, William Steadman. "Neutronic design of a fission converter-based ephithermal beam for neutron capture theory". Thesis, Massachusetts Institute of Technology, 1996. http://hdl.handle.net/1721.1/39761.
Testo completo
32
Sutharshan, Balendra 1966. "Engineering design of a fission converter-based epithermal beam for neutron capture therapy". Thesis, Massachusetts Institute of Technology, 1998. http://hdl.handle.net/1721.1/49671.
Testo completoAbstract (sommario):
Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Nuclear Engineering, 1998.Includes bibliographical references.
There is a need for high intensity and low contamination epithermal neutron beams, for boron neutron capture therapy research and for routine treatment if this becomes a successful modality for treating cancers. A fission converter based design for high performance epithermal neutron beam was developed at the Nuclear Reactor Laboratory of the Massachusetts Institute of Technology. This epithermal beam has the capability of delivering a treatment within minutes with negligible background contamination. This thesis deals with the engineering design of this facility. This fission converter based epithermal facility will be installed in the present thermal column and hohlraum of the MITR-II. The fission converter tank, which contains eleven MITR-II fuel elements, was designed with minimum front plate thickness to minimize neutron absorption. The converter tank structural analysis was performed analytically, and numerically with the finite element code ADINA. Three heat removal designs, natural convection, forced convection with bypass channel and simple forced convection, were considered to remove heat from eleven fuel elements in the fission converter tank. For all three designs, steady state and transient analyses were performed. The transient analysis includes loss of flow, loss of flow with shutter failure (converter scram) and loss of coolant. Most of these steady state and transient analyses were performed by both analytically and numerically. The results show that the fuel disruption will not occur during credible and incredible accident scenarios. Three shutters, cadmium shutter, water shutter and mechanical fast acting shutter were designed to control the beam at the patient position. The shutters were designed using the Monte Carlo radiation transport code MCNP. A medical irradiation room was designed for patient irradiation. The shielding computations for the medical room were also carried out with the MCNP. Fuel handling was investigated and proposed two options for refueling, which are similar to the MITR-II refueling.
by Balendra Sutharshan.
Ph.D.
33
Riley, Kent J. (Kent Jason). "Construction and characterization of a fission converter based epithermal neutron beam for BNCT". Thesis, Massachusetts Institute of Technology, 2001. https://hdl.handle.net/1721.1/123100.
Testo completoAbstract (sommario):
Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Nuclear Engineering, 2001.Includes bibliographical references.
This study demonstrates the first successful implementation of a fission converter to produce a source of epithermal neutrons suitable for BNCT. The final design, construction and characterization of a new epithermal neutron beam is presented. A high intensity source with low contamination is obtained using a fission converter driven by thermal neutrons from the MIT research reactor. The facility is housed in the experimental hall and operates in parallel with other user applications. The fission converter is powered by 10 spent MITR-II fuel elements and employs resonance scattering filters with thermal neutron absorbers to tailor the neutron energy distribution. A lead shield attenuates photon contamination in the beam and lead collimators direct the neutron beam toward the patient. A horizontal beamline leads to the new medical room which is built with 1.1 m thick, high density concrete walls and is large enough to permit various treatment configurations. Ambient dose equivalent rates outside the shielded room are < I mrem/hr with the converter operating at full power and do not interfere with other experimental users and reactor operations.
(cont.) Beam delivery is controlled with three in-line shutters that allow unrestricted access to the medical room while the reactor is at full power. Patient irradiations are controlled by redundant programmable logic controllers that automatically close the beam shutters when the prescribed monitor counts have been accumulated. Measurements were performed on central axis to assess beam performance. An in-air epithermal neutron flux of 8.4 +/- 0.8 E+09 n/cm2s was obtained with concomitant fast neutron and photon absorbed dose rates of 3.9 +/- 0.5 and 11.8 +/- 0.8 cGy/min. Depth dose profiles measured in-phantom are in general agreement with those determined from Monte Carlo calculations and indicate that normal tissue tolerance can be reached in treatment times of less than 10 minutes. The in-beam fast neutron and photon contaminants account for less than 10% of the dose received by normal tissue surrounding the target volume, which approaches the clinical optimum.
by Kent J. Riley.
Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Nuclear Engineering, 2001.
34
Dobrowolski, Artur. "Influence of differences in the proton and neutron distributions on nuclear fusion and fission". Université Louis Pasteur (Strasbourg) (1971-2008), 2006. https://publication-theses.unistra.fr/public/theses_doctorat/2006/DOBROWOLSKI_Artur_2006.pdf.
Testo completoAbstract (sommario):
Dans ce travail de thèse des ingrédients essentiels pour la description théorique de la dynamique des réactions de fusion et fission nucléaires sont étudiés, tel que le potentiel d'interaction entre noyaux cible et projectile pour le processus de fusion et l'énergie de déformation dans un espace multidimensionnel pour la fission. Nous avons évalué en particulier l'importance de la différence des distributions de densités protons et neutrons. Pour le processus de fusion, le potentiel d'interaction entre les noyaux peut être déterminé à travers des densités obtenues d'une façon auto-consistante par des calculs variationnels semi-classiques à partir d'une interaction nucléon-nucléon effective de type Skyrme. Les barrières de fusion ainsi obtenues permettent d'évaluer des sections efficaces de fusion dans le cadre du formalisme de Langevin. Pour le processus de fission il est essentiel de tenir compte de la grande richesse de formes nucléaires qui apparaissent tout au long du chemin de fission de l'état fondamental jusqu'au point de scission. Nous montrons qu'une paramétrisation tenant compte de l'élongation, ainsi que de la possible constriction, asymétrie gauche-droite et non-axialité du noyau, est effectivement capable dans le cadre de notre approche macroscopique-microscopique de donner une description précise de ce phénomène. On peut ainsi enrichir l'expression de l'énergie de type goutte liquide par un terme qui décrit la variation de l'énergie de liaison nucléaire due aux différentes déformations des distributions protons et neutrons. La réduction des hauteurs des barrières de fission qui en résulte est seulement de l'ordre du MeV, mais il peut facilement en résulter un changement de la section efficace de fission d'un ordre de grandeur et ainsi jouer un rôle capital pour la stabilité des noyaux super-lourds ou des noyaux exotiquesThis thesis work is centred on some essential ingredients of a theoretical description of the reaction dynamics of the nuclear fusion and fission process, such as the interaction potential between projectile and target nuclei for fusion and the deformation energy landscape in a multidimensional space for the fission process. We have in particular evaluated the importance of the difference between the neutron and proton density distributions on these two precesses. The fusion potential between the two interacting nuclei is obtained through the nucleon densities, determined in a self-consistent way through semiclassical density variational calculations for a given effective nucleon-nucleon effective interaction of the Skyrme type. These fusion barriers can then be used in a Langevin formalism to evaluation fusion cross sections. For the fission process it turns out to be essential to allow for the large variety of shapes which appear between the nuclear ground state and the scission configuration. We show that a shape parametrisation taking into account elongation, as well as possible neck formation, left-right asymmetry and non-axiality allows, indeed, for a precise description of this phenomena in the framework of the macroscopic-microscopic approach. We are thus able to enrich the expression of the liquid-drop type energy through a term which describes the variation of the nuclear energy due to a deformation difference between the proton and neutron distribution. The resulting reduction of the fission barriers is only of the order of an MeV but this can easily cause an change in the fission cross section by an order of magnitude and thus play an capital role for the stability of super-heavy of exotic nuclei
35
Dobrowolski, Artur Bartel Johann Pomorski Krzysztof. "Influence of differences in the proton and neutron distributions on nuclear fusion and fission". Strasbourg : Université Louis Pasteur, 2008. http://eprints-scd-ulp.u-strasbg.fr:8080/894/01/DOBROWOLSKI_Artur_2006.pdf.
Testo completoAbstract (sommario):
Thèse de doctorat : Physique Nucléaire Théorique : Strasbourg 1 : 2006. Thèse de doctorat : Physique Nucléaire Théorique : Université Marie Curie-Skłodowska de Lublin (Pologne) : 2006.Thèse soutenue en co-tutelle. Titre provenant de l'écran-titre. Bibliogr. 5 p.
36
De, Bona Emanuele. "Grain size effects on radiogenic Helium gas in the nuclear fuel UO2". Thesis, Université Paris-Saclay (ComUE), 2019. http://www.theses.fr/2019SACLC084.
Testo completoAbstract (sommario):
Dû à l’environnement extrême dans lequel il est exploité, le combustible nucléaire présente des modifications des propriétés thermophysiques ainsi que microstructurales. En particulier se forme en périphérie de l’élément de combustible une structure appelée Structure Haut Burnup (HBS) caractérisée par la subdivision des grains originaux en grains d’environ 100 nm. Dans ce travail, la synthèse d’échantillons denses de UO2 et ThO2 avec des grains d’environ 100 nm a été réalisée. De tels matériaux permettront de faire des études à effets séparés sur l’impact de la HBS sur le comportement du combustible en conditions normales et accidentelles. UO2 dopé avec 238Pu a également été synthétisé et ce dans le but d’étudier en accéléré l’impact de l’activité alpha sur l’évolution de la microstructure et des propriétés thermophysiques du combustible tel qu’en situation de stockage où ce type de décroissance prédomine pendant des millénaires. L’UO2 auto-irradié jusqu’à 0.41 dpa, correspondant à un combustible usagé standard de 300 ans , a été caractérisé périodiquement par un ensemble de techniques. La DRX a mis en évidence une saturation du paramètre de maille à 0.3% tandis que la désorption thermique d’hélium a montré une rétention complète du gaz. Le MEB a montré que l’intégrité de la structure est préservée ; en MET la formation de boucles de dislocations et de bulles d’hélium a été mis en évidence. La conductivité thermique mesurée par flash laser décroît de 40% dès 0.03 dpa et aucune restauration de défauts n’a été observée par calorimétrie différentielle aux températures caractéristiques de l’entreposage. Des spectres RAMAN ont été obtenus pour la première fois sur (U, Pu)O2 en fonction de la dose d’autoirradiation. Cette étude a permis d’évaluer certains aspects du comportement du combustible usagé en conditions d’entreposage pendant 300 ansDue to the extreme environment in which it is operated, nuclear fuel shows changes of its microstructure and thermophysical properties . In particular it develops a rim structure also named high burnup structure characterized by the subdivision of the original micrometer sized grains into 100 nm grains. In this work, the synthesis of dense UO2 and ThO2 with grains size down to 100 nm was designed. Such material would allow studying through single effect studies the impact of the high burnup structure on the fuel behavior in and out of normal operation. UO2 doped with 238Pu was also synthesised to study the accelerated effect of alpha-decays on fuel microstructure and thermophysical properties since alpha activity will be dominating in spent nuclear fuels for millenaries. Self-irradiated UO2 cumulating up to 0.41 dpa, the same reached by a LWR spent fuel after few centuries, was characterized periodically with a broad set of techniques. XRD showed a saturation of the lattice parameter increase around 0.3 %, while thermal desorption spectroscopy proved that the totality of the radiogenic He is still retained. SEM observations highlighted the integrity of the fuel is preserved, while TEM evidenced the ingrowth of dislocation loops and He bubbles within the matrix. Thermal conductivity as measured by laser flash had already decreased by 40 % at 0.03 dpa, and no defects annealing was detected by differential scanning calorimetry at the temperatures foreseen for spent fuel storage. Raman spectra were acquired for the first time on (U,Pu)O2 as a function of the self-irradiation dose. This study allowed assessing some aspects of the behaviour of LWR spent nuclear fuel during the first 300 years of storage time
37
Mauer, Andrew N. "A superconducting fusion transmutation of waste reactor". Thesis, Georgia Institute of Technology, 2002. http://hdl.handle.net/1853/15970.
Testo completo
38
Andrews, Nathan Christopher. "Development of fission gas swelling and release models for metallic nuclear fuels". Thesis, Massachusetts Institute of Technology, 2012. http://hdl.handle.net/1721.1/76958.
Testo completoAbstract (sommario):
Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Nuclear Science and Engineering, 2012.Cataloged from PDF version of thesis.
Includes bibliographical references (p. 145-146).
Fuel swelling and fission gas generation for fast reactor fuels are of high importance since they are among the main limiting factors in the development of metallic fast reactor fuel. Five new fission gas and swelling modules for the fast reactor metallic fuel code FEAST-METAL were developed. This increases the number of degrees of freedom in the code and enhances the science -based modeling options for fuel swelling. All of the modules developed were benchmarked against data from EBRII. Particularly, the code was benchmarked against U-19Pu-lOZr fuel and was applied to U-6Zr fuel. The modifications made still kept the overall GRSIS algorithm present in the code. The GRSIS model tracks "closed" and "open" bubbles. The new modifications increased the number of closed bubble groups used in the algorithm, inserted a model that changed the bubble groups from being based on constant volumes to ones with constant numbers of atoms, added phase dependence and reexamined closed bubble spacing through the implementation of a Monte-Carlo algorithm to calculate the effective distance between the nearest bubbles. All model options added to the code predicted the swelling, fission gas release and cladding strain effectively for the benchmark cases. However, significant differences in the results were fotind when the codes were applied to long-term U-6Zr fuel. The differences in the results cannot be resolved without more data on fuel behavior under irradiation; particularly, breeder fuel (blanket) data is needed to develop effective benchmarks. Until more data becomes available, it is advisable to use the original two group constant volume version of the code and the phase dependent version of the code and compare the results. The latter offers a much more scientifically based version of the code. Sensitivity analysis to the number of bubble groups indicate limited benefit may be obtained by using more than 2 bubble sizes. Additionally, care should be taken to ensure that the axial nodding of the fuel be such that the axial mesh length is smaller than 10% of the fuel length. Furthermore, if the FEAST code is to be used in a coupled fashion with the coolant sub-channel analysis code COBRA, the accuracy of the results depend on the model used for fuel swelling.
by Nathan Christopher Andrews.
S.M.
39
Hunter, Jessica Lynn. "Improvements and applications of the Uniform Fission Site method in Monte Carlo". Thesis, Massachusetts Institute of Technology, 2014. http://hdl.handle.net/1721.1/95598.
Testo completoAbstract (sommario):
Thesis: S.M., Massachusetts Institute of Technology, Department of Nuclear Science and Engineering, 2014.Cataloged from PDF version of thesis.
Includes bibliographical references (pages 61-63).
Monte Carlo methods for reactor analysis have been in development with the eventual goal of full-core analysis. To attain results with reasonable uncertainties, large computational resources are needed. Variance reduction methods have been developed in order to reduce the computational resources required to obtain results in a practical amount of time. This work seeks to expand research in the Uniform Fission Site (UFS) method, a variance reduction technique recently developed that causes uniformity in uncertainty distributions by forcing uniformity in source distributions. This work aims to both improve the method as well as investigate its use with a source acceleration method, Coarse Mesh Finite Difference (CMFD) acceleration. Both techniques have been implemented into OpenMC, a continuous energy Monte Carlo code. The UFS method uses weights to alter the number of neutrons born at a fission site. It operates on a superimposed mesh, in which each mesh cell contains a different weight. These weights use an estimate of the source fraction and fuel volume fraction within the cell to produce uniformity. In current implementations, the fuel volumes are assumed to be dispersed equally over all mesh cells. This work aims to provide an estimate of the fuel volume fraction in each cell in order to improve the accuracy of the method for irregular geometries. The new fuel volume approximation method is tested on a toy problem and on a model of the Advanced Test Reactor, a core with highly irregular geometry. Figures of merit were calculated for a basic Monte Carlo simulation, a simulation with the standard UFS implementation, and the new UFS method with estimated volume fractions. With the toy problem, the new method showed significant improvement and had the highest figure of merit. In the case of the ATR, the long run time for the approximation lowered the figure of merit. Both problems demonstrated that the use of the standard UFS implementation on an irregular geometry produced higher uncertainties than not using the method at all. The UFS method, when used with the estimated volume fractions, behaved as expected and produced uniform uncertainty distributions. The investigation of the use of the UFS method with CMFD acceleration was conducted using the 3-D BEAVRS benchmark. Results showed that keeping CMFD acceleration on during active batches maintained a stationary source and reduced the variance for assembly results. The UFS method stacked on this, reducing the maximum relative uncertainties. The UFS method had variable results with different tallies, but no interference between the two methods was observed.
by Jessica Lynn Hunter.
S.M.
40
Sweeney, David J. "Reactor power history from fission product signatures". Thesis, [College Station, Tex. : Texas A&M University, 2008. http://hdl.handle.net/1969.1/ETD-TAMU-3209.
Testo completo
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Henning, Gregoire. "Stability of Transfermium Elements at High Spin : Measuring the Fission Barrier of 254No". Phd thesis, Université Paris Sud - Paris XI, 2012. http://tel.archives-ouvertes.fr/tel-00745915.
Testo completoAbstract (sommario):
Super heavy nuclei provide opportunities to study nuclear structure near three simultaneous limits: in charge Z, spin I and excitation energy E∗. These nuclei exist only because of a fission barrier, created by shell effects. It is therefore important to determine the fission barrier and its spin dependence Bf(I), which gives information on the shell energy Eshell(I). Theoretical calculations predict different fission barrier heights from Bf(I = 0) = 6.8 MeV for a macro-microscopic model to 8.7 MeV for Density Functional Theory calculations using the Gogny or Skyrme interactions. Hence, a measurement of Bf provides a test for theories.To investigate the fission barrier, an established method is to measure the rise of fission with excitation energy, characterized by the ratio of decay widths Γfission/Γtotal, using transfer reactions. However, for heavy elements such as 254No, there is no suitable target for a transfer reaction. We therefore rely on the complementary decay widths ratio Γγ/Γfission and its spin dependence, deduced from the entry distribution (I, E∗).Measurements of the gamma-ray multiplicity and total energy for 254No have been performed with beam energies of 219 and 223 MeV in the reaction 208Pb(48Ca,2n) at ATLAS (Argonne Tandem Linac Accelerator System). The 254No gamma rays were detected using the Gammasphere array as a calorimeter - as well as the usual high resolution γ-ray detector. Coincidences with evaporation residues at the Fragment Mass Analyzer focal plane separated 254No gamma rays from those from fission fragments, which are > 10^6 more intense. From this measurement, the entry distribution - i.e. the initial distribution of I and E∗ - is constructed. Each point (I,E∗) of the entry distribution is a point where gamma decay wins over fission and, therefore, gives information on the fission barrier.The measured entry distributions show an increase in the maximum spin and excitation energy from 219 to 223 MeV of beam energy. The distributions show a saturation of E∗ for high spins. The saturation is attributed to the fact that, as E∗ increases above the saddle, Γfission rapidly dominates. The resulting truncation of the entry distribution at high E∗ allows a determination of the fission barrier height.The experimental entry distributions are also compared with entry distributions calculated with decay cascade codes which take into account the full nucleus formation process, including the capture process and the subsequent survival probability as a function of E∗ and I. We used the KEWPIE2 and NRV codes to simulate the entry distribution.
42
Gao, Wei Ph D. "Lithium-6 filter for a fission converter-based Boron Neutron Capture Therapy irradiation facility beam". Thesis, Massachusetts Institute of Technology, 2005. http://hdl.handle.net/1721.1/34653.
Testo completoAbstract (sommario):
Includes bibliographical references (p. 164-165).Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Nuclear Engineering, 2005.
(cont.) A storage system was designed to contain the lithium-6 filter safely when it is not in use. A mixed field dosimetry method was used to measure the photon, thermal neutron and fast neutron dose. The measured advantage depth is 9.3 ± 0.1cm without filter and 9.9 ± 0.1cm with 8mm lithium-6 filter. The result is consistent with the result of Monte Carlo calculation.
The design of a lithium-6 filter to be used in Boron Neutron Capture Therapy was developed. The lithium-6 filter increases the average energy of the epithermal neutrons in the epithermal neutron beam. This filter allows the beam to be used for effective BNCT treatment at greater depth in tissue. Based on Monte Carlo calculations, 8mm thick lithium-6 filter was found to be the optimum filter thickness for the MIT fission converter based epithermal neutron beam (FCB). The highly reactive lithium metal filter is sealed with aluminum covers against the humidity and surrounding air. A well shielded and convenient frame was also designed to hold the lithium-6 filter. The frame is separated into two parts. The fixed part of the frame will be mounted into the patient collimator of the FCB and provides a slot for the lithium-6 filter. The filter itself will be connected to the movable part of the frame and slid in and out of the beam through a pair of roller bearing tracks like a vertical drawer. Both parts of the frame are built with borated polyethylene (RICORAD) and steel to insure good shielding. Many safety issues have been considered in the design including tritium production, nuclear heating, pressure from released gases and radiation leakage on the side of the collimator.
by Wei Gao.
S.M.
43
Schmitt, Christelle. "Fission des Noyaux Lourds et Superlourds". Phd thesis, Université Louis Pasteur - Strasbourg I, 2002. http://tel.archives-ouvertes.fr/tel-00001529.
Testo completoAbstract (sommario):
Ce travail de thèse porte sur l'étude de la dynamique des réactions de fusion-fission sous ses aspects aussi bien théoriques qu'expérimentaux. Dans le cas de systèmes nucléaires fortement excités, nous décrivons la fission symétrique (seule présente à haute énergie) par un modèle macroscopique unidimensionnel basé sur la résolution de l'équation de Langevin couplée aux équations gouvernant l'évaporation de particules légères. Un accord satisfaisant entre prédictions théoriques et résultats expérimentaux de multiplicités de neutrons de pré-scission a alors été obtenu. A plus basse énergie la coexistence éventuelle de modes de fission symétrique et asymétrique(s), ou fission multimodale, est à prendre en compte. Notre description théorique est alors généralisée à un traitement multidimensionnel de l'équation de Langevin. L'influence des différentes prescriptions théoriques (effets de couche et d'appariement, friction, évaporation) sur les probabilités et les temps de fission ainsi que sur les multiplicités de pré-scission est étudiée. Dans une seconde partie, les données expérimentales obtenues lors d'une campagne de mesures menée auprès du cyclotron U$400$ au JINR de Dubna sont analysées. Le noyau composé de $^{227}$Pa résulte du bombardement d'une cible de $^{209}$Bi par un faisceau d'ions d'$^{18}$O. Le détecteur d'ions lourds CORSET, constitué de galettes à microcanaux, détecte les fragments de fission et le multidétecteur DéMoN permet la mesure des neutrons émis en co\"{\i}ncidence. Une recons\-truction cinématique fournit la distribution en masse et en vitesse des fragments de fission et une analyse en source donne accès aux neutrons de pré-scission. Il est en particulier intéressant d'étudier l'évolution de la multiplicité de pré-scission avec l'asymétrie en masse des fragments. La confrontation théorie-expérience nous permet alors de mettre en évidence les atouts mais également des défauts du modèle et de proposer dans certains cas (des\-cription de la friction, théorie d'évaporation) des moyens pour y remédier (dépendance en température des coefficients de transport, paramétrisation de la surface nucléaire).
44
Bourne, Geoffrey S. "Actinide and fission product activity variations in inter-tidal marine macrophytes". Thesis, Cardiff Metropolitan University, 1992. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.306854.
Testo completo
45
Khan, Md Gulam Musawwir. "Nuclear import mechanism of Php4 under iron deprivation in fission yeast Schizosaccharomyces pombe". Mémoire, Université de Sherbrooke, 2015. http://hdl.handle.net/11143/6008.
Testo completoAbstract (sommario):
Php4 is a subunit of the CCAAT-binding protein complex that has a negative regulatory function during iron deprivation in the fission yeast Schizosaccharomyces pombe. Under low iron conditions, Php4 fosters the repression of genes encoding iron using proteins. In contrast, under iron-replete conditions, Php4 is inactivated at both transcriptional and post-transcriptional levels. Our group has already described that Php4 is a nucleo-cytoplasmic shuttling protein, which accumulates into the nucleus during iron deficiency. On the contrary, Php4 is exported from the nucleus to the cytoplasm in response to iron abundance. Php4 possesses a leucine-rich NES (93LLEQLEML100) that is necessary for its nuclear export by the exportin Crm1. Our current study aims at understanding the mechanism by which Php4 is imported in the nucleus during iron starvation. Through microscopic analyses using different mutant strains, we showed that the nuclear localization of Php4 is independent of the other subunits of the CCAAT-binding core complex namely Php2, Php3 and Php5. Deletion mapping analysis of Php4 identifies two putative nuclear localization sequences (NLSs) in Php4 (171KRIR174 and 234KSVKRVR240). Using chimeric proteins that consist of GFP fused to Php4, we engineered substitutions of the basic amino acid residues 171AAIA174 and 234ASVAAAA240 and analyzed the functionality of both NLSs. We observed that both monopartite NLSs play critical role for Php4 nuclear localization. We also observed that
mutant strains of cut15+, imp1+ or sal3+ exhibited defects in nuclear targeting of Php4, revealing that nuclear accumulation of Php4 is dependent on two karyopherin α (Imp1 and Cut15) and one karyopherin β (Sal3) receptors. Consistently, the Php4-mediated repression activity is abolished in the absence of two functional NLSs. Moreover, loss of Imp1, Cut15 or Sal3 resulted in increased expression of isa1+, which is a target gene of Php4. Co-immunoprecipitation assay (Co-IP) reveals physical interaction of Php4 with Imp1, Cut15 and Sal3 in vitro. Collectively, our results demonstrate that Php4 has two distinct NLS regions responsible for its nuclear localization. Furthermore, karyopherin α and β receptors play a role in the nuclear import of Php4. Because Php4 is essential for growth under low iron conditions, the presence of two NLSs would ensure the protein to reach its nuclear destination when cells undergo a transition from iron-sufficient to iron-limiting conditions.
46
Goddard, Philip M. "A microscopic study of nuclear fission using the time-dependent Hartree-Fock method". Thesis, University of Surrey, 2014. http://epubs.surrey.ac.uk/806610/.
Testo completoAbstract (sommario):
This thesis is concerned with the application of the time-dependent Hartree-Fock method, adopting the Skyrme effective nuclear interaction, to investigate induced nuclear fission processes. Three-dimensional, symmetry-unrestricted constrained Hartree-Fock calculations of ²⁴⁰Pu are presented to obtain the static potential energy surface corresponding to an increasing quadrupole deformation. The time-dependent Hartree-Fock method is applied to evolve in time these deformed states. Different types of fission process are considered. Firstly, the evolution of static configurations which are deformed sufficiently such that they fission upon time evolution, without any external excitation, ('deformation-induced' fission) is investigated. The fragments smoothly form during a gradual evolution of the static state (taking up to ≈ 1500 fm/c for scission to occur), and the resulting fission products agree well with experimental neutron-induced fission data. The kinetic energy released in the process is also shown to compare well to experimental results. Secondly, the effect of providing an external excitation field to static configurations which would not otherwise fission ('boost-induced' fission) is investigated. Upon application of an instantaneous excitation, the evolution of the densities displays a violent oscillatory behaviour as the state evolves to fission, in contrast what was seen for the case of deformation-induced fission. The resulting fission products lie at the edges of typical mass distributions obtained from experiment. The evolution of the densities following the application of a gradual excitation field demonstrates behaviour similar to deformation-induced fission, suggesting that the timescale for the energy deposition has consequences regarding the fission dynamics. The comparison to experimental results following the application of a gradual excitations field is improved compared to an instantaneous excitation. Overall, the results provide a significant exploratory investigation of fission treated as a dynamic process, and suggest that the time-dependent Hartree-Fock method has potential to be applied further to describe the dynamics of nuclear fission.
47
Peck, Marius [Verfasser], Joachim [Akademischer Betreuer] Enders e Norbert [Akademischer Betreuer] Pietralla. "Correlation experiments in photon-induced nuclear fission / Marius Peck ; Joachim Enders, Norbert Pietralla". Darmstadt : Universitäts- und Landesbibliothek Darmstadt, 2020. http://d-nb.info/1216997799/34.
Testo completo
48
Calviani, Marco. "Measurement of fission cross-section of actinides at n_TOF for advanced nuclear reactors". Doctoral thesis, Università degli studi di Padova, 2009. http://hdl.handle.net/11577/3427217.
Testo completoAbstract (sommario):
The subject of this thesis is the determination of high accuracy neutron-induced fission cross-sections of various isotopes - all of which radioactive - of interest for emerging nuclear technologies. The measurements had been performed at the CERN neutron time-of-flight facility n_TOF. In particular, in this work, fission cross-sections on 233U, the main fissile isotope of the Th/U fuel cycle, and on the minor actinides 241Am, 243Am and 245Cm, have been analyzed. Data on these isotopes are requested for the feasibility study of innovative nuclear systems (ADS and Generation IV reactors) currently being considered for energy production and radioactive waste transmutation.
The measurements have been performed a high performance Fission Ionization Chamber (FIC), in conjunction with an innovative data acquisition system based on Flash-ADCs.
The first step in the analysis has been the reconstruction of the digitized signals, in order to extract the information required for the discrimination between fission fragments and the background, as well as for the determination of the neutron's energy from its time-of-flight.
Fission cross-sections for the various isotopes have been determined relative to the 235U(n,f) reaction, which is considered a standard of measurement in a wide energy range. In order to minimize systematic uncertainties, this reaction has been measured with the same detector and at the same time of the reactions subject of this thesis.
A fundamental part of the thesis work has been the analysis of the 235U(n,f) reaction, which has allowed to study the response of the fission chamber, thanks also to the use of detailed Monte Carlo simulations, performed with state-of-the-art codes for neutron transport and interaction. Moreover, the analysis of the 235U(n,f) reaction has allowed the energy calibration of the neutron beam, the determination of the incident neutron flux and an accurate estimate of the background.
In the present thesis the final results for the 233U(n,f) cross-section are shown, as well as the preliminary results for the 241Am(n,f), 243Am(n,f) and 245Cm(n,f) cross-sections. The characteristics of the n_TOF neutron beam have allowed to obtain results in a wide energy range, from about 30 meV to 1 MeV, in a single measurement. For the 233U(n,f) case, the final uncertainties on the cross-section are slightly larger than 3\%, a value required for the development of innovative nuclear systems. In order to reach such an accuracy, corrections for sample-dependent efficiencies, as well as for reaction-related dead-time effects, have been applied.Il lavoro oggetto della presente tesi ha riguardato la determinazione ad alta accuratezza della sezione d'urto per la reazione di fissione indotta da neutroni su vari isotopi - tutti radioattivi - di interesse per le tecnologie nucleari emergenti. Le misure erano precedentemente state effettuate presso la facility per tempi di volo di neutroni n_TOF, presso il CERN di Ginevra. In questo lavoro sono state in particolare analizzate le misure di fissione per il 233U, il principale isotopo fissile alla base del ciclo di combustibile Th/U, e quelle sugli attinidi minori 241Am, 243Am e 245Cm, le cui sezioni d'urto sono richieste per lo sviluppo di sistemi nuclear innovativi (ADS e reattori di IV Generazione) attualmente in fase di studio per la produzione di energia e per la trasmutazione delle scorie radioattive. Le suddette reazioni sono state misurate con una camera a ionizzazione per frammenti di fissione (Fission Ionization Chamber), in combinazione con un sistema di acquisizione basato su Flash-ADC. Il primo passo dell'analisi ha riguardato la ricostruzione dei segnali della camera di fissione, al fine di estrarre informazioni necessarie per la discriminazione dei frammenti di fissione dal background, nonché per la determinazione dell'energia dei neutroni, a partire dal loro tempo di volo. Le sezioni d'urto di fissione per i vari isotopi sopra citati sono state determinate relativamente alla sezione d'urto della reazione 235U(n,f), considerata standard di misura in un vasto intervallo energetico. Al fine di minimizzare gli errori sistematici, tale reazione è stata misurata con lo stesso rivelatore, e contemporaneamente alle reazioni oggetto della tesi. Una parte fondamentale del lavoro di tesi ha quindi riguardato l'analisi dei dati della reazione 235U(n,f), che ha permesso in primo luogo di studiare la risposta della camera a fissione, grazie anche all'utilizzo di dettagliate simulazioni effettuate con i più sofisticati codici Monte Carlo per il trasporto dei neutroni. L'analisi della reazione di fissione sul 235U ha permesso inoltre la calibrazione energetica del fascio di neutroni, la determinazione del flusso neutronico incidente sul rivelatore ed una stima accurata del background. Nella presente tesi sono presentati i risultati finali della sezione d'urto della reazione 233U(n,f), nonché i risultati preliminari della sezione d'urto per le reazioni 241Am(n,f), 243Am(n,f) e 245Cm(n,f). Le caratteristiche del fascio di neutroni di n_TOF hanno permesso di ottenere risultati in un vasto intervallo energetico, da circa 30 meV fino a 1 MeV, in una singola misura. Nel caso del 233U(n,f), la sezione d'urto è stata determinata con una incertezza di circa il 3\%, un valore prossimo a quello attualmente richiesto per lo sviluppo di sistemi nucleari innovativi. Al fine di raggiungere l'accuratezza richiesta, è stato necessario correggere i dati per effetti dipendenti dal bersaglio, legati all'efficienza del rivelatore. Inoltre sono state applicate correzioni dovute al dead-time, anch'esso dipendente dalla reazione studiata.
49
Régnier, David. "Contribution à l'étude des gammas prompts de fission". Thesis, Grenoble, 2013. http://www.theses.fr/2013GRENI065.
Testo completoAbstract (sommario):
Cette thèse a essentiellement été motivée par la montée en puissance de la thématique des échauffements nucléaires en réacteurs. Elle avait pour objectif la mise en place de méthodes permettant la simulation de l'émission des gammas prompts de fission. Pour répondre à cette attente, différents algorithmes de désexcitation d'un noyau ont été implémentés. Ils ont été testés avec succès via plusieurs calculs d'application (rapport d'embranchement isomérique, largeur radiative totale, etc). Ces méthodes ont ensuite été mises en oeuvre dans le cadre du code de fission FIFRELIN. L'outil qui en résulte, permet la détermination d'un grand nombre d'observables de fission, dans le cadre d'un modèle unique. Une étude de sensibilité des résultats à différents éléments du calcul a été réalisée. Enfin, des simulations de la fission spontanée du 252Cf, et des fissions induites par neutrons thermiques sur l' 235U et le 239Pu ont été menées. Les spectres des gammas prompts pour ces trois systèmes fissionnants ont pu être déterminés. Les résultats obtenus sont en très bon accord avec les données expérimentales disponibles, dont certaines ont été publiées récemment (2012 et 2013)This PhD thesis has essentially been motivated by the nuclear heating problematic in reactors. The main goal of this work was the production of methods capable of simulating the prompt gamma emission from fission. First of all, several algorithms for the treatment of the nucleus deexcitation were implemented. They have been succesfully tested through various calculations (isomeric branching ratio, total radiative width, etc). These methods were then incorporated in the frame of the fission code FIFRELIN. The tool which results from this work, enables the determination of numerous fission observables in the frame of a single consistent model. A sensitivity study of the results to several numerical and nuclear models has been realized. At last, calculation have been lead for the 252Cf spontaneous fission and the thermal neutron induced fission of 235U and 239Pu. The prompt gamma spectra obtained for those three fissionning systems have been determined. The results are in good agreement with available experimental data, including recent measurements published in 2012 and 2013
50
Lee, Terry Tak-Keon. "Safety analysis report and technical specifications of the MITR fission converter facility for neutron capture therapy". Thesis, Massachusetts Institute of Technology, 1997. http://hdl.handle.net/1721.1/43345.
Testo completo