<p>Fission is the most important nuclear reaction for society at large today due to its use in energy production. However, this has raised the problem of how to treat the long-lived radioactive waste from nuclear reactors. A radical solution would be to change the composition of the waste into stable or short-lived nuclides, which could be done through nuclear transmutation. Such a concept requires accelerator-driven systems to be designed, where those for transmutation are reactor hybrids. This thesis is a contribution to the knowledge base for developing transmutation systems, specifically with respect to the computational modeling of the underlying nuclear reactions, induced by the incident and secondary particles. Intermediate energy fission cross sections are one important type of such data. Moreover, they are essential for understanding the fission process itself and related nuclear interactions. </p><p>The experimental part of this work was performed at the neutron beam facility of the The Svedberg Laboratory in Uppsala. Fission cross sections of ²³⁸U, ²⁰⁹Bi, ^natPb, ²⁰⁸Pb, ¹⁹⁷Au, ^natW, and ¹⁸¹Ta were measured for neutrons in the range <i>E</i>_n = 30-160 MeV using thin-film breakdown counters for the fission fragment detection. A model was developed for the determination of the efficiency of such detectors. </p><p>A compilation of existing data on proton-induced fission cross sections for nuclei from ¹⁶⁵Ho to ²³⁹Pu was performed. The results, which constitute the main body of information in this field, were added to the worldwide EXFOR database. The dependences of the cross sections on incident energy and target nucleus were studied, which resulted in systematics that make it possible to give estimates for unmeasured nuclides. </p><p>Nucleon-induced fission cross sections were calculated using an extended version of the cascade exciton model. A comparison with the systematics and the experimental data obtained in the present work revealed significant discrepancies. A modification of the model was made that significantly improved the predictions. </p>

Fission is the most important nuclear reaction for society at large today due to its use in energy production. However, this has raised the problem of how to treat the long-lived radioactive waste from nuclear reactors. A radical solution would be to change the composition of the waste into stable or short-lived nuclides, which could be done through nuclear transmutation. Such a concept requires accelerator-driven systems to be designed, where those for transmutation are reactor hybrids. This thesis is a contribution to the knowledge base for developing transmutation systems, specifically with respect to the computational modeling of the underlying nuclear reactions, induced by the incident and secondary particles. Intermediate energy fission cross sections are one important type of such data. Moreover, they are essential for understanding the fission process itself and related nuclear interactions. The experimental part of this work was performed at the neutron beam facility of the The Svedberg Laboratory in Uppsala. Fission cross sections of 238U, 209Bi, natPb, 208Pb, 197Au, natW, and 181Ta were measured for neutrons in the range En = 30-160 MeV using thin-film breakdown counters for the fission fragment detection. A model was developed for the determination of the efficiency of such detectors. A compilation of existing data on proton-induced fission cross sections for nuclei from 165Ho to 239Pu was performed. The results, which constitute the main body of information in this field, were added to the worldwide EXFOR database. The dependences of the cross sections on incident energy and target nucleus were studied, which resulted in systematics that make it possible to give estimates for unmeasured nuclides. Nucleon-induced fission cross sections were calculated using an extended version of the cascade exciton model. A comparison with the systematics and the experimental data obtained in the present work revealed significant discrepancies. A modification of the model was made that significantly improved the predictions.

Nuclear pre-mRNA splicing proceeds via two mechanistically conserved consecutive trans-esterification reactions catalyzed by the spliceosome. The ordered coalescence of spliceosomal snRNPs and splicing factors on the pre-mRNA, coupled with essential spliceosomal rearrangements poise the splice-sites in proximity for the two catalytic reactions, ensuring intron removal and exon ligation to yield functional mRNA (reviewed in Will and Lührmann, 2006). Scope of the study The S. cerevisiae splicing factors Prp18 and Slu7 and their human homologs function during second catalytic reaction. In S. cerevisiae, Slu7 is essential, whereas Prp18 is dispensable at temperatures <30°C (Vijayraghavan et al., 1989; Vijayraghavan and Abelson, 1990; Frank et al., 1992; Horowitz and Abelson, 1993b; reviewed in Umen and Guthrie, 1995). Slu7 acts in concert with Prp18 and their direct interaction is required for their stable spliceosomal association (Zhang and Schwer, 1997; James et al., 2002). In vitro studies indicate that both the factors are dispensable for splicing of introns with short distances between branch nucleotide to 3’ splice-site (Brys and Schwer, 1996; Zhang and Schwer, 1997). Furthermore, mutational analyses of Slu7 and Prp18 have defined their functional domains/motifs (Frank and Guthrie, 1992; Bacíková and Horowitz, 2002; James et al., 2002). In this study, we have examined functions for the predicted homologs of Slu7 and Prp18 in fission yeast; an evolutionarily divergent organism where splicing mechanisms are not well understood and whose genome harbors genes with predominantly multiple introns with degenerate splice-junction sequences. Towards this goal, a combinatorial approach employing genetic and biochemical methods was undertaken to understand splicing functions and interactions of SpSlu7 and SpPrp18. Our mutational analysis of these protein factors provided an overview of the domains/motifs critical for their in vivo functions. Lastly our analysis of components of the budding yeast Cef1p-associated complex show novel interactions and splicing functions for two uncharacterized, yet evolutionarily conserved proteins. Conserved fission yeast splicing proteins SpSlu7 and SpPrp18 are essential for pre-mRNA splicing but have altered spliceosomal associations and functions Analyzing conserved splicing factors in evolutionarily divergent organisms is an important means to gain deeper functional insights on splicing mechanisms in genomes with varied gene architecture. We initiated our analysis of the ‘predicted’ S. pombe second-step splicing factors: spprp18+ and spslu7+, by genetically depleting these factors. We find spprp18+ is essential for viability, unlike budding yeast PRP18; while SLU7 is essential in both yeasts. The complete essentiality of both these fission yeast factors, prompted us to create conditional-lethal thiamine repressible ‘switch-off’ strains to probe their splicing functions. Through semi-quantitative RT-PCR and northern blot analysis we demonstrate splicing defects for tfIId+ pre-mRNAs upon metabolic depletion of spprp18+ or spslu7+, thus linking their essentiality to a role in pre-mRNA splicing. Further we examined whether their requirement as splicing factors is governed by specific intronic features. We find both factors are required in vivo for removal of several introns. However, for the introns tested, their functions are not strictly correlated with intron length, number, position or the branch-nucleotide to 3’ splice-site distance. The latter features dictate the need for their S. cerevisiae homologs. Strikingly the lack of either one of these essential proteins, arrests splicing before the first catalytic step; implicating possible functions early in spliceosome assembly even before any catalytic event, as opposed to budding yeast Slu7 and Prp18, which are second-step factors assembling late onto the spliceosome after the first splicing reaction. Given the different splicing arrest point, on depletion of SpSlu7 and SpPrp18, we investigated through yeast two-hybrid and co-immunoprecipitation assays whether the direct interaction between these proteins is conserved. We find despite being nuclear localized these proteins do not interact in either of the assays employed. A structural basis for the lack of interaction was provided by our homology modeling of SpPrp18, that was based on the crystal structure of S. cerevisiae Prp1879 (Jiang et al., 2000). Together these data raise the possibility of contextual functions and interactions for these conserved proteins that varies with changes in gene architecture. This likelihood is strengthened by our reciprocal genetic complementation tests; wherein we find that SpSlu7 and SpPrp18 cannot complement the corresponding S. cerevisiae null alleles and vice versa. Additionally, the human homologs, hSlu7 and hPrp18 also failed to rescue null alleles for spslu7+ and spprp18+. To understand the likely point of coalescence of SpSlu7 and SpPrp18 on assembling spliceosomes, we probed their snRNP associations through co-immunoprecipitation analysis. Our data revealed interaction of SpSlu7 with the U2, U5 and U6 snRNPs at moderate salt concentrations with the interaction with U5 snRNP being retained at higher salt conditions. SpPrp18, on the other hand, showed only a very weak association with U5 snRNP. Our analysis thus indicates that the assembly and step of action for “predicted” late-acting splicing factors in fission yeast differs from that in budding yeast, implicating novel interactions and functions for these fission yeast splicing factors. Mutational analysis of fission yeast SpPrp18 and SpSlu7 identifies functional domains To examine the protein domains/motifs critical for the functions of SpPrp18 and SpSlu7, we have performed a mutational study. This analysis was important after our findings that these factors are early acting and do not interact. The data gathered would shed light on the contribution of different domains/motifs in the functional diversification of these factors. Guided by the findings of Bacíková and Horowitz (2002); site-specific missense mutants were created in the highly conserved carboxyl-terminus (CR domain and helix 5) of SpPrp18. Additionally, site-specific missense mutants were generated in a conserved amino-terminus domain that is absent in budding yeast Prp18. Our data showed mutants in the highly conserved helix 5 and the CR domain of SpPrp18 to be recessive and non-functional, despite being stably expressed. This contrasts with the temperature-sensitivity conferred by similar mutants in homologous residues in budding yeast Prp18 (Bacíková and Horowitz, 2002). We speculate that the essentiality of the CR domain and helix 5 mutants of SpPrp18 arises due to a defect in spliceosomal association. However, the mutants in conserved residues in the protein’s amino-terminal domain are phenotypically wild type at various growth temperatures tested, suggesting redundant functions for these residues. Our data, based on analysis of a single missense mutant in the highly conserved zinc knuckle motif of SpSlu7, ascribes essential functions for the zinc knuckle motif. We find the mutant to be recessive and non-functional despite stable expression and normal cellular localization of the mutant protein. This contrasts with the behavior of zinc knuckle mutants in budding yeast and human Slu7. Budding yeast Slu7 mutants are functionally wild type and human Slu7 mutants have an altered cellular localization (Frank and Guthrie, 1992; James et al., 2002; Shomron et al., 2004). Possible roles for the zinc knuckle motif of SpSlu7 could be in facilitating interaction of SpSlu7 with U5 snRNA or even with some protein factor. Functional analysis of budding yeast Cef1p-associated complex SpSlu7 and its budding yeast homolog ScSlu7 co-purify with Cdc5/Cef1 in a complex of ~30 proteins together with U2, U5 and U6 snRNAs (Gavin et al., 2002; Ohi et al., 2002). Functional characterization of six proteins of the budding yeast Cef1p complex: Ydl209c (Cwc2/Ntc40), Ycr063w (Cwc14/Bud31), Yju2 (Cwc16), Ygr278w (Cwc22), Ylr424w (Spp382/Ntr1) and Ygl128c (Cwc23) was initiated using a combination of genetic and biochemical approaches. We probed direct protein-protein interactions between members of the Cef1p-associated complex by yeast two-hybrid assays. We also examined the pre-mRNA splicing roles for an essential factor, Yju2/Cwc16 and for a non-essential factor, Ycr063w/Cwc14. Our data reveals direct interaction between Yju2 and early acting factors, Syf1/Ntc90 and Clf1/Ntc77. Similarly interaction of Ydl209c/Cwc2 with early acting splicing factors, Prp19, Syf1/Ntc90 and Clf1/Ntc77 was noted. We created a temperature-sensitive expression strain for YJU2 using a temperature-sensitive Gal4 transcription trans-activator (Chakshusmathi et al., 2004; Mondal et al., 2007) to interrogate the splicing functions of YJU2. RT-PCR and northern blot assays show that depletion of YJU2 causes splicing defects for intron containing pre-mRNAs. We predict early splicing functions for YJU2 as is known for its interacting partners. Furthermore, we find that genetic depletion of the non-essential factor YCR063w causes temperature-sensitivity as has been reported for a few other factors (for e.g. Prp17, Lea1, Snt309/Ntc25, Ecm2) of Cef1p-associated complex (Jones et al, 1995; Chen et al., 1998). Although our yeast two-hybrid data does not reveal any direct interactions between Ycr063w and other proteins of the Cef1p-associated complex, we probed its functions through in vitro splicing assays. Splicing extracts from ycr063w/ycr063w cells show compromised second-step splicing at higher temperatures, thereby implying an auxiliary function for Ycr063w in stabilizing some functionally critical interactions during splicing. These studies employing complementary genetic and biochemical approaches implicate functional divergence of conserved predicted ‘second-step’ fission yeast factors, SpSlu7 and SpPrp18, suggesting co-evolution of splicing factors with changes in genome architecture and intron-exon structure. Our studies on Cef1p-associated complex show novel interactions and implicate pre-mRNA splicing functions for two previously uncharacterized proteins.

The Monte Carlo method is, practically, the only approach capable of giving detail insight into complex neutron transport problems. In reactor physics, the method has been used mainly for determining the keff in criticality calculations. In the last decade, the continuously growing computer performance has allowed to apply the Monte Carlo method also on simple burnup simulations of nuclear systems. Nevertheless, due to its extensive computational demands the Monte Carlo method is still not used as commonly as deterministic methods. One of the reasons for the large computational demands of Monte Carlo criticality calculations is the necessity to carry out a number of inactive cycles to converge the fission source. This thesis presents a new concept of fission matrix based Monte Carlo criticality calculations where inactive cycles are not required. It is shown that the fission matrix is not sensitive to the errors in the fission source, and can be thus calculated by a Monte Carlo calculation without inactive cycles. All required results, including keff, are then derived via the final fission matrix. The confidence interval for the estimated keff can be conservatively derived from the variance in the fission matrix. This was confirmed by numerical test calculations of Whitesides's ``keff of the world problem'' model where other Monte Carlo methods fail to estimate the confidence interval correctly unless a large number of inactive cycles is simulated.   Another problem is that the existing Monte Carlo criticality codes are not well shaped for parallel computations; they cannot fully utilise the processing power of modern multi-processor computers and computer clusters. This thesis presents a new parallel computing scheme for Monte Carlo criticality calculations based on the fission matrix. The fission matrix is combined over a number of independent parallel simulations, and the final results are derived by means of the fission matrix. This scheme allows for a practically ideal parallel scaling since no communication among the parallel simulations is required, and no inactive cycles need to be simulated.   When the Monte Carlo criticality calculations are sufficiently fast, they will be more commonly applied on complex reactor physics problems, like non-linear steady-state calculations and fuel cycle calculations. This thesis develops an efficient method that introduces thermal-hydraulic and other feedbacks into the numerical model of a power reactor, allowing to carry out a non-linear Monte Carlo analysis of the reactor with steady-state core conditions. The thesis also shows that the major existing Monte Carlo burnup codes use unstable algorithms for coupling the neutronic and burnup calculations; therefore, they cannot be used for fuel cycle calculations. Nevertheless, stable coupling algorithms are known and can be implemented into the future Monte Carlo burnup codes. / QC 20100709

Pour le géologue, l'application de la méthode des traces de fission permet d'obtenir des renseignements précieux sur l'histoire thermique des roches. Dans ce travail, cette méthode est appliquée à l'étude des Alpes franco-italiennes, sur une transversale d'Ivrée au Mont Blanc, recoupant le massif du Grand Paradis et les ophiolites de la zone Piémontaise. La comparaison des résultats obtenus dans ces différentes unités, nous apporte des contraintes thermiques et temporelles pour les phases de métamorphisme Haute Pression et Schiste Vert. Nous proposons un modèle géodynamique de l'orogenèse alpine dans ce secteur des Alpes internes .

pas de résumé

The closure of the nuclear fuel cycle is a topic of interest in the sustainability context of nuclear energy. The implication of such closure includes considerations of nuclear waste management. This originates from the fact that a closed fuel cycle requires recycling of useful materials from spent nuclear fuel and discarding of non-usable streams of the spent fuel, which are predominantly the fission products. The fission products represent the near-term concerns associated with final geological repositories for the waste stream. Long-lived fission products also contribute to the long-term concerns associated with such repository. In addition, an ultimately closed nuclear fuel cycle in which all actinides from spent nuclear fuels are incinerated will result in fission products being the only source of radiotoxicity. Hence, it is desired to develop a transmutation strategy that will achieve reduction in the inventory and radiological parameters of significant fission products within a reasonably short time. In this dissertation, a transmutation strategy involving the use of the VHTR is developed. A set of specialized metrics is developed and applied to evaluate performance characteristics. The transmutation strategy considers six major fission products: 90Sr, 93Zr, 99Tc, 129I, 135Cs and 137Cs. In this approach, the unique core features of VHTRs operating in equilibrium fuel cycle mode of 405 effective full power days are used for transmutation of the selected fission products. A 30 year irradiation period with 10 post-irradiation cooling is assumed. The strategy assumes no separation of each nuclide from its corresponding material stream in the VHTR fuel cycle. The optimum locations in the VHTR core cavity leading to maximized transmutation of each selected nuclides are determined. The fission product transmutation scenarios are simulated with MCNP and ORIGEN-S. The results indicate that the developed fission product transmutation strategy offers an excellent potential approach for the reduction of inventories and radiological parameters, particularly for long-lived fission products (93Zr, 99Tc, 129I and 135Cs). It has been determined that the in-core transmutation of relatively short-lived fission products (90Sr and 137Cs) has minimal advantage over a decay-only scenario for these nuclides. It is concluded that the developed strategy is a viable option for the reduction of radiotoxicity contributions of the selected fission products prior to their final disposal in a geological repository. Even in the cases where the transmutation advantage is minimal, it is deemed that the improvement gained, coupled with the virtual storage provided for the fission products during the irradiation period, makes the developed fission product transmutation strategy advantageous in the spent fuel management scenarios. Combined with the in-core incineration options for TRU, the developed transmutation strategy leads to potential achievability of engineering time scales in the comprehensive nuclear waste management.

Les réacteurs hybrides, bases sur le couplage entre un accélérateur de particules et un coeur sous-critique via une cible<br />de spallation, présentent des possibilités de réduction de la radiotoxicité des déchets de haute activité et a vie<br />longue promis au stockage. Les différents concepts proposes ces dernières années dans la communauté scientifique montrent<br />la nécessite de réaliser un démonstrateur.<br />Ce travail de thèse a porte sur l'optimisation par simulation Monte Carlo a l'aide du code MCNPX, de la neutronique d'un tel<br />système dans le but de réaliser un réacteur pilote.<br />Tout d'abord, nous avons indique les principales caractéristiques neutroniques d'un réacteur hybride avant de présenter le<br />concept de démonstrateur refroidi au gaz base sur le remontage effectue par la société Framatome ANP. Nous avons<br />caractérise puis optimise la neutronique a travers la géométrie et les matériaux utilises pour ce démonstrateur.<br />Dans le cadre de l'incinération des actinides mineurs, nous avons calcule l'évolution des combustibles envisageables suivant<br />les phases de démonstration prévues. Les grandeurs liées a l'incinération des actinides mineurs sont rapportées. En vue<br />de la transmutation du 99Tc et de l'129I, nous avons calcule les temps caractéristiques et les taux de transmutation<br />a l'équilibre.<br />D'autre part, nous avons analyse le passage du démonstrateur vers un réacteur incinérateur de puissance a partir de<br />critères physiques tels que les facteurs de forme et les niveaux de flux. A partir de cette analyse, des solutions innovantes sont<br />proposées pour améliorer les facteurs de forme d'un incinérateur de puissance.<br />Enfin, dans des perspectives a plus long terme, l'utilisation des réacteurs hybrides dans le cadre de la génération<br />d'233U pour accélérer le démarrage d'une filière de réacteurs a sels fondus basée sur le cycle<br />232Th/233U a été explorée et s'avère particulièrement efficace.

L'Albanie occupe une position critique au sein de la ceinture plissée alpine Dinaro-Hellénique. Cet orogène est caractérisé par trois composants fondamentaux: une ceinture de plis et chevauchements externe à l'Ouest, un domaine central caractérisé par la présence de nappes ophiolitiques, et un complexe interne oriental. <br />Certains points clés pour la connaissance de l'évolution géodynamique de cette chaîne ont été éclairé par l'application de la thermochronologie de basse température (trace de fission et (U-Th)/He sur apatite et zircon), plus précisément en ce qui concerne l'histoire du refroidissement des Albanides Internes. <br />Un taux d'exhumation (<0,1 km/Ma) durant l'Eocène et le début de l'Oligocène caractérise la limite entre les zones externes et zones Internes. Il est probablement lié au soulèvement isostatique, conséquence de l'épaississement de la croûte près du front de chevauchement durant l'emplacement tectonique du complexe interne sur la ceinture de plis et chevauchements externe.<br />Dans le domaine interne le refroidissement et la surrection sont beaucoup plus récents et rapides. Les thermochronomètres (U-Th)/He et traces de fission nous ont permis de déterminer un début de la phase d'extension ~ 20 Ma. Vers 3-6 Ma, le taux d'exhumation des roches de la zone de Korabi s'est accéléré jusqu'à 1,2 km/Ma. Nous proposons que la structure actuelle symétrique des unités ophiolitique dans les Albanides Internes soit le résultat d'un régime de collapse en extension, affectant la partie oriental des ophiolites et la zone de Korabi.<br />Ces études montrent que la complémentarité des méthodes de thermochronologie trace de fission et (U-Th)/He est un moyen puissant de mieux contraindre l'histoire de l'exhumation d'un orogène.

A high-efficiency gamma-ray counting system has been built at Los Alamos National Laboratory for use in analyzing nuclear forensics samples. This system consists of two clover high-purity germanium detectors and is surrounded by a thallium-doped sodium iodide annulus. Special precautions have been taken to ensure the system has a low background. The system is connected to XIA Pixie-4 fast digitizers and collects data in list-mode. This work is split into two main parts. The first part describes the proper steps and techniques to initialize the settings of a detector system connected to fast digitizers in order to optimize the system for resolution and throughput. The various counting modes for this particular system are described in detail, including the benefits and drawbacks of each mode. Steps are then shown to characterize the system by obtaining efficiency curves for various counting modes and sample geometries. Because of the close counting geometry involved with this system, true-coincidence summing factors must be calculated, and are done so in part by measuring the peak-to-total ratios of the system in its various counting modes across a wide energy range. The dead-time for the system can be complicated due to the multiple inputs of the system. Techniques for calculating the dead-time of multiple-detector systems are discussed. The second part of this work shows the system's usefulness in analyzing nuclear forensics samples, specifically irradiated enriched uranium. Three fission product parent-daughter pairs of different lifetimes are analyzed over a course of six months. The activities of each nuclide are calculated at each time step. Age dating techniques using the parent-daughter pairs are discussed, as well as the detection limits of each nuclide for a range of sample ages. Finally, avenues for further research are presented, as well as potential sources of error or uncertainty for this work. / text