Des recherches sur de nouvelles sources énergétiques ont été initiées pour faire face à la hausse de la demande mondiale. Dans le domaine nucléaire, la filière Th-U est envisagée pour compléter ou remplacer la filière U-Pu. Toutefois, des améliorations technologiques sont nécessaires avant l’aboutissement industriel de ce projet, notamment sur l’adaptation du procédé de retraitement du combustible U-Pu à celui du cycle Th-U, qui consiste à dissoudre le combustible en milieu acide nitrique, suivi d’une extraction liquide-liquide. Or, contrairement à l’uranium, l’oxyde de thorium n’est pas soluble dans HNO₃ concentré. Un ajout de petites quantités de fluorures est nécessaire à l’obtention d’une dissolution totale. Cependant, la dissolution reste lente et le milieu HNO₃-HF est très corrosif. L’objectif de cette étude est de comprendre les réactions mises en jeu afin d’optimiser la composition de la solution pour diminuer la corrosion et améliorer la cinétique de dissolution. La poudre d’oxyde de thorium synthétisée en laboratoire est solubilisée dans des conditions expérimentales et selon un protocole prédéfinis. L’effet des caractéristiques du solide, de la méthode de dissolution et du milieu de dissolution ont été étudiés. La morphologie de l’oxyde a une forte influence sur la vitesse de dissolution. L’impact majeur des ultrasons de puissance est l’augmentation de la température, qui améliore considérablement la vitesse de dissolution. Le remplacement des fluorures par d’autres complexants du thorium n’a pas permis d’atteindre une dissolution quantitative de l’oxyde. L’augmentation des concentrations de HF et de HNO₃ accélère la dissolution, mais une concentration trop importante de fluorure mène à la formation d’un précipité blanc qui passive la surface. Sur la base de cette étude, un mécanisme réactionnel a été proposé. L’étape limitante est la formation d’un complexe activé. L’écriture et la résolution des équations cinétiques et des équilibres thermodynamiques a permis d’établir une relation analytique exprimant le taux de dissolution de ThO₂ en fonction des paramètres expérimentaux. La simulation des résultats expérimentaux à l’aide de cette relation a ensuite permis de déterminer les constantes thermodynamiques et cinétiques rendant compte de tous les phénomènes observés, validant ainsi le mécanisme proposé. / Studies of new energy sources are necessary to meet the rising global demand. In the nuclear area, Th-U cycle has been reinvestigated to supplement or replace the currently used U-Pu cycle. This project though needs further improvement to be operated in an industrial plant, especially on the reprocessing process, which consists in fuel dissolution in nitric acid medium, followed by liquid-liquid extraction. Still, unlike uranium, thorium oxide does not dissolve in concentrated nitric acid. Small amounts of fluoride are required to achieve the dissolution. The dissolution is rather slow and HNO₃-HF mixture is very corrosive. The aim of this project is thus to find an efficient dissolution method which both decreases corrosion and improves dissolution rate. The synthetized thorium oxide powder has been dissolved in chosen conditions. Effects of solid parameters, dissolution method and dissolution medium have been studied. Results show a strong dependence on oxide crystallinity. No improvement on dissolution rate was observed with power ultrasounds, except for the temperature increase, which greatly enhances dissolution rate. No other complexing agents than fluoride allows total dissolution. Rising HNO₃ and HF concentrations increases dissolution rate until the amount of fluorides is so high that a precipitate forms at the surface. This study led to the proposal of a dissolution mechanism whose limiting step is the formation of an activated complex. Based on kinetics and equilibrium equations, initial dissolution rate was then written as a function of the different studied parameters. Experimental results were finally fitted by this relation to find kinetics and thermodynamics constants, proving the accuracy of the proposed mechanism.

With current energy demands globally and locally, nuclear energy remains one of the top competitors for cleaner and sustainable energy. The nuclear industry requires more inherent safety and proliferation resistance in reactor design. Thorium has therefore been identified as a possible fuel for future nuclear reactors that can comply with these requirements. However current extraction techniques are expensive, time consuming and generate large quantities of hazardous waste. A possible alternative to conventional solvent extraction of thorium is SFE (Supercritical Fluid Extraction). A monazite sample from the Steenkampskraal mine was investigated using SEM (Scanning Electron Microscope) analysis methods to determine the distribution of thorium in the grains that could potentially complicate the effectiveness of the SFE extraction method if zoning is present. The results show a homogeneous distribution with no discernable zonation in the grains. The concentration of Th, Ce and Nd was determined by quantitative MPA (Micro Probe Analysis). The results obtained from the MPA point analysis on several grains show average Th, Ce and Nd concentrations of 6.5 wt. %, 24.1 wt. % and 9.7 wt. % respectively. The extraction of Th+4 from a filter paper was conducted to verify the extraction procedure and extractability of transition elements employing SFE. The extraction was conducted using supercritical CO2 and methanol as co-solvent with TBP (Tributyl Phosphate) and HFA (Hexafluoroacetylacetone) added in situ as chelates. ICP-MS results for the Th+4 extraction procedure showed extraction efficiency of 53 % compared to 83 % in literature (Kumar et al. 2009). This marked difference in extraction efficiency is attributed to ineffective trapping methods employed and lack of prior maintenance and support on the extraction apparatus. Subsequently all further extracted samples of Th from monazite were tested using XRF analysis methods. Due to the lack of prior maintenance on the extraction apparatus several technical breakdowns were encountered and addressed from a mechanical engineering standpoint. The operational effectiveness of the modified apparatus was verified through the extraction of marula seed oil and compared with another supercritical fluid (SF) extractor to show 50 % extraction efficiency in each case. A review of the literature indicated that the crystal chemical requirements for substitution of trivalent (Ce+3) for tetravalent (Th+4) may be fulfilled during SFE processes. Experimental substitution extractions were conducted by addition of different chelates and were conducted by subjecting the monazite samples to 20 MPa pressure for 180 min static flow and 10 min continuous flow extraction times with a CO2 flow rate of 2 mL/min with 10 % co-solvent flow rate. The results of the two sets of substitution extractions namely α and β show no clear indication of Th extraction. The maximum theoretical efficiency obtainable under current extraction equipment limitations was calculated as 12%. The XRF analysis error margin was given by the analytical laboratory as 10 %. The literature has shown the substitution of trivalent cations for tetravalent cations in the monazite structure to be a valid reaction mechanism. The experimental results showed little or no success in extracting thorium from monazite. In order to prove the practical feasibility of thorium extraction several changes to the experimental operating conditions is required. / MIng (Mechanical Engineering), North-West University, Potchefstroom Campus, 2015

With current energy demands globally and locally, nuclear energy remains one of the top competitors for cleaner and sustainable energy. The nuclear industry requires more inherent safety and proliferation resistance in reactor design. Thorium has therefore been identified as a possible fuel for future nuclear reactors that can comply with these requirements. However current extraction techniques are expensive, time consuming and generate large quantities of hazardous waste. A possible alternative to conventional solvent extraction of thorium is SFE (Supercritical Fluid Extraction). A monazite sample from the Steenkampskraal mine was investigated using SEM (Scanning Electron Microscope) analysis methods to determine the distribution of thorium in the grains that could potentially complicate the effectiveness of the SFE extraction method if zoning is present. The results show a homogeneous distribution with no discernable zonation in the grains. The concentration of Th, Ce and Nd was determined by quantitative MPA (Micro Probe Analysis). The results obtained from the MPA point analysis on several grains show average Th, Ce and Nd concentrations of 6.5 wt. %, 24.1 wt. % and 9.7 wt. % respectively. The extraction of Th+4 from a filter paper was conducted to verify the extraction procedure and extractability of transition elements employing SFE. The extraction was conducted using supercritical CO2 and methanol as co-solvent with TBP (Tributyl Phosphate) and HFA (Hexafluoroacetylacetone) added in situ as chelates. ICP-MS results for the Th+4 extraction procedure showed extraction efficiency of 53 % compared to 83 % in literature (Kumar et al. 2009). This marked difference in extraction efficiency is attributed to ineffective trapping methods employed and lack of prior maintenance and support on the extraction apparatus. Subsequently all further extracted samples of Th from monazite were tested using XRF analysis methods. Due to the lack of prior maintenance on the extraction apparatus several technical breakdowns were encountered and addressed from a mechanical engineering standpoint. The operational effectiveness of the modified apparatus was verified through the extraction of marula seed oil and compared with another supercritical fluid (SF) extractor to show 50 % extraction efficiency in each case. A review of the literature indicated that the crystal chemical requirements for substitution of trivalent (Ce+3) for tetravalent (Th+4) may be fulfilled during SFE processes. Experimental substitution extractions were conducted by addition of different chelates and were conducted by subjecting the monazite samples to 20 MPa pressure for 180 min static flow and 10 min continuous flow extraction times with a CO2 flow rate of 2 mL/min with 10 % co-solvent flow rate. The results of the two sets of substitution extractions namely α and β show no clear indication of Th extraction. The maximum theoretical efficiency obtainable under current extraction equipment limitations was calculated as 12%. The XRF analysis error margin was given by the analytical laboratory as 10 %. The literature has shown the substitution of trivalent cations for tetravalent cations in the monazite structure to be a valid reaction mechanism. The experimental results showed little or no success in extracting thorium from monazite. In order to prove the practical feasibility of thorium extraction several changes to the experimental operating conditions is required. / MIng (Mechanical Engineering), North-West University, Potchefstroom Campus, 2015

No description available.

No description available.

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.

M.Sc. / Please refer to full text to view abstract

O sistema Cananéia-Iguape é um grande complexo de canais lagunares e estuarinos, localizado no sul do estado de São Paulo. Este sistema passou a receber as águas do rio Ribeira de Iguape após a abertura do canal do Valo Grande, finalizado no ano de 1852. Este canal tornou-se a principal rota de transporte das produções da região do Vale do Ribeira, e nas décadas de 1940 a 1990 as atividades de mineração da região contaminaram o rio Riberia de Iguape, causando alterações nas características originais do sistema Cananéia-Iguape. Este estudo avaliou os níveis de urânio e tório nas amostras de sedimentos superficiais deste sistema. Esta avaliação foi feita através de um método espectrofotométrico utilizando Arsenazo III. As amostras foram digeridas em um forno micro-ondas com aplicação de HNO3, HF e H2O2. As concentrações de urânio variam de 1,3 mg·kg-1 a 5,8 mg·kg-1 e as de tório de 0,82 mg·kg-1 a 12,1 mg·kg-1. Estes resultados foram comparados com os dados da região, e observou-se que as concentrações determinadas são similares. O índice de geoacumulação foi calculado, classificando a região como \"Não contaminada a moderadamente contaminada\" para urânio e \"Não contaminada\" para tório, indicando que não há contaminação por estes metais. Estes resultados podem ser considerados como a concentração basal da região. As avaliações estatísticas identificaram uma maior afinidade do urânio e do tório com a fração de argila e silte. As concentrações de U e Th não apresentaram um comportamento homogêneo ao longo do sistema estudado. / The Cananéia-Iguape system is a huge complex of lagoon and estuarine channels, located in the south of São Paulo State. This system received the waters of Ribeira de Iguape River, after the construction of the Valo Grande Channel, an artificial channel which was concluded in 1852. This channel became an important route of transportation of the agricultural production from Ribeira Valley, reducing the transportation time and cost. From 1940 and 1990, the mining activities of the region contaminated the waters from Ribeira de Iguape River, changing the original characteristic of the Cananéia-Iguape system. This study evaluated the uranium and thorium concentration in superficial sediments samples from Cananéia-Iguape system. The evaluation was performed by a spectrophotometric method with Arsenazo III. The samples were digested with HNO3, HF and H2O2, on a microwave. The concentration of uranium varied between 1.3 mg·kg-1 and 5.8 mg·kg-1, and thorium concentration varied between 0.82 mg·kg-1 and 12.1 mg·kg-1. The results obtained were compared with literature values and were very similar. The geo-accumulation index was applied, and the region was classified as \"No contaminated or moderately contaminated\" for uranium and \"No contaminated\" for Th; indicating that the region is not contaminated by these elements. The results obtained can be considered as the baseline of the region. The statistical treatment of the results showed that U and Th presented a good correlation with the fraction clay and silt. The concentrations of U and Th did not present a homogeneous behavior along the system studied.

O objetivo do presente trabalho é determinar a concentração de atividade de isótopos de ocorrência natural de urânio e tório e o polônio em perfis de sedimento marinho da Baixada Santista. A Baixada Santista é uma região populosa e abrange os municípios de Bertioga, Guarujá, Santos, São Vicente, Cubatão, Praia Grande, Mongaguá, Itanhaém e Peruíbe. Esta é uma das mais importantes regiões industriais do Brasil, dado ao grande número de indústrias, como as de fertilizantes, petroquímicas e do aço. A grande atividade industrial, juntamente com a grande população da região, exerce forte pressão sobre a biota marinha. O presente trabalho analisou 10 perfis de sedimentos marinhos extraídos da região. Foram coletados perfis com tamanhos variando de 41 a 98 cm, na região de Santos, Cubatão, São Vicente e Bertioga. Os perfis foram fatiados a cada 2 cm e passaram por um pré-tratamento físico e químico. Para a dissolução total e digestão foram utilizados ataques com ácidos fortes. As técnicas radioquímicas utilizadas para a separação dos radionuclídeos e preparação das fontes radioativas, para a contagem alfa, foram separação por cromatografia de troca iônica e eletrodeposição para urânio e tório e deposição espontânea para o polônio. Os resultados obtidos para as concentrações de atividade dos radionuclídeos são semelhantes aos de sedimentos marinhos em regiões não contaminadas, exceto em alguns pontos amostrados, nos quais foi possível identificar influência antrópica. / The objective of the present work is to determine the activity concentration of uranium and thorium isotopes and polonium in estuarine sediments from BaixadaSantista. The area comprehends nine cities in the Brazilian coast (Bertioga, Guarujá, Santos, São Vicente, Cubatão, Praia Grande, Mongaguá, Itanhaém and Peruíbe). It is one of the most important industrial areas in Brazil due to the large number of industries operating in Cubatão, such as steel, petrochemical and fertilizer industries. That, together with a large population, causes a negative impact on the marine biota. In the present work 10 profiles of sediment cores, with depth varying from 41 to 98 cm, were analyzed. The cores were obtained from Santos, Cubatão, São Vicente and Bertioga. The cores were sliced every 2 cm and were treated with both physical and chemical processes, including drying, grinding and sieving. The samples were dissolved by acid digestion, till total dissolution and destruction of organic matter. After the acid digestion, the samples were filtered and evaporated till dryness. The radiochemical techniques used to separate the radionuclides and to prepare the final source for alpha counting were ion exchange chromatography and spontaneous deposition for polonium and electrodeposition for U and Th. The final sources were counted in a surface barrier alpha spectrometer. Values obtained for the radionuclides analyzed were similar to the activity concentrations of sediments in non polluted areas, with the exception of a few samples that showed anthropic influence.

O Oceano Austral é o maior repositório dos oceanos, com concentrações elevadas de nutrientes e baixas de clorofila. A região tem papel fundamental para regular a transferência líquida de dióxido de carbono entre o oceano e a atmosfera, em parte devido à assimilação anual de CO2 pelo fitoplâncton, que resulta na exportação do carbono orgânico particulado (POC) para o oceano profundo. O elemento tório apresenta múltiplos isótopos que tem emergido como traçadores poderosos dos processos associados à transferência de material particulado nos oceanos. De todos os isótopos de Th, o 234Th (meia-vida de 24,1 d) tem sido foco de destaque e aplicação nos últimos anos. A produção do 234Th a partir do 238U, além do comportamento conservativo solúvel do 238U na água do mar, tornam fácil a caracterização das fontes de 234Th neste cenário. Além disso, a meiavida do 234Th é suficientemente curta para torná-lo sensível a mudanças a curto prazo (sazonais) que ocorrem na coluna dágua superficial dos oceanos, ou nas colunas dágua e de sedimentos nas regiões costeiras. Devido ao seu comportamento partículo-reativo, o 234Th é removido da água de duas formas, por decaimento ou pelo fluxo de material particulado. Desta forma, um modelo de balanço uni-dimensional no estado estacionário pode ser utilizado para estimar seu fluxo. Os isótopos naturais de Ra também tem sido amplamente empregados em estudos marinhos para traçar massas dágua e quantificar processos de mistura. Este trabalho apresenta os resultados de um projeto de pesquisa multidisciplinar que investigou a distribuição dos fluxos de carbono orgânico no Estreito de Bransfield com a finalidade de avaliar sua influência na remoção de CO2 da atmosfera local. As distribuições de macro-nutrientes, micro-nutrientes e clorofila-a foram utilizadas para examinar possíveis fontes e vias de transferência. Os isótopos de Ra foram aplicados como traçadores de massas dágua, enquanto o 234Th foi utilizado como traçador do fluxo particulado no oceano, uma vez que o afundamento deste material é o principal mecanismo de seqüestro de carbono no Oceano Austral. As amostras foram coletadas no período de 9 a 14 de Dezembro de 2007, a bordo do Navio de Apoio Oceanográfico Ary Rongel, da Marinha do Brasil. / The Southern Ocean is the largest of several high-nutrient, low-chlorophyll (HNLC) regions in the worlds oceans. This region plays a major role in regulating the global net transfer of carbon dioxide between the ocean and the atmosphere, in part because the annual photosynthetic uptake of CO2 by phytoplankton and resulting export of particulate organic carbon (POC) to the deep ocean. The element thorium has multiple radioisotopes that have emerged collectively as a powerful set of tracers for particle associated processes in the oceans. Of all the Th isotopes, 234Th (half-life 24.1 d) has been the focus of increasing attention and application in the past years. The production of 234Th from 238U, coupled with the conservative behavior of 238U in seawater, makes the source of 234Th easy to characterize. Moreover, the half-life of 234Th is sufficiently short to make it sensitive to the short-term (e.g. seasonal) changes that occur in the upper water column of the open ocean or in sediments or water column in coastal areas. Because of its very particle reactive behavior, 234Th is removed from a parcel of water in only two ways, through decay and through particle flux. Therefore, a steady-state 1D activity balance can be used to calculate its flux. Natural Ra isotopes have been also widely used in marine studies to trace water masses and to quantify mixing processes. This work presents results of a collaborative research on organic carbon fluxes distribution in the Bransfield Strait in order to evaluate its influence in the CO2 drawdown. Macro-nutrients, micro-nutrients and chlorophyll-a distributions were used to examine the pathway sources. Natural radium isotopes were applied as tracers to study the movement of shelf water, while 234Th was used as a tracer of particle flux in the upper ocean, since POC export via sinking particles is the primary mechanism of carbon sequestration in the Southern Ocean. Sea water samples for total 234Th and natural Ra isotopes were collected from 9 to 14 December 2007 on the RV Ary Rongel (Brazilian Navy). Analyses of total 234Th activities were performed by gross beta counting using a gas-flow low background proportional counter. The longlived Ra isotopes were determined by low-background high efficiency gamma spectrometry.