3. Electronic Theses and Dissertations (ETDs) - All submissions
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Item Dose reduction benefits of zinc injection into the primary cooling system of a pressurized water reactor(2018) Saaymans, Martin AnthonyKoeberg Nuclear Power Station has set itself the objective of achieving world top quartile performance in terms of dose to workers. Achieving this goal requires implementation of a number of dose reduction initiatives. One such initiative is the continuous injection of depleted zinc acetate into the primary cooling system of the plant as a means of cobalt reduction, the principal contributor to out-of-core radiation fields. The purpose of this research report is to investigate the measure of success in reducing the radiation dose that nuclear plants world-wide have achieved through zinc injection, and how Koeberg measures up to international results. The investigation method included an extensive literature study on international experiences and results of other cobalt reduction methods, the benefits of zinc injection, as well as the potential negative impact it may have on major plant components. The report also presents the results of Koeberg’s zinc monitoring programme while comparing it to international trends. A recommendation is made for a more coordinated monitoring programme at Koeberg in order to obtain maximum benefits in the long term. Notwithstanding, the research has led to the conclusion that Koeberg is achieving the desired results compared to industry performances for plants that are in the same phase of their respective zinc injection programmes.Item Hydrogenation of carbon monoxide over modified cobalt-based catalysts(1991) Colley, Saul EricA disadvantage of the Fischer-Tropsch synthesis is that a broad product spectrum is obtained. Economic considerations however require an improvement in the optimization of the reaction to maximize the production of high value commercial products, in·particular, short chain olefins and high molecular weight hydrocarbons. [Abbreviated abstract. Open document to view full version]Item The CIS influence of the corrin ring in cobalt corrins(2016) Ghadimi, NafiseIt is well-established that there is electronic communication between the equatorial and axial ligands in the cobalt corrins. It can therefore be anticipated that the electronic structure of the corrin ligand will affect the chemistry of the axial coordination sites of Co(III) in these complexes. To probe this cis-influence the electronic structure of the corrin was perturbed by substituting the H atom at C10 by Br (which is π electron-donating towards the corrin) in aquacobalamin ([H2OCbl]+), and by NO2 (which is strongly electron-withdrawing) and NH2 (which is strongly electron-donating) in aquacyanocobester ([ACCbs]+). The first part of this study was dedicated to aqua-10-bromocobalamin ([H2O-(10-Br)Cbl]+) and the second part to aquacyano-10-nitrocobester ([AC-(10-NO2)Cbs]+) and aquacyano-10-aminocobester ([AC-(10-NH2)Cbs]+). The successful synthesis of [H2O-(10-Br)Cbl]+, was verified by ESI-MS, 1H and 13C NMR, uv-vis spectroscopy and XRD. The stability constants for the substitution of coordinated H2O by a series of anionic (N3 –, NO2 –, SCN–, SO3 2–) and neutral N-donor ligands (imidazole, DMAP) were obtained for [H2OCbl]+, [H2O-(10-Br)Cbl]+ and [H2O-(10-Cl)Cbl]+ under the same conditions. Substitution of the C10 H by Cl or Br favours the coordination of anionic ligands, but discriminates against the binding of neutral N-donor ligands. The anionic ligands bind more strongly to [H2O-(10-Br)Cbl]+ than to [H2OCbl]+ with log K values between 0.05 and 0.62 (average 0.33) larger. Conversely, neutral ligands bind less strongly to [H2O-(10-Br)Cbl]+ than to [H2OCbl]+ with log K values between 0.29 and 0.36 (average 0.33) smaller. DFT (BP83/TZVP) calculations were used to rationalise these observations. When H is changed to Cl or Br, the metal ion becomes less positive. When the β ligand changes from a neutral to an anionic ligand, the partial charge on the C10 substituent becomes more negative. Replacing C10 H by Cl or Br discriminates against a neutral ligand because of the greater electron richness of the metal. If the ligand is an anion, however, the charge donation can be accepted by delocalisation onto the C10 substituent. The reaction kinetics of the substitution of H2O in [H2O-(10-Br)Cbl]+ were determined for the ligands N3 – and imidazole and were compared with values available for [H2OCbl]+ and [H2O-(10- Cl)Cbl]+. The results showed that both N3 – and imidazole react more slowly with [H2O-(10- Br)Cbl]+ than with [H2OCbl]+, consonant with the previous observations for [H2O-(10-Cl)Cbl]+. Although ΔH‡ values are smaller, they do not compensate for significantly more negative values of ΔS‡, indicative of a transition state that occurs earlier along the reaction coordinate in [H2O- (10-Br)Cbl]+ and [H2O-(10-Cl)Cbl]+ whereas the transition state occurs later along the reaction coordinate with [H2OCbl]+. It is argued that this is a consequence of the lower charge density on the metal, making it a better electrophile both towards the incoming and the departing ligand. Dicyano-10-nitrocobester ([DC-(10-NO2)Cbs]) and dicyano-10-aminocobester ([DC-(10- NH2)Cbs]) were synthesised from dicyanocobester [DCCbs] by established methods and converted to the aquacyano form so that the thermodynamics and kinetics of the substitution of coordinated H2O by a variety of ligands could be investigated. The stability constants for the substitution of coordinated H2O by a number of neutral (imidazole, DMAP, methylamine) and anionic (N3 –, NO2 –, SCN–, SO3 2–, CN–) ligands were determined for [ACCbs]+, [AC-(10-NO2)Cbs]+ and [AC-(10-NH2)Cbs]+ in 50% isopropanol. The soft anions (SO3 2– and CN–) bind better to the softer Co(III) metal centre in [AC-(10-NH2)Cbs]+ and [ACCbs]+ than in [AC-(10-NO2)Cbs]+ and the converse is true for the hard anions (N3 –, NO2 – and SCN–). The case is less clear for the N-donor ligands; DMAP clearly has a higher affinity for [AC-(10- NH2)Cbs]+ and [ACCbs]+ than for [AC-(10-NO2)Cbs]+, but there is little discrimination in the case of imidazole and methylamine. This implies that the affinity of the metal for an exogenous ligand depends on the electron density at the metal centre. DFT calculations showed that as the C10 substituent is changed from NH2 to H to NO2, the charge density on the metal centre decreases and the metal becomes harder. The kinetics of the substitution of H2O by CN– in [ACCbs]+, [AC-(10-NO2)Cbs]+ and[AC-(10- NH2)Cbs]+ in 50% isopropanol were determined. The results showed that the substitution of coordinated H2O proceeded with biphasic kinetics and through a dissociative interchange (Id) mechanism where there is nucleophilic participation of the entering ligand in the transition state. The slower phase corresponds to the substitution of coordinated H2O trans to OH– in the aqua hydroxo species, which, together with the dicyano species, is inevitably present in solutions of [ACCbs]+, and the faster phase corresponds to the substitution of the coordinated H2O trans to CN– in the aquacyano species. The difference in rate of the reaction of the [AC-(10-Z)Cbs] (Z = H, NH2 and NO2) was not very large, the ratio between the largest (for Z = H) and the smallest (for Z = NO2) is just over 40, and does not follow the electron donor properties of Z. This is misleading, however, because of a compensation effect between ΔH‡ and ΔS‡. As values of ΔH‡ become smaller, which causes an increase in the reaction rate, ΔS‡ becomes less positive (or more negative), which causes a decrease in the reaction rate. Hence, comparing rate constants at any particular temperature is not very informative and the compensation effect masks the very significant differences in the reactivity of the metal ion towards the entering CN– ligand. The compensation effect is attributed to the position of the transition state along the reaction coordinate, which depends on the charge density on the metal ion. Indeed, if all three reactions had the same value of ΔS‡ then the values of the rate constant would be in the approximate ratio 109:106:1 for Z = NH2, H and NO2, respectively. This study shows that how profoundly the perturbation of the electronic structure of the corrin affects the thermodynamic and kinetic properties of the Co(III) ion, and provides further evidence that the unusual chemistry of Co(III) in the cobalt corrins is a consequence of the cis-influence of the equatorial macrocyclic ligand.Item The alpha-eta transformation in cobalt with particular reference to the use of cobalt in cemented carbides(2015) Rees, G JA study of the reduction of black cobalt oxide, using hydrogen as the reductant, showed that the temperature and time used for reduction exerted a profound influence on the proportions of face centred cubic (a) and hexagonal close packed (e) cobalt allotropes obtained in the reduced metal powder. Reduction under certain conditions yielded a faulted hexagonal cobalt powder product which contained stacking faults. The influence of oxygen and titanium on cobalt materials showed that these impurities did not stabilise the proportions of either the cubic or hexagonal phases under the conditions, which were used. The grinding of cobalt powders from different sources showed different tendencies towards agglomeration, when acetome was used as the milling fluid. It was found that powders which contained an initially higher proportion of the hexagonal phase -had a strong tendency towards agglomeration during the initial stages of grinding. The importance of the different phases of cobalt present in the metal powders, and their different agglomeration tendencies on grinding, on the milling required for satisfactory densification of cobalt - tungsten carbide compacts has been shown; it was (iv) found that hard metal alloys prepared using powders with an initially higher proportion of the hexagonal phase required less milling to achieve a given sintered density, than did mixtures prepared with other cobalt powder*;. A correlation analysis between the properties of the milled hard metal powders and the properties of the sintered compacts was carried out; it was found that the most important correlation was between the specific surface area of the milled hard metal powder and the coercivity of the sintered product.Item Studies on hemin and cobalt corrinoids in aqueous solution(2015-01-13) Campbell, Vivien Mary