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Fissile materials in the form of aqueous homogeneous solutions are used during the chemical processing of nuclear materials. In this form there exists the possibility of an accidental criticality. A one-dimensional multi-region model was developed for simulating fast power excursions in a cylinderical homogeneous aqueous fissile solution. The model includes a novel method for computing reactivity feedback that accounts for the axial distribution of power and solution density. The feedback method is based on the change in neutron leakage due to a change in solution density. The model also includes an equation of state derived from quasi-static thermodynamic theory for a solution containing gas bubbles. The model was compared to the KEWB-5 (Kinetic Experiments on Water Boilers) series of experiments. The model could approximate the values of peak power and pressure found in the experiments, but the pressure curves did not match the shapes of the experimental pressure traces.

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A dissertation submitted to the Faculty of Engineering and the Built Environment, University of Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Master of Science in Engineering Johannesburg, 29 August 2016 / This study discusses underground coal gasification (UCG) and the analysis thereof. Two main methods were used. The first is the Bond Equivalent Diagram, which gives an ideal of where operations should take place in relation to their coal and product gas compositions. This method was used to analyze several real life sites for their idealized and actual operations. The second consisted of a comparative exergy simulation study. This was done for an air-blown UCG plant with a downstream Fischer-Tropsch reactor and an oxygen-blown UCG plant with upstream air separation. The plants were analyzed by their overall exergy efficiency as well as their exergy outputs with respect to coal inputs (fuel). It was discovered that the air-blown simulation with downstream Fischer-Tropsch was the better choice from an exergy point of view due to it having higher efficiencies (1.5 for overall, 1.38 for fuel) as opposed to the oxygen-blown simulation (0.77 overall, 0.8 for fuel). This coupled with other design and safety factors led to the conclusion that the air-blown simulation was better. / MT2017

Thesis. 1979. M.S.--Massachusetts Institute of Technology. Alfred P. Sloan School of Management. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND DEWEY. / Includes bibliographical references. / M.S.

Artificial photosynthesis is broadly defined as the process of solar energy conversion into chemical fuels and represents a promising route towards alleviating the global energy crisis. In this context, the development of photocathodes for the use in photoelectrochemical cells is an attractive approach for the storage of solar energy in the form of a chemical energy carrier (e.g. H$_{2}$ and CO$_{2}$-reduction products from H$_{2}$O and CO$_{2}$). However, molecular catalyst-based photocathodes remain scarcely reported and typically suffer from low efficiencies and/or stabilities due to inadequate strategies for interfacing the molecular component with the light-harvesting material, with benchmark systems continuing to rely on precious metal components. In this thesis, the straightforward preparation of a p-silicon|mesoporous titania|molecular catalyst photocathode assembly that is active towards proton reduction in aqueous media is first established. The mesoporous TiO$_{2}$ scaffold acts as an electron shuttle between the silicon and the catalyst, while also stabilising the silicon from passivation and enabling a high loading of molecular catalysts. When a Ni bis(diphosphine)-based catalyst is anchored on the surface of the electrode, a catalytic onset potential of +0.4 V vs. RHE and a high turnover number of 1 $\times$ 10$^{3}$ was obtained from photoelectrolysis under UV-filtered simulated solar irradiation at 1 Sun after 24 hours. Notwithstanding its aptitude for molecular catalyst immobilisation, the Si|TiO$_{2}$ photoelectrode showed great versatility towards different types of catalysts and pH conditions, highlighting the flexible platform it represents for many potential reductive catalysis transformations. The Si|TiO$_{2}$ scaffold was extended towards solar CO$_{2}$ reduction via the immobilisation of a novel phosphonated cobalt bis(terpyridine) catalyst to achieve the first precious metal-free, CO$_{2}$-reducing molecular hybrid photocathode. Reducing CO$_{2}$ in both organic-water and purely aqueous conditions, the activity of this photocathode was shown to be affected by its environment and reached record turnover numbers for CO production by a molecular photocathode under optimal conditions, maintaining stable activity for more than 24 hours. Critically, in-depth electrochemical and in situ resonance Raman and infrared spectroelectrochemical investigations provided key insights into the nature of the surface-bound Co complex under reducing conditions. While demonstrating the power and precision offered by such in situ spectroelectrochemical techniques, these studies ultimately alluded to a catalytic mechanism that contrasts with that reported for the in-solution (homogeneous) catalyst. Overall, this affords a distinct mechanistic pathway that unlocks an earlier catalytic onset and enables photoelectrochemical activity. Finally, in the context of improving product selectivity in molecular-based CO$_{2}$ reduction, polymers based on the cobalt bis(terpyridine) motif were synthesised and immobilised on inverse opal-type electrodes designed specifically to accommodate large molecules. Rational design of the polymers' co-monomers was aimed towards the provision of an artificial environment for the active complex that would influence product selectivity, which was ultimately demonstrated by the improvement of a H$_{2}$:CO product ratio of 1:2 (molecule) to 1:6 (polymer). Further studies of this all-in-one system included modulating its degree of cross-linkage as well as a CO$_{2}$ reducing demonstration photocathode on a Si|inverse-opal TiO$_{2}$ scaffold.

Experimental and kinetic modeling studies were carried out on the conversion a wood-oil obtained from high pressure liquefaction of aspen poplar wood to liquid hydrocarbon fuels and useful chemicals in a fixed bed micro-reactor using HZSM-5 catalyst. Similar experiments were conducted using silicalite, H-mordenite, H-Y and amorphous silica-alumina catalysts. <p> Preliminary vacuum distillation studies showed that the wood-oil was made up of volatile and non-volatile fractions. A maximum yield of 62 wt% volatiles at 200 °C, 172 Pa was obtained. The volatile fraction consisted of over 80 compounds. These compounds were comprised of acids, alcohols, aldehydes, ketones, esters, ethers, furans, phenols and some hydrocarbons. The characteristics of the oil showed that it was unstable with time, i.e., its physical properties and chemical composition changed with time probably due to the reaction of free radicals or the oxidative coupling of some of the wood-oil components. However, when the oil was mixed with tetralin, the stability improved. <p> Upgrading studies were first conducted over inert berl saddles in the presence and absence of steam (i. e. non-catalytic treatment/blank runs). Yields of hydrocarbons were between 16 and 25 wt% of the wood-oil. High residue fractions of between 32 to 56 wt% were obtained after processing. Some portions of wood-oil formed a carbonaceous material (char or coke) when exposed to the experimental temperatures. The chars (coke) fraction increased with temperature from 4.7 to 12.5 wt% when processing with steam and 8.0 to 20.4 wt% when processing without steam. <p> Catalytic upgrading studies were first carried out using HZSM-5 catalyst in the presence and absence of steam. The results showed that approximately 40 to 65 wt% of the oil could be converted to a hydrocarbon-rich product (i.e. desired organic liquid product (distillate). This contained about 45 to 70 wt% hydrocarbons with selectivities ranging between 0.47 to 0.88. This fraction was highly aromatic in nature and consisted mainly of benzene, toluene, xylene (BTX compounds) and other alkylated benzenes within the gasoline boiling point range. The yield and selectivities were strong functions of the process time and temperature. A comparison between the two processes, i.e. upgrading in the presence and absence of steam, showed that about 30 to 45 % reduction in coke formation and 5 to 18 wt% increase in organic distillate could be achieved when processing in the presence of steam. These changes were probably due to changes in the rates of cracking, deoxygenation, aromatization and polymerization reactions resulting from the competitive adsorption processes between steam and wood-oil molecules in addition to changes in contact time of molecules. However, the selectivity for hyqrocarbons decreased in the presence of steam. <p> Yields of organic distillate fractions of between 72 to 93 wt% and hydrocarbon yields and selectivities of 44 to 51 wt% and 0.93 to 1.13, respectively, were obtained when wood-oil volatile fraction was upgraded over HZSM-5 after separation from the non-volatile fraction by vacuum distillation. <p> The spent HZSM-5 catalyst could be easily regenerated and reused with little change in its performance. <p> The yields and selectivities for hydrocarbons when upgrading with the other catalysts were between 9 and 22 wt%, and 0.12 and 0.29, respectively for silicalite, 16 and 28 wt%, and 0.22 and 0.28, respectively for H-mordenite, 15.5 and 21 wt%, and 0.17 and 0.21, respectively for H-Y and S.5 and 26.2, and 0.13 and 0.36, resrectively for silica-alumina. Compared to HZSM-5 (yield between 34 and 43 wt%, selectivity of 0.66 to O.SS) these yields and selectivities were much lower. These experiments also showed that the pore size, acidity and shape selectivity of the catalyst influenced the distribution of hydrocarbons in terms of the carbon number. The yield and selectivity of H-mordenite and H-Y (large pore zeolites) were mostly for kerosene range hydrocarbons (C_{<font size=2>9</font>} to C_{<font size=2>15</font>}) and for silicalite and HZSM-5 (medium pore zeolites) for gasoline range hydrocarbons. The hydrocarbon fraction from amorphous silica-alumina did not show any defined distribution. The performance followed the order: HZSM-5> H-mordenite> H-Y> Silicalite, Silica-alumina.<p> With the aid of model compound reactions involving acetic acid methyl ester, propanoic acid, 4-methylcyclohexanol, methylcyclopentanone, 2-methylcyclopentanone, methoxybenzene, ethoxybenzene, phenol, 2-methoxy-4-(2-propenyl) phenol, a synthetic and wood-oil volatile, two reaction pathways were proposed to explain the chemical steps through which the final products of upgrading were obtained. Also, reaction pathways were proposed for each chemical group. These experiments showed that the final products were formed probably through cracking, deoxygenation, olefin formation, oligomerization, hydrogen and hydride transfer, cyclization, isomerization, alkylation and polymerization reactions. <p> Rate models were derived based upon the two reaction pathways and the power law rate model. The rates of formation of products followed the general order: Organic distillate> Hydrocarbons> Residue> Coke> Gas >Aqueous Fraction. Estimates of the values of the kinetic parameters showed that the rate constants ranged between 10^{<font size=2>-6</font>} (aqueous fraction) and 1.81 (volatile fraction), activation energies between 6.7-76.0 x 10^{<font size=2> 3</font>} KJ/Kmol and reaction orders from 0.7 (gas formation) to 2.5 (residue formation). Two mathematical models were derived based on the integral reactor design equation and on the two reaction pathways. This was used to estimate the yield of products. The models predicted the experimental results fairly accurately. Model discrimination showed that the model based on coke and residue formation from both volatile and non­-volatile fractions of the wood-oil best predicted the experimental results.<p> Hydrocarbon selectivity relations which were based on coke, residue and combined coke and residue as undesired products were also derived. Application of these relations showed that lower temperatures and concentrations were most appropriate for higher hydrocarbon selectivity. However, this was at the expense of higher conversions.