Approximation of the Neutron Diffusion Equation on Hexagonal Geometries

La ecuación de la difusión neutrónica describe la población de neutrones de un reactor nuclear. Este trabajo trata con este modelo para reactores nucleares con geometría hexagonal. En primer lugar se estudia la ecuación de la difusión neutrónica. Este es un problema diferencial de valores propios, llamado problema de los modos Lambda. Para resolver el problema de los modos Lambda se han comparado diferentes métodos en geometrías unidimensionales, resultando como el mejor el método de elementos espectrales. Usando este método discretizamos los operadores en geometrías bidimensiones y tridimensionales, resolviendo el problema algebraica de valores propios resultante con el método de Arnoldi. La distribución de neutrones estado estacionario se utiliza como condición inicial para la integración de la ecuación de la difusión neutrónica dependiente del tiempo. Se utiliza un método de Euler implícito para integrar en el tiempo. Cuando un nodo está parcialmente insertado aparece un comportamiento no físico de la solución, el efecto ``rod cusping'', que se corrige mediante la ponderación de las secciones eficaces con el flujo del paso de tiempo anterior. Cuando la solución de los sistemas algebraicos que surgen en el método hacia atrás, un método de Krylov se utiliza para resolver los sistemas resultantes, y diferentes estrategias de precondicionamiento se evalúan se. La primera consiste en el uso de la estructura de bloque obtenido por los grupos de energía para resolver el sistema por bloques, y diferentes técnicas de aceleración para el esquema iterativo de bloques y un precondicionador utilizando esta estructura de bloque se proponen. Además se estudia un precondicionador espectral, que hace uso de la información en un subespacio de Krylov para precondicionar el siguiente sistema. También se proponen métodos exponenciales de segundo y cuarto orden integrar la ecuación de difusión neutrónica dependiente del tiempo, donde la exponencial de la matriz del sistema tiene quGonzález Pintor, S. (2012). Approximation of the Neutron Diffusion Equation on Hexagonal Geometries [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/17829Palanci

Micellar Electrokinetic Remediation of TNT From Soil.

An assessment of the feasibility of micellar electrokinetic (EK) remediation of TNT from soils is presented. Batch solubilization tests and bench scale EK tests were conducted to select the best candidate surfactant and to evaluate the efficiency of removal of TNT from a real world soil. From batch tests, it was found that SDS gave the best desorption results among a group of candidate surfactants, which includes DOWFAX 8390 (anionic), Tween 80 and Brij 35 (nonionic) and CTAB and CTAC (cationic). Unenhanced and surfactant enhanced EK tests was unable to move substantial amounts of TNT across the specimen to the electrodes. TNT concentrations reduced significantly in the soil sample at sections closest to the cathode after EK processing. This was postulated to be due to transformation of TNT to TNT anions (Jackson-Meisenheimer anions or the Janovsky\u27s complex). EK tests with neutralization at the electrodes achieved both characteristics, i.e. improvement of electroosmotic flow and preventing TNT transformation to TNT anions. Changing the process parameters, such as improving electroosmotic flow, maintaining a constant pH across the medium and extending processing period also did not improve the process in terms of efficiency is evaluated by pulsing the process. The pulse type processing also did not improve electroosmotic flow nor it improved TNT transport for 5% Tween 80 solution. Electroosmotic flow significantly increased in pulse type loading with 20% methanol solution. From the limited data obtained, it is demonstrated that a pulse type loading may be a more efficient technique to drive pore fluid into the soil compared to a continuous current method. There is also evidence of improved transport of TNT in the soil specimen in this type of system. This study provides further evidence to demonstrate that nonpolar species such as TNT cannot be transported from soils by electroosmosis. The attempt to desorb, solubilize and polarize TNT by the formation of micelles with SDS, although successful in batch extraction through vigorous shaking, becomes ineffective in extracting TNT in bench scale EK experiments

Monolithic Perimeter Gated Single Photon Avalanche Diode Based Optical Detector in Standard CMOS

Since the 1930\u27s photomultiplier tubes (PMTs) have been used in single photon detection. Single photon avalanche diodes (SPADs) are p-n junctions operated in the Geiger mode. Unlike PMTs, CMOS based SPADs are smaller in size, insensitive to magnetic fields, less expensive, less temperature dependent, and have lower bias voltages. Using appropriate readout circuitry, they measure properties of single photons, such as energy, arrival time, and spatial path making them excellent candidates for single photon detection. CMOS SPADs suffer from premature breakdown due to the non-uniform distribution of the electric field. This prevents full volumetric breakdown of the device and reduces the detection effciency by increasing the noise. A novel device known as the perimeter gated SPAD (PGSPAD) is adopted in this dissertation for mitigating the premature perimeter breakdown without compromising the fill-factor of the device. The novel contributions of this work are as follows. A novel simulation model, including SPICE characteristics and the stochastic behavior, has been developed for the perimeter gated SPAD. This model has the ability to simulate the static current-voltage and dynamic response characteristics. It also simulates the noise and spectral response. A perimeter gated silicon photomultiplier, with improved signal to noise ratio, is reported for the first time. The gate voltage reduces the dark current of the silicon photomultiplier by preventing the premature breakdown. A digital SPAD with the tunable dynamic range and sensitivity is demonstrated for the first time. This pixel can be used for weak optical signal application when relatively higher sensitivity and lower input dynamic range is required. By making the sensitivity-dynamic range trade-off the same detector can be used for applications with relatively higher optical power. Finally, an array has been developed using the digital silicon photomultiplier in which the dead time of the pixels have been reduced. This digital photomultiplier features noise variation compensation between the pixels

Effects of sterilization with calcium hypochlorite and of low oxygen pressures

The greater part of the work deals with the effects produced by reduction of the oxygen pressure in the atmosphere surrounding seeds kept in germinative conditions. This was studied with the view to the establishment of correlations between the value of the minimum oxygen pressure necessary for germination and 1) the nature of the food reserve or 2) the extent of aeration of the soil of the usual habitat. After the trial of several forms of apparatus the 'continuous-current. method was adapted to the needs of this problem. Atmospheres with reduced oxygen pressures were obtained by introducing oxygen, produced electrolytically, into a stram of nitrogen supplied from a cylinder. The seeds used in the investigations combined some of the following characteristics:- 1) Presence of absence of endosperm. 2) Carbohydrate or oil as food-reserve. 3) Soil of habitat usually stiff and poorly-aerated or light and well aerated. The results indicate that high minimum oxygen values may be expected for seeds storing oils and usually growing in well- aerated soils whilst low minima are connected with the storage of carbohydrates and the usual occurrence in badly aerated soils. Whilst working on the above it was found necessary to sterilize the seeds. The method recommended by Wilson (A.J.B.1915) was used; but bleaching powder is a strong oxidizing agent and its action upon seed reserves might affect the rate of germination of seeds sterilized in this way. Accordingly a section of the paper embodies attempts made to solve this problem and to determine whether a solution of the specified strength is capable of any action upon the oils stored in seeds. The results indicate a slight stimulatory effect upon the rate of germination of seeds storing oils after sterilization for short periods of time and that such a solution does net react with some classes of oils found stored, in seeds.<p