Estudo da homogeneidade e determinação de parâmetros básicos do experimento de leito fluidizado den-01

Neste trabalho foi feito um estudo hidráulico de um experimento a leito fluidizado onde aplicou-se a técnica de absorção gama para a determinação da porosidade do leito. Verificou-se a influência da colocação de uma tela para limitação da altura do leito bem como do comportamento do mesmo com a introdução de um anel metálico de diâmetro próximo do diâmetro do tubo de fluidização. As medidas foram realizadas em um modelo hidráulico especialmente construído para esta finalidade. No modelo foram utilizadas esferas de aço com diâmetro de 8 mm que fluidizaram em água à temperatura ambiente em circuito fechado. Foram feitos testes com 20 kg, 30 kg e 40 kg de esferas. Para as medidas utilizou-se a radiação gama proveniente de uma fonte de cobalto 60 com atividade de 700 mCi colimada com chumbo. O detector utilizado foi um cristal de NaI (Tl). Os resultados obtidos mostram que tanto a tela como o anel provocam a divisão do leito fluidizado em duas partes. A tela faz formar um leito fixo invertido na sua superfície inferior. O anel faz com que o leito seja transportado para a parte de cima do mesmo a medida em que ele é introduzido no leito. Verificou-se que a maior expansão do leito se dá para uma relação entre a altura do leito compactado e o diâmetro do tubo de fluidização de 0,55, correspondendo ao teste com 30 kg de esferas. A formação de jorro acontece sempre para a mesma vazão, em torno de 16 l/s, independente da relação entre a altura do leito compactado e o diâmetro do tubo de fluidização. Na fase de leito particulado a porosidade é menor no centro do leito e aumenta nas camadas superiores e inferiores.In this work an experimental hydraulical study of the fluidized bed reactor concept was performed, where the absorption gamma ray technique was applied for the determination of bed porosity. The influence of the use of a screen for the limitation of the bed height was investigated as well as the behavior of the bed by introducing a metal ring with a diameter close to the diameter of the fluidization tube. Measurements were performed in a hydraulical model built with this purpose. In the model, steel spheres with 8 mm diameter were fluidized in water at room temperature in a closed loop. Tests with 20, 30 and 40 kg of spheres were made. Bed porosity was measured with a cobalt-60 gamma ray source, with 700 mCi activity, and a NaI (Tl) crystal detector. Results show that both screen and ring split the bed in two parts. The screen leads to the formation of a fixed bed in the top of the test section. By moving the ring downwards, the bed is displaced upwards. It was observed that the largest bed expansion occurs for a value of 0.55 ofthe ratio between the height ofthe compacted bed and the tube diameter, which corresponds to a mass of 30 kg of spheres. Jet formation occurred always for the same flow rate, about 16 l/h, without dependence on the ratio between the height ofthe fixed bed and tube diameter. For the particulate bed regime, porosity is lower at the center o f the bed, increasing at the upper and lower layers

Fuga de radiação de unidades de radiologia pediátrica

Em radiologia pediátrica, é necessário que uma pessoa segure os pacientes durante a exposição, já gue os pacientes são crianças e recém-nascidos. Portanto, torna-se importante uma apropriada determinação e minimização da radiação de fuga das unidades de radiodiagnóstico. Fez-se medidas de radiação de fugas em cinco unidades de raios X, sendo quatro unidades produzidas pela mesma companhia. Três das quatro unidades produzidas pela mesma companhia apresentaram uma contribuição anormal da radiação de fuga. Discute-se a não-adequação dos atuais cabeçotes para radiologia pediátrica e apresenta-se sugestões para um novo enfoque em radiação pediátrica.In pediatric radiology, it is necessary that a person stay with the patients, as they are children and newly bom, during radiation exposure. Therefore, the proper evaluation and minimization of radiation leakage from radiodiagnostic units becomes important. Measurements of leakage radiation were carried out in five X-ray units, where four of which are manufactured by the same company. Three of the four units produced by the same company, showed an abnormal contribution of the leakage radiation. The inadequacy of the presently available housing tubes for pediatric radiology is discussed. Suggestions regarding new approach in pediatric radiology are presented

Estudo da homogeneidade e determinação de parâmetros básicos do experimento de leito fluidizado den-01

Neste trabalho foi feito um estudo hidráulico de um experimento a leito fluidizado onde aplicou-se a técnica de absorção gama para a determinação da porosidade do leito. Verificou-se a influência da colocação de uma tela para limitação da altura do leito bem como do comportamento do mesmo com a introdução de um anel metálico de diâmetro próximo do diâmetro do tubo de fluidização. As medidas foram realizadas em um modelo hidráulico especialmente construído para esta finalidade. No modelo foram utilizadas esferas de aço com diâmetro de 8 mm que fluidizaram em água à temperatura ambiente em circuito fechado. Foram feitos testes com 20 kg, 30 kg e 40 kg de esferas. Para as medidas utilizou-se a radiação gama proveniente de uma fonte de cobalto 60 com atividade de 700 mCi colimada com chumbo. O detector utilizado foi um cristal de NaI (Tl). Os resultados obtidos mostram que tanto a tela como o anel provocam a divisão do leito fluidizado em duas partes. A tela faz formar um leito fixo invertido na sua superfície inferior. O anel faz com que o leito seja transportado para a parte de cima do mesmo a medida em que ele é introduzido no leito. Verificou-se que a maior expansão do leito se dá para uma relação entre a altura do leito compactado e o diâmetro do tubo de fluidização de 0,55, correspondendo ao teste com 30 kg de esferas. A formação de jorro acontece sempre para a mesma vazão, em torno de 16 l/s, independente da relação entre a altura do leito compactado e o diâmetro do tubo de fluidização. Na fase de leito particulado a porosidade é menor no centro do leito e aumenta nas camadas superiores e inferiores.In this work an experimental hydraulical study of the fluidized bed reactor concept was performed, where the absorption gamma ray technique was applied for the determination of bed porosity. The influence of the use of a screen for the limitation of the bed height was investigated as well as the behavior of the bed by introducing a metal ring with a diameter close to the diameter of the fluidization tube. Measurements were performed in a hydraulical model built with this purpose. In the model, steel spheres with 8 mm diameter were fluidized in water at room temperature in a closed loop. Tests with 20, 30 and 40 kg of spheres were made. Bed porosity was measured with a cobalt-60 gamma ray source, with 700 mCi activity, and a NaI (Tl) crystal detector. Results show that both screen and ring split the bed in two parts. The screen leads to the formation of a fixed bed in the top of the test section. By moving the ring downwards, the bed is displaced upwards. It was observed that the largest bed expansion occurs for a value of 0.55 ofthe ratio between the height ofthe compacted bed and the tube diameter, which corresponds to a mass of 30 kg of spheres. Jet formation occurred always for the same flow rate, about 16 l/h, without dependence on the ratio between the height ofthe fixed bed and tube diameter. For the particulate bed regime, porosity is lower at the center o f the bed, increasing at the upper and lower layers

Experimental hydraulic study of a fluidized-bed reactor

This article reports the results of an experimental rig that was developed to study the bed fluidization in a fluidized-bed nuclear reactor model by measuring bed porosity using gamma absorptiometry. This hydraulic study reveals some problems with the fluidized bed control resulting from the original design concept for some reactor components. As a consequence, the limiting screen and control ring will have to be redesigned, making possible the formation of a stable fluidization for fuel/moderator aspect ratio between 0.75 and 1.0

The PLATO Mission

International audiencePLATO (PLAnetary Transits and Oscillations of stars) is ESA's M3 mission designed to detect and characterise extrasolar planets and perform asteroseismic monitoring of a large number of stars. PLATO will detect small planets (down to <2 R_(Earth)) around bright stars (<11 mag), including terrestrial planets in the habitable zone of solar-like stars. With the complement of radial velocity observations from the ground, planets will be characterised for their radius, mass, and age with high accuracy (5 %, 10 %, 10 % for an Earth-Sun combination respectively). PLATO will provide us with a large-scale catalogue of well-characterised small planets up to intermediate orbital periods, relevant for a meaningful comparison to planet formation theories and to better understand planet evolution. It will make possible comparative exoplanetology to place our Solar System planets in a broader context. In parallel, PLATO will study (host) stars using asteroseismology, allowing us to determine the stellar properties with high accuracy, substantially enhancing our knowledge of stellar structure and evolution. The payload instrument consists of 26 cameras with 12cm aperture each. For at least four years, the mission will perform high-precision photometric measurements. Here we review the science objectives, present PLATO's target samples and fields, provide an overview of expected core science performance as well as a description of the instrument and the mission profile at the beginning of the serial production of the flight cameras. PLATO is scheduled for a launch date end 2026. This overview therefore provides a summary of the mission to the community in preparation of the upcoming operational phases

The PLATO Mission

International audiencePLATO (PLAnetary Transits and Oscillations of stars) is ESA's M3 mission designed to detect and characterise extrasolar planets and perform asteroseismic monitoring of a large number of stars. PLATO will detect small planets (down to <2 R_(Earth)) around bright stars (<11 mag), including terrestrial planets in the habitable zone of solar-like stars. With the complement of radial velocity observations from the ground, planets will be characterised for their radius, mass, and age with high accuracy (5 %, 10 %, 10 % for an Earth-Sun combination respectively). PLATO will provide us with a large-scale catalogue of well-characterised small planets up to intermediate orbital periods, relevant for a meaningful comparison to planet formation theories and to better understand planet evolution. It will make possible comparative exoplanetology to place our Solar System planets in a broader context. In parallel, PLATO will study (host) stars using asteroseismology, allowing us to determine the stellar properties with high accuracy, substantially enhancing our knowledge of stellar structure and evolution. The payload instrument consists of 26 cameras with 12cm aperture each. For at least four years, the mission will perform high-precision photometric measurements. Here we review the science objectives, present PLATO's target samples and fields, provide an overview of expected core science performance as well as a description of the instrument and the mission profile at the beginning of the serial production of the flight cameras. PLATO is scheduled for a launch date end 2026. This overview therefore provides a summary of the mission to the community in preparation of the upcoming operational phases