Artículos (Física Aplicada I)

URI permanente para esta colecciónhttps://hdl.handle.net/11441/10849

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Diffusion equations for nonhomogeneous media. Existence of similarity solutions
(Elsevier, 1985) Romero Romero, Francisco; Romero Romero, J.L.; Archilla, Juan F. R.; Universidad de Sevilla. Departamento de Física Aplicada I
We study the invariance of the diffusion equation δP(x,t)/δt = (δ/δx)[D(x)δP(x,t)/δx] under continuous groups of transformations. We show the conditions which D(x) must satisfy for the existence of similarity solutions.
An Easy Method for the Determination of Ra Isotopes and Actinide Alpha Emitters from the Same Water Sample
(Elsevier, 1986) Morón Romero, María del Carmen; García-Tenorio García-Balmaseda, Rafael; García Montaño, Estrella; García León, Manuel; Madurga, G.; Universidad de Sevilla. Departamento de Física Aplicada I; Universidad de Sevilla. Departamento de Física Aplicada II; Universidad de Sevilla. Departamento de Física Atómica, Molecular y Nuclear; Comisión Asesora de Investigación Científica y Técnica (CAICYT). España
Radium isotopes and actinide α emitters are easily determined from the same water sample. The Ra fraction is obtained by coprecipitation with Ba, while a Fe coprecipitation is used for the actinides. Both fractions are measured with a gas-flow proportional counter. Additionally the isotopic Ra composition is obtained by measuring at two or three different times the α activity from the Ra-fraction. The method is applied to rain water and drinking water samples.
Diffusion in a class of exactly solvable non-harmonic potentials. Intrinsic effects induced by non-linearities
(Elsevier, 1990) Romero Romero, Juan Luis; Ramírez, J.; Romero Romero, Francisco; Archilla, Juan F. R.; Universidad de Sevilla. Departamento de Física Aplicada I
This paper deals with the problem of a particle that diffuses in a potential with a reflecting barrier and has a point of stable equilibrium and a point of unstable equilibrium. Based on the exact solutions obtained earlier for the Fokker-Planck equation of a class of these models, we analyze the behavior of the probability density, the mean path and the onset time which determines the transition from unimodal to bimodal probability densities. The study is made over different initial positions, two of them very close to the unstable point, which permits a clear comparison among the subsequent evolutions, and the observation of some intrinsic effects induced by non-linearities.
The use of 137-cs in marine and lacustrine sediment dating
(Pergamon Press, 1992) Sánchez, C. I.; García-Tenorio García-Balmaseda, Rafael; García León, Manuel; Abril Hernández, José María; El-Daoushy, F.; Universidad de Sevilla. Departamento de Física Aplicada II
En este trabajo, la técnica de datación 137 Cs con precisión mejorada se muestra como una herramienta poderosa para establecer cronologías en diferentes depósitos acuáticos jóvenes (lacustres, marinos). El método tiene especial importancia cuando el método de 210 Pb da resultados parciales o poco claros
Low-level radioactivity studies in the marine environment of the South of Spain
(Elsevier, 1992) Manjón Collado, Guillermo; Martínez Aguirre, Aránzazu; García León, Manuel; Universidad de Sevilla. Departamento de Física Aplicada I; Universidad de Sevilla. Departamento de Física Aplicada II; Universidad de Sevilla. Departamento de Física Atómica, Molecular y Nuclear
In this paper preliminary levels of natural and man-made radionuclides in the South Spanish marine environment are presented. Local man-made sources are found to be the main contributors to the natural environmental radioactivity in such areas while fallout is mainly responsible for the occurrence of artificial radioactivity in our zone.
Self-diffusion in freely evolving granular gases
(American Institute of Physics, 1994) Brey Abalo, José Javier; Ruiz Montero, María José; Cubero Gómez, David; García Rojo, Ramón; Universidad de Sevilla. Departamento de Física Atómica, Molecular y Nuclear; Universidad de Sevilla. Departamento de Física Aplicada I; Dirección General de Investigación Científica y Técnica (DGICYT). España
A self-diffusionequation for a freely evolving gas of inelastic hard disks or spheres is derived starting from the Boltzmann–Lorentz equation, by means of a Chapman–Enskog expansion in the density gradient of the tagged particles. The self-diffusion coefficient depends on the restitution coefficient explicitly, and also implicitly through the temperature of the system. This latter introduces also a time dependence of the coefficient. As in the elastic case, the results are trivially extended to the Enskog equation. The theoretical predictions are compared with numerical solutions of the kinetic equation obtained by the direct simulation Monte Carlo method, and also with molecular dynamics simulations. An excellent agreement is found, providing mutual support to the different approaches.
A modeling study of Ra-226 dispersion in an estuarine system in south-west Spain
(Elsevier, 1994) Periáñez Rodríguez, Raúl; Abril Hernández, José María; García León, Manuel; Universidad de Sevilla. Departamento de Física Aplicada I; Universidad de Sevilla. Departamento de Física Atómica, Molecular y Nuclear; Comisión Interministerial de Ciencia y Tecnología (CICYT). España
A numerical model to study 226Ra dispersion in the Odiel River, in which two fertilizer plants release their wastes, has been developed. The hydrodynamic equations have been solved to obtain the instantaneous water state which allows us to describe the advective and diffusive transport of the radionuclides. The hydrodynamic model has been calibrated in order to reproduce field data. Diffusion coefficients have been formulated taking into account the shear effect. Good agreement between calculated and experimentally measured 226Ra concentrations has been achieved. The source term, which was unknown, has been investigated and found to be comparable to that of other fertilizer plants.
U and Th distribution in solution and suspended matter from rivers affected by the phosphate rock processing in southwestern Spain
(Elsevier, 1994) Martínez Aguirre, Aránzazu; García León, Manuel; Ivanovich, M.; Universidad de Sevilla. Departamento de Física Aplicada I; Universidad de Sevilla. Departamento de Física Atómica, Molecular y Nuclear; Dirección General de Investigación Científica y Técnica (DGICYT). España
A study is presented on the distribution of U and Th isotopes in the solid and liquid fractions of waters collected along the Odiel and Tinto rivers (southwest Spain). Such rivers are affected by the operation of phosphate fertilizer factories. The analysis of activity concentrations and activity ratios in both fractions provides interesting information on the way in which the factories disturb their close environment.
Identification and effects of anthropogenic emissions of U and Th on the composition of sediments in a river/estuarine system in Southern Spain
(Elsevier, 1994) Martínez Aguirre, Aránzazu; García León, Manuel; Ivanovich, M.; Universidad de Sevilla. Departamento de Física Aplicada I; Universidad de Sevilla. Departamento de Física Atómica, Molecular y Nuclear; Dirección General de Investigación Científica y Técnica (DGICYT). España
A study of the distribution of natural radionuclides in different fractions of river bottom sediments has been carried out. The study has shown that the majority of the total U in the sediment is located in the non-residual fraction of the sediment, while Th is more suitable to be present in the residual fraction of the sediments. Also, it has been found that coprecipitation with amorphous ferromanganese oxyhydroxides is the main process of incorporation of U- and Th-isotopes from the water column to suspended matter or bottom sediments. The distribution of the radionuclides and the analysis of some relevant activity ratios in different fractions of sediments has made an unequivocal connection between the enhanced U content in river sediments and the waste discharged into the Odiel and Tinto rivers by the operation in the vicinity of phosphate fertilizer industries.
The distribution of U, Th and 226Ra derived from the phosphate fertilizer industries on an estuarine system in Southwest Spain
(Elsevier, 1994) Martínez Aguirre, Aránzazu; García León, Manuel; Ivanovich, M.; Universidad de Sevilla. Departamento de Física Aplicada I; Universidad de Sevilla. Departamento de Física Atómica, Molecular y Nuclear; Dirección General de Investigación Científica y Técnica (DGICYT). España
This paper reports an extensive study of the presence of natural radioactivity around a phosphate fertilizer factory complex situated in an estuarine area of southwest Spain. The study has concluded that the wastes from such industries are the cause of the enhancement of natural radioactivity in the immediate environment. Thus, significantly high levels of U- and Th-isotopes and 226Ra are detected in water and sediment samples collected in this area. These conclusions based on the enhanced isotopic concentrations are further supported by the measured different from any observed elsehwere in undisturbed estuaries. These isotope activity ratios appear to be very sensitive indicators of waste disposal practices in such environments.
Resonant behavior of a Poisson process driven by a periodic signal
(American Physical Society, 1995) Brey Abalo, José Javier; Casado Pascual, Jesús; Sánchez-Rey, Bernardo; Universidad de Sevilla. Departamento de Física Atómica, Molecular y Nuclear; Universidad de Sevilla. Departamento de Física Aplicada I; Dirección General de Investigación Científica y Técnica (DGICYT). España
The statistical properties of the residence times of a periodically modulated Poisson process (time-dependent shot noise) on a line segment are analyzed. They show the characteristic features of a resonant behavior, which is similar in many aspects to the stochastic resonance taking place in systems confined in monostable or multistable potentials. The dependence of the mean residence time on both the frequency of the periodic stimulus and its amplitude is studied in detail. The behavior of this parameter also displays the effects associated with the interplay between noise and deterministic modulation.
Intersection of crisis loci in a driven nonlinearly damped oscillator
(Elsevier, 1995) Palmero Acebedo, Faustino; Romero Romero, Francisco; Romero Romero, Juan Luis; Archilla, Juan F. R.; Universidad de Sevilla. Departamento de Física Aplicada I
We report on a phenomenon observed in a driven nonlinearly damped oscillator when two control parameters, the frequency of the external excitation and the nonlinear damping coefficient, are varied simultaneously. An interior crisis locus and a boundary crisis locus, corresponding to two different chaotic attractors, intersect in a point of the parameter space. There exists an interchange in the type of crisis that each attractor suffers after crossing the intersection point.
U and Th speciation in river sediments
(Elsevier, 1995) Martínez Aguirre, Aránzazu; García León, Manuel; Ivanovich, M.; Universidad de Sevilla. Departamento de Física Aplicada I; Universidad de Sevilla. Departamento de Física Atómica, Molecular y Nuclear
A study of the distribution of some natural radionuclides in different chemical fractions of river bottom sediments has been carried out. The study has shown that the majority of the total U in sediments is located in the non-residual fractions, while Th is more likely to be present in the residual ones. This difference between U and Th reflects largely a much higher mobility of U relative to Th in surface and near-surface environments. Coprecipitation with amorphous ferromanganese oxyhydroxides is the main process of U and Th incorporation from the water column into the soil particles. Moreover, the distribution of the radionuclides and the analysis of the Th/U mass ratios in different chemical fractions of sediments has made an unequivocal connection between the enhanced U content in river sediments and the wastes discharged into the rivers by the operation of fertilizer industries.
Modelling the suspended matter distribution in an estuarine system : Application to the Odiel river in southwest Spain
(Elsevier, 1996) Periáñez Rodríguez, Raúl; Abril Hernández, José María; García León, Manuel; Universidad de Sevilla. Departamento de Física Aplicada I; Universidad de Sevilla. Departamento de Física Atómica, Molecular y Nuclear; Comisión Interministerial de Ciencia y Tecnología (CICYT). España
A numerical model which solves the advective-diffusive dispersion equation for suspended matter and includes the deposition and resuspension is presented. The model requires the simultaneous solution of the hydrodynamic equations under tidal dynamics and atmospheric forcing, using time steps of a few seconds. The model has been applied to the Odiel river (southwest Spain). The hydrodynamic module has been widely validated for neap and medium tides, whereas the dispersion was calibrated against the dissolved 226Ra dispersion pattern. This 226Ra is discharged to the Odiel river from a phosphate fertilizer factory. The model was able to reproduce the observed behaviour of the suspended matter in the estuary. The sedimentation rates have shown that a net, although slow, sedimentation is being produced. Sensitivity tests were inconclusive with respect to parameters describing settling and resuspension, as internal processes within the estuary are overridden by the high influx and efflux of particulate material from the sea.
Homogeneous cooling state of a low-density granular flow
(American Physical Society, 1996) Brey Abalo, José Javier; Ruiz Montero, María José; Cubero Gómez, David; Universidad de Sevilla. Departamento de Física Atómica, Molecular y Nuclear; Universidad de Sevilla. Departamento de Física Aplicada I; Dirección General de Investigación Científica y Técnica (DGICYT). España
The homogeneous cooling state of a granular flow of smooth spherical particles described by the Boltzmann equation is investigated by means of the direct simulation Monte Carlo method. The velocity moments and also the velocity distribution function are obtained and compared with approximate analytical results derived recently. The accuracy of a Maxwell-Boltzmann approximation with a time-dependent temperature is discussed. Besides, the simulations show that the state of uniform density is unstable to long enough wavelength perturbations so that clusters and voids spontaneously form throughout the system. The instability has the characteristic features of the clustering instability which has been observed in molecular dynamics simulations of dense fluids and predicted by hydrodynamic models of granular flows.
Transfer of natural radionuclides from soils to plants in a wet marshland
(Elsevier, 1996) Martínez Aguirre, Aránzazu; García León, Manuel; Universidad de Sevilla. Departamento de Física Aplicada I; Universidad de Sevilla. Departamento de Física Atómica, Molecular y Nuclear; European Union (UE)
Soils and two wild plant species (Spartina densiflora and Spartina maritima) have been collected from the Odiel river marsh and analysed for 210Po and 238U. The radioactivity concentrations in plants and soils reflect the impact of the operation of two phosphoric acid production factories located close to the marsh. Concentrations ratios (CR) of these radionuclides, between plant and underlying soil, have been studied. First results show that the concentration ratios (CR) seem to be dependent on radionuclide soil concentration.
Modelling the dispersion of non-conservative radionuclides in tidal waters—Part 1: Conceptual and mathematical model
(Elsevier, 1996) Periáñez Rodríguez, Raúl; Abril Hernández, José María; García León, Manuel; Universidad de Sevilla. Departamento de Física Aplicada I; Universidad de Sevilla. Departamento de Física Atómica, Molecular y Nuclear; European Union (UE)
A 2D four-phase model to study the dispersion of non-conservative radionuclides in tidal waters, in conditions of disequilibrium for ionic exchanges, has been developed. At disequilibrium conditions, ionic exchanges cannot be formulated using distribution coefficients kd. Thus, kinetic transfer coefficients have been used. The model includes ionic exchanges among water and the solid phases (suspended matter and two grain size fractions of sediments), the deposition and resuspension of suspended matter and advective plus diffusive transport. In the second part of this work, which is presented in a separate paper, the model is applied to simulate 226Ra dispersion, discharged from a fertilizer processing plant, in an estuarine system in the south-west of Spain.
Anthropogenic emissions of Po-210, Pb-210 and Ra-226 in an estuarine environment
(Springer, 1996) Martínez Aguirre, Aránzazu; García León, Manuel; Gascó, C.; Travesi, A.; Universidad de Sevilla. Departamento de Física Aplicada I; Universidad de Sevilla. Departamento de Física Atómica, Molecular y Nuclear; European Union (UE)
An extensive study on the distribution of natural radionuclides in an estuarine ecosystem located in Southwestern Spain is presented. This environment is highly affected by the wastes released by a phosphoric acid industry which uses phosphate rocks as raw material for fertilizer production. This rock has generally high concentrations of U and its daughters. The estuary is formed by two rivers, Odiel and Tinto, which have a common mouth into the Atlanic Ocean and a salt marsh (Odiel marsh) affected by the income of Odiel riverwaters. This river receives directly the liquid and part of the solid (gypsum) wastes released from the industries. Besides that, most of the phosphogypsum wastes are stored in uncovered piles at the right margin of the Tinto river. The study has concluded that the wastes from such industries are the cause of the enhanced concentrations found at the bed of both river channels as well as the enhancement found in surface soils in certain zones of the Odiel wet marshland. Indeed, the Northern marsh and the Mojarrera channel at the Odiel marsh seem to be the main sinks of the contaminant released by the phosphoric acid industry.
210Pb distribution in riverwaters and sediments near phosphate fertilizer factories
(Elsevier, 1996) Martínez Aguirre, Aránzazu; García León, Manuel; Universidad de Sevilla. Departamento de Física Aplicada I; Universidad de Sevilla. Departamento de Física Atómica, Molecular y Nuclear; European Union (UE)
two fertilizer factories operate at the estuary formed by the Odiel and Tinto rivers in Southwestern Spain. In previous work the authors have shown that such operation enhances the natural radionuclide background of the sea. Thus, fairly high levels of U, Th and Ra-isotopes were found in water and sediment samples taken from the estuary. In this paper, we have extended the study to investigate the presence of 210Pb in the dissolved aqueous phase of the river and in bottom sediments. Significant enhancement of 210Pb concentrations have been detected confirming the previously reported results.
Modelling the dispersion of non-conservative radionuclides in tidal waters .2. Application to Ra-226 dispersion in an estuarine system
(Elsevier, 1996) Periáñez Rodríguez, Raúl; Abril Hernández, José María; García León, Manuel; Universidad de Sevilla. Departamento de Física Aplicada I; Universidad de Sevilla. Departamento de Física Atómica, Molecular y Nuclear; European Union (UE)
The Odiel river is an estuary system located in the south-west of Spain and affected by tidal dynamics. A phosphate fertilizer processing plant releases part of its wastes directly into the Odiel river. The model described in a previous paper (J. Environ. Radioactivity, 31(2), 127–141) has been applied to study 226Ra (discharged from the fertilizer plant) dispersion in the Odiel river. The model adequately reproduces the observed behaviour of 226Ra in suspended matter and the dissolved phase along the Odiel river, as well as the general behaviour of the measured distribution coefficients. Some predictive studies as well as the corresponding sensitivity studies are also included in this paper.

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