Monte Carlo Neutronics and Thermal-hydraulics coupling applied to Fast Reactors

Un escenario habitualmente considerado para el uso sostenible y prolongado de la energ��a nuclear contempla un parque de reactores r��pidos refrigerados por metales l��quidos (LMFR) dedicados al reciclado de Pu y la transmutaci��n de act��nidos minoritarios (MA). Otra opci��n es combinar dichos reactores con algunos sistemas subcr��ticos asistidos por acelerador (ADS), exclusivamente destinados a la eliminaci��n de MA. El dise��o y licenciamiento de estos reactores innovadores requiere herramientas computacionales pr��cticas y precisas, que incorporen el conocimiento obtenido en la investigaci��n experimental de nuevas configuraciones de reactores, materiales y sistemas. A pesar de que se han construido y operado un cierto n��mero de reactores r��pidos a nivel mundial, la experiencia operacional es todav��a reducida y no todos los transitorios se han podido entender completamente. Por tanto, los an��lisis de seguridad de nuevos LMFR est��n basados fundamentalmente en m��todos deterministas, al contrario que las aproximaciones modernas para reactores de agua ligera (LWR), que se benefician tambi��n de los m��todos probabilistas. La aproximaci��n m��s usada en los estudios de seguridad de LMFR es utilizar una variedad de c��digos, desarrollados a base de distintas teor��as, en busca de soluciones integrales para los transitorios e incluyendo incertidumbres. En este marco, los nuevos c��digos para c��lculos de mejor estimaci��n ("best estimate") que no incluyen aproximaciones conservadoras, son de una importancia primordial para analizar estacionarios y transitorios en reactores r��pidos. Esta tesis se centra en el desarrollo de un c��digo acoplado para realizar an��lisis realistas en reactores r��pidos cr��ticos aplicando el m��todo de Monte Carlo. Hoy en d��a, dado el mayor potencial de recursos computacionales, los c��digos de transporte neutr��nico por Monte Carlo se pueden usar de manera pr��ctica para realizar c��lculos detallados de n��cleos completos, incluso de elevada heterogeneidad material. Adem��s, los c��digos de Monte Carlo se toman normalmente como referencia para los c��digos deterministas de difusi��n en multigrupos en aplicaciones con reactores r��pidos, porque usan secciones eficaces punto a punto, un modelo geom��trico exacto y tienen en cuenta intr��nsecamente la dependencia angular de flujo. En esta tesis se presenta una metodolog��a de acoplamiento entre el conocido c��digo MCNP, que calcula la generaci��n de potencia en el reactor, y el c��digo de termohidr��ulica de subcanal COBRA-IV, que obtiene las distribuciones de temperatura y densidad en el sistema. COBRA-IV es un c��digo apropiado para aplicaciones en reactores r��pidos ya que ha sido validado con resultados experimentales en haces de barras con sodio, incluyendo las correlaciones m��s apropiadas para metales l��quidos. En una primera fase de la tesis, ambos c��digos se han acoplado en estado estacionario utilizando un m��todo iterativo con intercambio de archivos externos. El principal problema en el acoplamiento neutr��nico y termohidr��ulico en estacionario con c��digos de Monte Carlo es la manipulaci��n de las secciones eficaces para tener en cuenta el ensanchamiento Doppler cuando la temperatura del combustible aumenta. Entre todas las opciones disponibles, en esta tesis se ha escogido la aproximaci��n de pseudo materiales, y se ha comprobado que proporciona resultados aceptables en su aplicaci��n con reactores r��pidos. Por otro lado, los cambios geom��tricos originados por grandes gradientes de temperatura en el n��cleo de reactores r��pidos resultan importantes para la neutr��nica como consecuencia del elevado recorrido libre medio del neutr��n en estos sistemas. Por tanto, se ha desarrollado un m��dulo adicional que simula la geometr��a del reactor en caliente y permite estimar la reactividad debido a la expansi��n del n��cleo en un transitorio. ��ste m��dulo calcula autom��ticamente la longitud del combustible, el radio de la vaina, la separaci��n de los elementos de combustible y el radio de la placa soporte en funci��n de la temperatura. ��ste efecto es muy relevante en transitorios sin inserci��n de bancos de parada. Tambi��n relacionado con los cambios geom��tricos, se ha implementado una herramienta que, automatiza el movimiento de las barras de control en busca d la criticidad del reactor, o bien calcula el valor de inserci��n axial las barras de control. Una segunda fase en la plataforma de c��lculo que se ha desarrollado es la simulaci�� din��mica. Puesto que MCNP s��lo realiza c��lculos estacionarios para sistemas cr��ticos o supercr��ticos, la soluci��n m��s directa que se propone sin modificar el c��digo fuente de MCNP es usar la aproximaci��n de factorizaci��n de flujo, que resuelve por separado la forma del flujo y la amplitud. En este caso se han estudiado en profundidad dos aproximaciones: adiab��tica y quasiest��tica. El m��todo adiab��tico usa un esquema de acoplamiento que alterna en el tiempo los c��lculos neutr��nicos y termohidr��ulicos. MCNP calcula el modo fundamental de la distribuci��n de neutrones y la reactividad al final de cada paso de tiempo, y COBRA-IV calcula las propiedades t��rmicas en el punto intermedio de los pasos de tiempo. La evoluci��n de la amplitud de flujo se calcula resolviendo las ecuaciones de cin��tica puntual. Este m��todo calcula la reactividad est��tica en cada paso de tiempo que, en general, difiere de la reactividad din��mica que se obtendr��a con la distribuci��n de flujo exacta y dependiente de tiempo. No obstante, para entornos no excesivamente alejados de la criticidad ambas reactividades son similares y el m��todo conduce a resultados pr��cticos aceptables. Siguiendo esta l��nea, se ha desarrollado despu��s un m��todo mejorado para intentar tener en cuenta el efecto de la fuente de neutrones retardados en la evoluci��n de la forma del flujo durante el transitorio. El esquema consiste en realizar un c��lculo cuasiestacionario por cada paso de tiempo con MCNP. La simulaci��n cuasiestacionaria se basa EN la aproximaci��n de fuente constante de neutrones retardados, y consiste en dar un determinado peso o importancia a cada ciclo computacial del c��lculo de criticidad con MCNP para la estimaci��n del flujo final. Ambos m��todos se han verificado tomando como referencia los resultados del c��digo de difusi��n COBAYA3 frente a un ejercicio com��n y suficientemente significativo. Finalmente, con objeto de demostrar la posibilidad de uso pr��ctico del c��digo, se ha simulado un transitorio en el concepto de reactor cr��tico en fase de dise��o MYRRHA/FASTEF, de 100 MW de potencia t��rmica y refrigerado por plomo-bismuto. ABSTRACT Long term sustainable nuclear energy scenarios envisage a fleet of Liquid Metal Fast Reactors (LMFR) for the Pu recycling and minor actinides (MAs) transmutation or combined with some accelerator driven systems (ADS) just for MAs elimination. Design and licensing of these innovative reactor concepts require accurate computational tools, implementing the knowledge obtained in experimental research for new reactor configurations, materials and associated systems. Although a number of fast reactor systems have already been built, the operational experience is still reduced, especially for lead reactors, and not all the transients are fully understood. The safety analysis approach for LMFR is therefore based only on deterministic methods, different from modern approach for Light Water Reactors (LWR) which also benefit from probabilistic methods. Usually, the approach adopted in LMFR safety assessments is to employ a variety of codes, somewhat different for the each other, to analyze transients looking for a comprehensive solution and including uncertainties. In this frame, new best estimate simulation codes are of prime importance in order to analyze fast reactors steady state and transients. This thesis is focused on the development of a coupled code system for best estimate analysis in fast critical reactor. Currently due to the increase in the computational resources, Monte Carlo methods for neutrons transport can be used for detailed full core calculations. Furthermore, Monte Carlo codes are usually taken as reference for deterministic diffusion multigroups codes in fast reactors applications because they employ point-wise cross sections in an exact geometry model and intrinsically account for directional dependence of the ux. The coupling methodology presented here uses MCNP to calculate the power deposition within the reactor. The subchannel code COBRA-IV calculates the temperature and density distribution within the reactor. COBRA-IV is suitable for fast reactors applications because it has been validated against experimental results in sodium rod bundles. The proper correlations for liquid metal applications have been added to the thermal-hydraulics program. Both codes are coupled at steady state using an iterative method and external files exchange. The main issue in the Monte Carlo/thermal-hydraulics steady state coupling is the cross section handling to take into account Doppler broadening when temperature rises. Among every available options, the pseudo materials approach has been chosen in this thesis. This approach obtains reasonable results in fast reactor applications. Furthermore, geometrical changes caused by large temperature gradients in the core, are of major importance in fast reactor due to the large neutron mean free path. An additional module has therefore been included in order to simulate the reactor geometry in hot state or to estimate the reactivity due to core expansion in a transient. The module automatically calculates the fuel length, cladding radius, fuel assembly pitch and diagrid radius with the temperature. This effect will be crucial in some unprotected transients. Also related to geometrical changes, an automatic control rod movement feature has been implemented in order to achieve a just critical reactor or to calculate control rod worth. A step forward in the coupling platform is the dynamic simulation. Since MCNP performs only steady state calculations for critical systems, the more straight forward option without modifying MCNP source code, is to use the flux factorization approach solving separately the flux shape and amplitude. In this thesis two options have been studied to tackle time dependent neutronic simulations using a Monte Carlo code: adiabatic and quasistatic methods. The adiabatic methods uses a staggered time coupling scheme for the time advance of neutronics and the thermal-hydraulics calculations. MCNP computes the fundamental mode of the neutron flux distribution and the reactivity at the end of each time step and COBRA-IV the thermal properties at half of the the time steps. To calculate the flux amplitude evolution a solver of the point kinetics equations is used. This method calculates the static reactivity in each time step that in general is different from the dynamic reactivity calculated with the exact flux distribution. Nevertheless, for close to critical situations, both reactivities are similar and the method leads to acceptable practical results. In this line, an improved method as an attempt to take into account the effect of delayed neutron source in the transient flux shape evolutions is developed. The scheme performs a quasistationary calculation per time step with MCNP. This quasistationary simulations is based con the constant delayed source approach, taking into account the importance of each criticality cycle in the final flux estimation. Both adiabatic and quasistatic methods have been verified against the diffusion code COBAYA3, using a theoretical kinetic exercise. Finally, a transient in a critical 100 MWth lead-bismuth-eutectic reactor concept is analyzed using the adiabatic method as an application example in a real system.

Problems and solutions; New York State professional engineering examinations, parts II and III. Hydraulics, machine design, thermodynamics, economics and practice, hygrometry.

[v.1.] 1950-1954.--v.2. 1955-1958.

Performance and stresses obtained on an isolated VTOL-type propeller operating in hovering, transitional and axial flight,

"USAAVLABS technical report 67-37."

A complete view of the British customs : containing, I. A perfect and distinct account of the several particular branches whereof that revenue consists. II. The former and additional books of the rates of merchandizes ... III. Directions for, and examples of, the method of computing the aforesaid duties ... IV. The several ports, members and creeks of Great. Britain ... V. An index, wherein is comprehended the substance of the several laws now in force, relating to the customs, &c. The whole being a complete system of His Majesty's revenue of customs /

Goldsmiths'-Kress no. 07604.1.

Determining propeller induced erosion alongside quay walls in harbours using Artificial Neural Networks

In shallow waters, such as those found close to berth structures, the wash from a manoeuvring ship���s propeller can cause erosion of the seabed. This erosion can be increased if the wash intersects a berth structure. A number of researchers have undertaken model studies and used regression analysis to develop predictive relationships for the scouring action. This paper presents an experimental investigation with Artificial Neural Networks (ANN���s), used to analyse the results. The purpose of using ANN���s was to examine the prediction accuracy of the Networks in comparison with previous regression analysis methods. ANN���s were found to provide a more accurate method of predicting propeller wash scour than the equations presented by previous investigators.

Scour and fill in cobble-bedded streams

Scour and deposition have been measured in two small cobble-bedded upland streams, for two years. Grids of scour chains were inserted in the bed and relocated after the passage of individual hydrographs. Scour, fill and the area of the bed affected by these processes were recorded. The relationship between mean scour or fill and maximum scour or fill is assessed. In addition, the relationship between the depth of scour and the sediment transport rate as bedload is discussed briefly.

A review of the equations used to predict the velocity distribution within a ship���s propeller jet

Predicting the velocity within the ship���s propeller jet is the initial step to investigate the scouring made by the propeller jet. Albertson et al. (1950) suggested the investigation of a submerged jet can be undertaken through observation of the plain water jet from an orifice. The plain water jet investigation of Albertson et al. (1950) was based on the axial momentum theory. This has been the basis of all subsequent work with propeller jets. In reality, the velocity characteristic of a ship���s propeller jet is more complicated than a plain water jet. Fuehrer and R��misch (1977), Blaauw and van de Kaa (1978), Berger et al. (1981), Verhey (1983) and Hamill (1987) have carried out investigations using physical model. This paper reviews the state-of-art of the equations used to predict the time-averaged axial, tangential and radial components of velocity within the zone of flow establishment and the zone of established flow of a ship���s propeller jet.

Langmuir turbulence and deeply penetrating jets in an unstratified mixed layer

The influence of surface waves and an applied wind stress is studied in an ensemble of large eddy simulations to investigate the nature of deeply penetrating jets into an unstratified mixed layer. The influence of a steady monochromatic surface wave propagating parallel to the wind direction is parameterized using the wave-filtered Craik-Leibovich equations. Tracer trajectories and instantaneous downwelling velocities reveal classic counterrotating Langmuir rolls. The associated downwelling jets penetrate to depths in excess of the wave's Stokes depth scale, ��s. Qualitative evidence suggests the depth of the jets is controlled by the Ekman depth scale. Analysis of turbulent kinetic energy (tke) budgets reveals a dynamical distinction between Langmuir turbulence and shear-driven turbulence. In the former, tke production is dominated by Stokes shear and a vertical flux term transports tke to a depth where it is dissipated. In the latter, tke production is from the mean shear and is locally balanced by dissipation. We define the turbulent Langmuir number Lat = (v*/Us)0.5 (v* is the ocean's friction velocity and Us is the surface Stokes drift velocity) and a turbulent anisotropy coefficient Rt = /( + ). The transition between shear-driven and Langmuir turbulence is investigated by varying external wave parameters ��s and Lat and by diagnosing Rt and the Eulerian mean and Stokes shears. When either Lat or ��s are sufficiently small the Stokes shear dominates the mean shear and the flow is preconditioned to Langmuir turbulence and the associated deeply penetrating jets.

Search for stealth supersymmetry in events with jets, either photons or leptons, and low missing transverse momentum in pp collisions at 8 TeV

Conselho Nacional de Desenvolvimento Cient��fico e Tecnol��gico (CNPq)

Search for squarks and gluinos with the ATLAS detector in final states with jets and missing transverse momentum using 4.7 fb����� of sqrts=7 TeV proton-pro collision data

A search for squarks and gluinos in final states containing jets, missing transverse momentum and no high-p(T) electrons or muons is presented. The data represent the complete sample recorded in 2011 by the ATLAS experiment in 7 TeV proton-proton collisions at the Large Hadron Collider, with a total integrated luminosity of 4.7 fb(-1). No excess above the Standard Model background expectation is observed. Gluino masses below 860 GeV and squark masses below 1320 GeV are excluded at the 95% confidence level in simplified models containing only squarks of the first two generations, a gluino octet and a massless neutralino, for squark or gluino masses below 2 TeV, respectively. Squarks and gluinos with equal masses below 1410 GeV are excluded. In minimal supergravity/constrained minimal supersymmetric Standard Model models with tan beta = 10, A(0) = 0 and mu > 0, squarks and gluinos of equal mass are excluded for masses below 1360 GeV. Constraints are also placed on the parameter space of supersymmetric models with compressed spectra. These limits considerably extend the region of supersymmetric parameter space excluded by previous measurements with the ATLAS detector.

Search for squarks and gluinos with the ATLAS detector in final states with jets and missing transverse momentum using ���s=8 TeV proton--proton collision data

A search for squarks and gluinos in final states containing high-pT jets, missing transverse momentum and no electrons or muons is presented. The data were recorded in 2012 by the ATLAS experiment in ���s = 8TeV proton-proton collisions at the Large Hadron Collider, with a total integrated luminosity of 20.3 fb���1. Results are interpreted in a variety of simplified and specific supersymmetry-breaking models assuming that R-parity is conserved and that the lightest neutralino is the lightest supersymmetric particle. An exclusion limit at the 95% confidence level on the mass of the gluino is set at 1330GeV for a simplified model incorporating only a gluino and the lightest neutralino. For a simplified model involving the strong production of first- and second-generation squarks, squark masses below 850GeV (440GeV) are excluded for a massless lightest neutralino, assuming mass degenerate (single light-flavour) squarks. In mSUGRA/CMSSM models with tan �� = 30, A0 = ���2m0 and �� > 0, squarks and gluinos of equal mass are excluded for masses below 1700GeV. Additional limits are set for non-universal Higgs mass models with gaugino mediation and for simplified models involving the pair production of gluinos, each decaying to a top squark and a top quark, with the top squark decaying to a charm quark and a neutralino. These limits extend the region of supersymmetric parameter space excluded by previous searches with the ATLAS detector.

Morphology and dynamics of the jets of comet 67P/Churyumov-Gerasimenko: Early-phase development

Aims. The OSIRIS camera onboard the Rosetta spacecraft obtained close-up views of the dust coma of comet 67P. The jet structures can be used to trace their source regions and to examine the possible effect of gas-surface interaction. Methods. We analyzed the wide-angle images obtained in the special dust observation sequences between August and September 2014. The jet features detected in different images were compared to study their time variability. The locations of the potential source regions of some of the jets are identified by ray tracing. We used a ring-masking technique to calculate the brightness distribution of dust jets along the projected distance. Results. The jets detected between August and September 2014 mostly originated in the Hapi region. Morphological changes appeared over a timescale of several days in September. The brightness slope of the dust jets is much steeper than the background coma. This might be related to the sublimation or fragmentation of the emitted dust grains. Interaction of the expanding gas flow with the cliff walls on both sides of Hapi could lead to erosion and material down-fall to the nucleus surface.

Compact multiport antenna with isolated ports

The small element spacing of compact multiport arrays introduces strong mutual coupling between the antenna ports. Due to this coupling, the input impedance of the array changes when elements excitations are varied, and consequently, the array cannot be matched for an arbitrary excitation. Decoupling networks have in the past been used to provide an additional connection between antenna ports in order to cancel the coupling between elements. An alternative approach is to design the antenna so that each port does not excite a single element, but all elements simultaneously instead. The geometry of the antenna is optimized so that this direct excitation of elements counteracts the mutual coupling, thus yielding decoupled ports. This paper describes the design of such a 4-port antenna.