Sharif Digital Repository

Numerical analysis method improvement is a topic of interest among all engineering disciplines. Regarding the fact that our ability to predict the behavior of a physical system is often limited by our computational resources, the efficiency of the employed numerical method is an important factor in the degree of approximation used in modeling. One of the recent numerical methods is the Cell Method (CM), which is also known as the Direct Discrete Method (DDM). This method combines some features of a variety of methods, especially finite volume and finite element, and gains some insight from the graph theory used in network analysis. The result is a method in which the set of equations which... 

Releasing radioactive materials in atmosphere and their damages on human health via emitting radioactivity, show the necessity of investigations on their behavior. To simulate behavior of radioactive materials in atmosphere, we use Gaussian plume model. Gaussian model as a mathematical tool can simulate diffusion of Gaseous matters. This model also can be employed for radioactive materials. Radioactive materials vanish by decaying and can be produced via decaying of their mothers! Therefore, later effect must be considered in the Gaussian model via a factor. Many factors like wind speed and weather conditions also are involved in diffusion of radioactive materials. By considering all of... 

In operating nuclear reactor core, fluctuations (deviations from normal operating conditions) are usually produced and propagated. These fluctuations can be due to control rod vibrations, inlet coolant temperature fluctuations, inlet coolant velocity fluctuations and so on. The induced neutron noise can be detected by in-core neutron detectors. Noise source identifications (such as the type, location and propagating velocity) as well as the calculation of the dynamical parameters (such as moderator temperature coefficient in PWRs and Decay Ratio in BWRs) are of the main applications of the neutron noise analysis in power reactors.
Investigating the propagation noise in PWRs (specifically... 

Analysis of neutronic and thermal-hydraulics fluctuations has various applications in calculation or measurement of the core dynamical parameters (temperature reactivity coefficients) furthermore it is applicable in thermal-hydraulics surveillance and diagnostics. Experience of the Fukushima Dai-ichi plant and the lessons learned from the European stress tests, demonstrated that alternative and various tools and methods are needed for the reactor's condition identification. In this paper the feasibility of development of an alternative accident monitoring via eminent coupling of neutron and thermal-hydraulics noise is proposed. and it is programming a simple simulator for calculating... 

Radiation detection techniques have important applications including prompt gamma neutron activation analysis (PGNAA), radiotherapy dose measurement, medical imaging and homeland security. Pulse height distribution is also a useful parameter in Radionuclide identification. Gamma ray spectroscopy analysis is a common technique in measuring of radiation field to identify the radioactive source.
Probabilistic methods are commonly used for numerical simulation of Pulse height distribution. In Monte Carlo methods, each history is individually tracked and details of interactions are available. Using the Monte Carlo methods for calculation of pulse height distribution leads to more real... 

The present ph.D. thesis consists of three sections including the static calculation, neutron noise calculation and neutron noise source unfolding in VVER-1000 reactor core. The multi-group, two dimensional neutron diffusion equations and corresponding adjoint equations are solved in the static calculation. The spatial discretization of equations is based on Galerkin Finite Element Method (GFEM) using unstructured triangle elements generated by Gambit software. The static calculation is performed for both linear and quadratic approximations of shape function; baesd on which results are compared. Using power iteration method for the static calculation, the neutron and adjoint fluxes with the... 

Neutron fluctuations in both low and high power reactors provide important information about the system. The origins of these fluctuations are different in both regimes. In low power the branching process is the reason of these fluctuations. But the random nature of technological processes such as boiling in the reactors cooling and mechanical structure vibrating such as control rod and fuel rod created the neutron noise. Mathematical descriptions and applications of neutron noise for each mode are different. In common mode, fluctuations in the neutron detection system, determine some parameters and determin the start abnormality in system. In this study the spatial dependence of the cross... 

Nowadays, the use of numerical methods is very common in solving complex equations, and various methods have been developed in this field. One of the new methods in this field, is the direct discrete method(DDM), Which was initially used to solve the electromagnetic field equations. In the past years, this method has been used to discretization some of the neutronics equations and acceptable results are recorded. So that the convergence order for the neutron diffusion equation in this method is higher than other numerical methods. In this method the geometry of the problem is divided into primary and secondary cell that primary cell platform is fixed but there is the possibility of making... 

Sharif University of Technology Plasma Focus (SUT-PF) is a Mather Type DPF device which has a 39 μF capacitor bank with 14 KV charging ability at its ultimate limit corresponding to 7 KJ stored energy. In the first step of this experiment, a test has been done at three distances 5, 7, 10 cm from anode tip with the different number of shots 20, 30, and 40 at 0 constant angles.In the following, a test is done at 5 and 10 degrees with 40 shots, and PF chamber has been filled up with nitrogen gas. Various tests have carried out for survey of this experiment, the result of which are as follow:The X-ray diffraction (XRD) analysis are used in order to reveal if the Nickel or Nickel nitride have... 

In this thesis, localization of a postulated noise from limited neutron detectors sparsely distributed throughout the core of a typical VVER-1000 reactor is investigated. For this purpose, developing a 2-D neutron noise simulator for hexagonal geometries based on the 2-group diffusion approximation, the reactor dynamic transfer function is calculated. The box-scheme finite difference method is first developed for hexagonal geometries, to be used for spatial discretisation of both 2-D 2-group static and noise diffusion equations. Using the discretised static equations, a 2-D 2-group static simulator (HEXDIF-2) is developed which its results are benchmarked against the well-known CITATION... 

This thesis examines various approaches of digital and analog methods of neutron - gamma discrimination. The analog detector used in this research is a type of 2inch and NE213 liquid scintillator. The applied analog discrimination method is the zero-crossing time and the Neutron Source is AmBe. In digital discrimination, the simulated pulses are generated and used for the experiment of the research. The method used to separate the data are two partitional clustering methods, namely FCM and K-Medoids. The FCM and K-Medoids are two data mining approaches in which the input data are grouped in two or more clusters based on their similarities. The merit of this approach is that this algorithms... 

The emergence of complex equations in engineering sciences leads to development of numerical methods for solving these equations. Obviously, each of the proposed Algorithms and various methods in this field were evaluated according to the limitations of time, software and hardware and based on these limitations the advantages and disadvantages of these methods were determined. Direct Discrete method is one of the modern methods used in solving the conservation equations. In this method, in addition to the advantages of numerical methods, equations will be produced in discrete space to avoid involving with differential equations. This method was first studied in the field equations. The... 

For many reasons the Montecarlo method is chosen for investigation of problems like shielding. Nowadays, superiority of utilizing the codes based upon Montecarlo methods such as using cross sections with continuous energy or using exact models of complex geometries, is obvious for everyone. But obtaining results with least possible variance is extremely important. variance reduction methods usedfor achieving exact results with less calculating time. Up to now in the MCNP codes, many methods have been utilized for reducing the variance. One of the methods which have recently attracted attentions is using the importance function. For obtaining importance function we need to solve the... 

The most common method for radioactivity measurements in the environmental sciences is gamma ray spectrometry. The traditional laboratory analysis is a time consuming method and demands special acilities and know-how for the chemical treatment of the samples and half-life limitations . NaI(Tl) are the most common crystals for long-term measurements due to low consumption, good efficiency and low cost ,but HPGe is not suitable due to the high cost of it and cooling system.The aim of this study was designing and making enclosure and basehold for a NaI(Tl) detector to use the water at a depth of 100meters and calibrate.Applications of this detector is measuring the pollution caused by the... 

The analysis of gamma-ray spectra of low resolution detectors is difficult and sometimes impossible, as a result of photo peak’s overlapping. The usual methods of radiation spectra analysis based on fitting of peaks to mathematical curves or detecting of peaks with numerical calculation, are valid for high resolution detectors. However these methods are less successful for lower resolution detectors such as the common scintillators. The wide peaks in the spectrum maybe overlap and make it difficult to analysis. To solve this problem, we test here a method, based on the use of the artificial neural network. At first spectra of elements are converted to the patterns which are suitable for the... 

This study consists of two parts: steady state calculations and neutron noise calculations in the frequency domain for two rectangular and hexagonal geometries. In the steady state calculation, the neutron diffusion and its adjoint equations are approximated by two-dimensional coordinates in two-group energy and are solved using the average current nodal expansion method. Then, by considering the node size in the dimensions of a fuel assembly, different orders of flux expansion are investigated. For verification purposes, the calculations have been performed by power iteration method for two test problems of BIBLIS-2D and IAEA-2D. For rectangular geometry with increasing flux expansion order... 

High level of safety, environmental issues, economic efficiency and on-line refueling give the potential to pebble bed reactors to have a unique significance among the new generation of reactors. Here the reactor core is an assembly of the fuel pebbles and moderators which are randomly located in the core. This feature makes it difficult and ingenious for simulating, moreover this kind of distribution makes us unable to know the exact structure of the core. Fuel pebble is made of many tiny particles called TRISO which are again randomly located in the graphite lattice, therefore makes the reactor's simulation too difficult. Due to these problems, simulation of this reactor with certain codes... 

Localization of a neutron point source using a designed computer program namely “MCMC-MATURE” is performed. The computer program analyses several detector responses in some certain media by Markov Chain Monte Carlo (MCMC) method and a new iteration algorithm. Identification of the possible regions of source position would be found by analyzing the initial fluxes generated by mesh tally of MCNPX computer code. The designed computer program is capable to generate the flux between detectors. “Regular-Sequential”, “Irregular-Sequential” and “Non-Sequential” are three methods used for sampling the generated random number in two dimensions. Each sample multiplied by a sampling function and lead to... 

Nowadays numerical solution of engineering equations and relationships is one of the most efficient and most important parts of scientific research. Hence, various methods have been developed and expanded for this purpose, and the choice of the best method for solving a problem based on its strengths and weaknesses is essential. Direct discrete method is one of the new numerical methods that has been used for solving magnetic equations for the first time and shown promising results in nuclear researches. In this method, after determining an interpolation function for the unknown quantity, the discrete form of mathematical operators such as integral and gradient is obtained in the grid... 

Precise knowledge of the laws which govern a system is of interest for two reasons. First, it paves the way to understand the subtle behaviors of the system, which are not understandable from simpler models of the system. Second, it helps in the design of experimental requirements needed to observe these behaviors. The behavior of neutrons in a system, which could be a reactor or a detector, is stochastic from two perspectives. First of all, since the place of the atoms of a medium are randomly sited, at least from a neutrons point of view, the collision sites are random places, which contributes to the stochasticity of the transport phenomena. This type of randomness is somewhat similar to...