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- Type of Document: Ph.D. Dissertation
- Language: Farsi
- Document No: 53361 (46)
- University: Sharif University of Technology
- Department: Energy Engineering
- Advisor(s): Vosoughi, Naser
- Abstract:
- In every nuclear reactor core, there are neutron flux fluctuations around the mean value. The neutron noise is the deviation between the time-dependent neutron flux and its expected value, assuming that all process is stationary and ergodic in time. These fluctuations can be caused by the stochastic nature of neutron interactions or mechanical oscillations in the reactor cores. In a reactor working at a high-power level, mechanical fluctuations are the major cause of the fluctuations measured by the detectors. These mechanical vibrations include control rod or fuel rod vibrations, changes in the incoming coolant speed, or changes in the coolant temperature. These perturbations are seen as fluctuations of neutron flux and referred to as noise, and the source of each of the perturbations is called the noise source.The power reactor noise is used for diagnostic purposes and surveillance and monitoring of the nuclear reactor core. The neutron noise is calculated using the Greens' function technique in this work. The transfer function, Green's function, has been obtained by this technique, depends on the parameters of the unperturbed core, so the derived Green's function is independent of the perturbation type. The neutron noise equations developed by diffusion, telegrapher, and SN transport theory have different accuracy. More accurate calculation of Green's function and, consequently, the neutron noise is essential in the interoperation of detector signals and localizing noise sources.This study's primary purpose is to develop an accurate and more comprehensive method for simulating power reactor noise. This study's secondary purpose is to validate the neutron noise results based on diffusion theory by comparing it with transport neutron noise as a validation criterion.So far, the widespread method base on diffusion has been used in neutron noise studies. The diffusion method has many simplifying assumptions that reduce its validity range in some issues such as small dimensions, near edges, high heterogeneity medium and high frequency source of perturbation, etc. In this study, the induced neutron noise derived from the transport equation. The Sn transport method is used to solve transport noise because of reasonable accuracy and programming capability.Before performing neutron noise calculations, it is necessary to calculate static values such as effective multiplication factor and neutron flux, so in the first step, a program for static calculations was written using the Sn method. In the next step, a program was developed for simulating neutron noise. Since the study of neutron noise based on diffusion theory is widespread, a comparison was made between these two methods to determine the validity of the diffusion method and also the extent of the difference between these two methods in different issues was analyzed.Locating the noise source and then determining its characteristics is called noise source reconstruction, which detects damages and defects in the reactor. There are different methods for reconstructing the noise source. In this study, inverse and scanning methods were used to reconstruct the noise source based on the transport and diffusion theory, and then the results were compared. Green's functions based on diffusion and transport have considerable differences in some cases such as small dimensions, near edges, high heterogeneity medium, and high frequency source of perturbation. These differences have a significant impact on the unfolding, localizing and identification, of the neutron noise source.Several computer codes based on the Fortran programming language developed to simulate induced neutron noise and reconstruct the noise source by both transport and diffusion methods.Comparisons between transport and diffusion noise show significant differences in the mentioned cases, confirming the need to use the transport noise method in a wide range of power reactor problems.In order to improve the calculated noise based on the diffusion method, the telegrapher theory was used, which corrected the propagation speed of diffusion noise
- Keywords:
- Power Reactors ; Transport Equation ; Nuclear Reactors ; Telegrapher"s Equation ; Noise Source Identification ; SN Method ; Noise Source Reconstruction
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