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A Monte Carlo Method for Neutron Noise Calculation in the Frequency Domain

Ghorbani Ashraf, Mahdi | 2021

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 53789 (46)
  4. University: Sharif University of Technology
  5. Department: Energy Engineering
  6. Advisor(s): Vosoughi, Naser
  7. Abstract:
  8. Neutron noise equations, which are obtained by assuming small perturbations of macroscopic cross sections around a steady-state neutron field and by subsequently taking the Fourier transform in the frequency domain, have been usually solved by analytical techniques or by resorting to diffusion theory, but in this thesis, in order to increase of accuracy of neutron noise calculation, has been used transport approximation for neutron noise calculation and the Monte Carlo method has been used to solve transport equation of the neutron noise in the frequency domain. Since the transport equation of the neutron noise is a complex equation, a new Monte Carlo technique for treating complex-valued weights has been introduced to solve the complex equation. To cancel the positive and negative values of complex-valued weights, an algorithm that is similar to the power iteration method has been implemented (weight cancellation method). The weight cancellation method is benchmarked to analytical solutions in an infinite homogeneous medium. The neutron noise spatial distributions have been obtained both with the newly-developed Monte Carlo method and the conventional diffusion method for an infinitely-long homogeneous cylinder. The results with the Monte Carlo method agree well with those of the diffusion method. However, near the noise source induced by a high frequency perturbation, significant differences are found between the diffusion method and Monte Carlo method. The weight cancellation method is expected to contribute to the improvement of calculation accuracy of the neutron noise. At the end, for improvement of the computational time of weight cancellation method for very low frequency perturbations has been used other method for neutron noise calculation. The second method relies on a modified collision operator and does not need any weight cancellation technique. In this thesis, both Monte Carlo methods are compared toghether for several noise frequencies. The second method shows better performances in the frequency region of interest and is easier to implement because it relies upon the conventional algorithm for fixed-source problems
  9. Keywords:
  10. Neutron Noise ; Monte Carlo Method ; Frequency Domain ; Transport Theory ; Finite Difference Method

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