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Simulation and Analysis of Neutron Noise Caused by the Bowing Phenomenon of Fuel Assemblies in VVER-1000 Reactor

Vosoughi, Javad | 2025

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 58195 (46)
  4. University: Sharif University of Technology
  5. Department: Energy Engineering
  6. Advisor(s): Salehi, Ali Akbar; Vosoughi, Naser
  7. Abstract:
  8. The Lateral deformation of Fuel Assembly (FA) under the operational conditions of the reactor cores is called FA bowing. This phenomenon is caused by factors such as thermal and hydraulic loads on FAs in the reactor core. It can lead to disturbances in the movement of control rods inside of FAs, cross-contact of FAs in refueling, and also changes in power distribution The bowing phenomenon causes a local change in the distance between the FAs. that results in a neutronic perturbation that affects the power distribution and its asymmetry. Considering the change of boric acid concentration during the operation cycle of the reactor, this neutronic perturbation is time-dependent. that is a very slow change compared to the dynamics of the system. Neutron noise theory is used in the analysis and investigation of time-dependent neutronic perturbation. In this research, first by developing a computational model for the bow VVER-1000 reactor FAs, the microscopic cross sections are calculated based on the local change of the distance and distribution of the water gap around the fuel. after the model validation with the MCNPX code, are used DRAGON cell calculations code, PARCS core calculations code, and COBRA thermohydraulic calculations code for the development of a neutronic simulator to perform neutronic calculations during the cycle. The neutronic simulator results show a difference of 0.12% in the cycle length, 2.71% in the boric acid concentration, and also a maximum difference of 6.03% in the power during the cycle compared to the reference values. After ensuring the accuracy of the calculations by using neutron noise theory with adiabatic approximation, the impact of the bowing phenomenon of the VVER-1000 reactor FA during the cycle is investigated. For this purpose, various modes of FA bowing as neutron noise sources have been considered. The results show that the neutron noise caused by the bowing phenomenon varies during the cycle. It depends on the changes in the boric acid concentration during the cycle, burnup, size, and direction of FAs bowing. Finally, using the neural network approach and having the power distribution and asymmetry caused by the FA bowing, the bow pattern of the FAs in the reactor core is determined based on the assumptions of the problem
  9. Keywords:
  10. Fuel Asembely ; Power Distribution ; Neutron Noise ; Bowing ; Macroscopic Cross Section ; Vodo-Vodyanoi Energetichesky (VVER)1000

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