Development of High-Speed Neutron Noise Simulator based on High order Nodal Expansion Method for Hexagonal Geometry in Frequency-Domain

Improvement of reactor safety using simulation and analysis of the power reactor noise needs to develop the neutron noise computation codes. The purpose of the current study is to develop the neutron noise simulator for hexagonal geometry reactors. Therefore, in this research, the SD-HACNEM (Sharif Dynamic - High order Average Current Nodal Expansion Method) simulator is improved to solve the steady-state neutron diffusion equation and neutron noise equation in the frequency-domain for two-dimensional hexagonal geometry by the high-order nodal expansion method. Firstly, calculation is performed for the steady-state. To reduce the discretization error, the flux expansion polynomials degree is increased from 3rd to 5th by considering the nodes with the size of a fuel assembly for either ACNEM (Average Current Nodal Expansion Method) or HACNEM (High-order Average Current Nodal Expansion Method). The verification of the ACNEM and HACNEM is performed by comparing the results with valid references for the IAEA-2D benchmark problem reactor. The steady-state numerical results show that the use of HACNEM provides more accuracy compared with ACNEM, without reducing the size of the nodes. In the main part of the present study, neutron noise calculations are performed in the frequency-domain for two types of noise sources including absorber with variable strength and ILOFAIP . The results are benchmarked by performing simulation at zero frequency and adjoint calculations. The numerical results show that the use of the high-order nodal expansion method is effective for the simulation of neutron noise in the hexagonal reactor.