Development of 3D neutron noise simulator based on GFEM with unstructured tetrahedron elements

• The neutron noise calculation is performed in the 3D geometries.
• The GFEM is used to solve the neutron diffusion and the neutron noise equations.
• Unstructured tetrahedron elements are used to discrete the equations.
• The absorber of variable strength noise source is considered in the calculation.
• The variation of the neutron noise in the axial direction of the reactor core is investigated.

In the present study, the neutron noise, i.e. the stationary fluctuation of the neutron flux around its mean value is calculated based on the 2G, 3D neutron diffusion theory. To this end, the static neutron calculation is performed at the first stage. The spatial discretization of the neutron diffusion equation is performed based on linear approximation of Galerkin Finite Element Method (GFEM) using unstructured tetrahedron elements. Using power iteration method, neutron flux and corresponding eigen-value are obtained. The results are then benchmarked against the valid results for VVER-1000 (3D) benchmark problem. In the second stage, the neutron noise equation is solved using GFEM and Green’s function method for the absorber of variable strength noise source. Two procedures are used to validate the performed neutron noise calculation. The calculated neutron noise distributions are displayed in the different axial layers in the reactor core and its variation in axial direction is investigated. The main novelty of the present paper is the solution of the neutron noise equations in the three dimensional geometries using unstructured tetrahedron elements.