Reconstruction of neutron flux distribution by nodal synthesis method using online in-core neutron detector readings

Abstract The safety and optimal performance of nuclear reactors require online monitoring in the core. The present paper describes a method that avoids the solution of the time-dependent neutron diffusion equation, and it uses online readings of the fixed in-core neutron detectors to reconstruct the three-dimensional (3D) neutron flux distribution. The essential idea of the nodal synthesis method is the separation of time and space-dependence of the neutron flux distribution. The time-dependent section of the flux distribution is determined by in-core neutron detector readings, and the space-dependent section is obtained from pre-computed harmonics of the neutron diffusion equation. In online computation, time-dependent coefficients are only computed, and flux distribution is reconstructed by pre-computed harmonics. To calculate the diffusion equation harmonics, the nodal expansion method and the Gram-Schmidt process are used. Online reconstruction of neutron flux distribution in the Bushehr Nuclear Power Plant with VVER-1000/446 reactor type is simulated, and the results are benchmarked by valid results. The maximum relative percent error between reconstructed results and the valid results is about 5%, and the computing time is about 0.5 s.