A study on regional stability analysis methodology with a one-point neutron kinetics model

A methodology for boiling water reactor (BWR) regional stability with a one-point neutron kinetics model is proposed from the higher harmonics viewpoint and is verified with the Ringhals-1 stability benchmark test data, A one-point neutron kinetics model for regional stability analysis is derived from the spatial neutron diffusion equation using the mode decomposition technique. From the derivation, the intermode coupled reactivity coefficient is defined and applied to a frequency-domain BWR stability analysis model. The analysis model traces a unit power perturbation and calculates the open-loop transfer function as the power response to the input perturbation. Combined with the aforementioned reactivity coefficient and the asymmetric shape perturbation that reflects the first azimuthal mode, the first azimuthal mode is excited exclusively without any assumption on the ex-core model. Therefore, the regional stability can be evaluated with a normal recirculation flow model, which is employed for core-wide stability analysis. The methodology is verified with the Ringhals-1 stability benchmark test data, whose stability conditions were widely distributed and suitable for verification, The results show that the proposed methodology is quite appropriate for BWR regional stability analysis.