The purpose of the present paper is to establish a new void reactivity coefficient (VRC) estimation method based on gray box modeling concept. The gray box model consists of a point kinetics model as the first principle model and a fitting model of core flow kinetics. Two approaches in frequency and time domain are applied to estimate VRC from steady state fluctuating data. With the VRC estimation, principal noise sources, which are important for assuring VRC estimation reliability, are also estimated. In the frequency domain approach, the estimation criterion is defined by power spectral density, transfer function and coherence function. This criterion is useful to estimate VRC and system noise sources at the same time. This approach gives better results than the time domain approach using Kalman filter and maximum likelihood. The effectiveness of the proposed method is shown by comparing the core performance calculation code based on actual BWR plant data measured through one year fuel cycle. The method can also provide related BWR core dynamics information such as core stability using an eigenvalue of the gray box model.
