On butterfly effect in the dynamics of Oskarshamn-2 instability event of 1999

Abstract

The current research is a further investigation on the OECD/NEA benchmark on the Oskarshamn-2 stability event of cycle 24, occurred on 25th February 1999, in order to study the effect of minor core design changes on both steady state and dynamic behavior of the reactor core, using SIMULATE-3K. The analysis of steady state and transient have been performed along with a systematic comparison between results of the reference and modified core. Results are presented in terms of different parameters such as power/flow ratio, 2D core maps for power and flow for steady state; and channels’ power time series, 2D core maps for maximum power, decay ratio, and resonance frequency, for the transient. The results show that, the introduction of minor changes in the core through the reshuffling of the two hottest channels, and even less limiting channels, has small effects on the steady state core behavior. However, a significant change in the core dynamic behavior is observed through a drastic reduction of the power oscillation amplitude and the simulation demonstrates that, if such minor core design changes had been introduced in the beginning of cycle 24, the stability event of February 25, 1999 might have been potentially avoided. This drastic change in dynamical behavior is explained by carrying out a detailed stability and bifurcation analysis, which concluded that the reason behind such significant reduction in the power oscillation amplitudes is due to a shift in the stability boundary, making the core more stable and also due to the fact that the event took place very close to the stability boundary, region in which the system, unexpectedly, found very sensitive to small changes in the unstable region where the supercritical Hopf bifurcation is expected.