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Numerical Study on Observed Decay Ratio of Coupled Neutronic-Thermal Hydraulic Instability in Ringhals Unit 1 under Random Noise Excitation

The core stability in the Ringhals Unit 1 was estimated under the numerical random noise that simulates indefinable two-phase flow noise sources in actual cores. This noise model is expressed as a product of band white amplitude and arbitrary shape functions. In evaluating decay ratios, the conventional free-decaying method based on a clean modal disturbance was replaced with the response analysis method based on a numerical moderator density noise. The stability monitoring procedure was reproduced numerically by giving the spatially random shaped noise disturbance and by linearly varying the moderator density reactivity multiplier. It was confirmed that the observed regional decay ratio based on differential LPRM signals proposed by Hagen sometimes shows a discontinuous jump from the stable to the unstable region as predicted by Pazsit. Nevertheless, the regional decay ratio based on the extracted modal response will show a continuous change under the same condition. It was clarified that this jumping is mainly induced by the local fluctuation of moderator density at the frequency range which overlaps with a predominance range of the fundamental mode. It was demonstrated that this kind of numerical noise analysis is useful in verifying the monitoring algorithm before applying it in actual plants.