Analytical study of nuclear-coupled density-wave instability in a natural circulation pressure tube type boiling water reactor

An analytical model has been developed to study the nuclear-coupled density -wave instability in the Indian advanced heavy water reactor (AHWR) which i s a natural circulation pressure tube type boiling water reactor. The model considers a point kinetics model for the neutron dynamics and a lumped par ameter model for the fuel thermal dynamics along with the conservation equa tions of mass, momentum and energy and equation of state for the coolant. I n addition, to study the effect of neutron interactions between different p arts of the core, the model considers a coupled multipoint kinetics equatio n in place of simple point kinetics equation. Linear stability theory was a pplied to reveal the instability of in-phase and out-of-phase modes in the boiling channels of the AHWR. The results indicate that the stability behav ior of the reactor is greatly influenced by the void reactivity coefficient , fuel time constant. radial power distribution and channel inlet orificing . The delayed neutrons were found to have a strong influence on the Type I and Type II instabilities observed at low and high channel powers, respecti vely. Also, it was found that the coupled multipoint kinetics model and the modal point kinetics model predict the same threshold power for out-of-pha se instability if the coupling coefficient in the former model is half the eigen value separation between the fundamental and the first harmonic mode in the latter model. Decay ratio maps were predicted considering various op erating parameters of the reactor, which are useful for its design. (C) 200 0 Elsevier Science S.A. All rights reserved.