In order to investigate the out-of-phase instability in the boiling channels of the AHWR, a linear analytical model has been developed. The model considers homogeneous two-phase flow for the coolant, a coupled multipoint kinetics model for the neutron field dynamics and a lumped fuel heat transfer model. The characteristic equation has been derived considering the interactions of multiple parallel channels of the reactor with the inlet and outlet feeders and the external loop while operating under natural circulation condition. The thermal hydraulic and coupled neutronic-thermohydraulic out-of-phase stability behaviour of the reactor has been studied. The effect of coupling coefficient which determines the reactivity contributed by the interaction between subcores during an out-of-phase instability has been predicted. The results of the present model has been compared with that of a modal point kinetics model. The effect of radial power distribution and channel inlet orificing on the stability of the reactor has also been investigated.
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