In this paper we study thermally induced instabilities in Indian Advanced Heavy Water Reactor (AHWR). One dimensional homogeneous equilibrium model has been used to simulate the two-phase flow. The nonlinear mass, momentum and energy conservation equations are solved along the characteristic directions by using implicit finite difference scheme. The virtue of this scheme is that it handles the boundary conditions naturally. This scheme is fast because the time steps can be greater than that given by Courant-Friedrichs-Lewy condition. The numerical scheme is sufficiently general and can handle axially varying heat flux and different combinations of inlet and exit boundary conditions of enthalpy, mass flux and pressure and multiple channels. No assumptions regarding constant properties and in-compressibility have been taken. The one dimensional fuel heat transfer model was then coupled to the thermal-hydraulics model to analyze out-of-phase instabilities in AHWR. Out-of-phase oscillations are studied by considering two parallel boiling channels. Further effects of radial power distribution, inlet orificing and axial power distribution were considered.

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