m saidi - Department of Physics, Science Faculty, Ilam University, Ilam, Iran
r Khodabakhsh - Department of Physics, Science Faculty, Urmia University, Urmia, Iran
s Behnia - Department of Physics, Urmia University of Technology, Urmia, Iran

Lyapunov exponent method is one of the best tools for investigating the range of stability and the transient behavior of the dynamical systems. In beryllium-moderated and heavy water-moderated reactors, photo-neutron plays an important role in dynamic behavior of the reactor. Therefore, stability analysis for changes in the control parameters of the reactor in order to guarantee safety and control nuclear reactor is important. In this work, the range of stability has been investigated using Lyapunov exponent method in response to step, ramp and sinusoidal external reactivities regarding six groups of delayed neutrons plus nine groups of photo-neutrons. The qualitative results are in good agreement with quantitative results of other works

photo-neutron, control parameter, delayed neutron, Lyapunov exponent