[en] It has been implemented in the attached code a new method of inducing instabilities in NPP with BWR reactor, through disturbances in the moderator density based on the shape and amplitude of the power modes. This method has been tested and verified with the simulations presented here. The results of the simulations under the conditions of the Record 9' of Ringhals-1 nuclear power plant with the coupled codes show that the type of stability depends on the perturbed mode and amplitudes of these disturbances, that is, the stability of the reactor not only depends on the conditions thermohydraulics previous to the swing, but also the disturbance that starts the swing.