Left: evolution of the neutron flux in the Flattop-Pu benchmark after a positive reactivity insertion of $2.2, according to the solution of the point-kinetics equations with one family of precursors. Middle: Evolution of the importance ratio for every time-step used for the kinetic simulation. The population control strategy is performed on a grid that becomes finer as the neutron flux increases. The values of I_n and I_c are calculated according to equations (6); consequently, they vary with the simulated time-step length and with the considered reactivity. Right: Evolution of the importance values (I_n and I_c) for the same simulation.