Subcriticality in nuclear systems is the state in which the neutron population is unable to sustain a fission chain reaction. For nuclear safety, it is important to develop calculation codes that can reproduce the behavior of subcritical systems. In France, the reference Monte Carlo code is TRIPOLI-4 developed at CEA (Commissariat à l’énegie atomique et aux énergies alternatives) Saclay. First, the convergence study of the estimation of the amplification factor of the system, defined as the ratio between fission neutrons and those produced by the external source, by TRIPOLI-4 code was carried out on a simplified geometry by analyzing the α-Pareto coefficient, the Kolmogorov-Smirnov distance and the trend of the mean value of the amplification factor.