  Neutron imaging is a non-destructive technique for analyzing a wide class of
samples, such as archaeological or industrial material structures. In recent
decades, technological advances have had a great impact on the neutron imaging
technique, which has meant an evolution from simple radiographs using films
(2D) to modern tomography systems with digital processing (3D). The 5 MW
research nuclear reactor IEA-R1, which is located at the Instituto de Pesquisas
Energ\'eticas e Nucleares (IPEN) in Brazil, possesses a neutron imaging
instrument with $1.0 \times 10^{6}$ $n/cm^{2}s$ in the sample position. IEA-R1
is over 60 years old and the future of neutron science in Brazil, including
imaging, will be expanded to a new facility called the Brazilian Multipurpose
Reactor (RMB, Portuguese acronym), which will be built soon. The new reactor
will house a suite of instruments at the Neutron National Laboratory, including
the neutron imaging facility, viz., Neinei. Inspired by recent author's works,
we model the Neinei instrument through stochastic Monte Carlo simulations. We
investigate the sensitivity of the neutron imaging technique parameter ($L/D$
ratio) with the neutron flux, and the results are compared to data from the
Neutra (PSI), Antares (FRM II), BT2 (NIST) and DINGO (OPAL) instruments. The
results are promising and provide avenues for future improvements.
