This is a Monte Carlo Neutron Transport Code, where results are valid only at equilibrium.
https://laws.lanl.gov/vhosts/mcnp.lanl.gov/pdf_files/la-ur-18-20247.pdf
https://laws.lanl.gov/vhosts/mcnp.lanl.gov/pdf_files/la-ur-16-29043.pdf
https://laws.lanl.gov/vhosts/mcnp.lanl.gov/pdf_files/la-ur-05-4983.pdf