The isotopic depletion calculation is typically solved by using the quasi-static approximation. In this approximation, the neutron flux is assumed as constant along the time steps in which the burn-up cycle is divided. This work proposes two different calculation methodologies to obtain the neutron flux, used in the isotopic depletion calculation in subcritical systems driven by an external neutron source, in a coarse mesh framework. The so-called depletion calculation methodology with Coarse Mesh Finite Difference (CMFD), and depletion calculation methodology with Pseudo-Harmonic Expansion (PHE). In these methodologies, the neutron flux at the beginning of each burn-up step is calculated with CMFD and the PHE methods, respectively. In order to verify the applicability of both methodologies to an ADStype reactor, it is simulated a 45 MW small-reactor with burn-up cycle of 90 days, and an 800 MW large-reactor with burn-up cycle of 600 days. The subcriticality adjustment mechanism is based on the effective multiplication factor, solving the neutron diffusion equation with the Nodal Expansion Method (NEM). In addition, the averaged power density and the intensity of the neutron source are evaluated throughout the burn-up cycle. The results obtained by both methodologies are in good agreement with each other.
