Accurate determination of isotopic densities in nuclear materials is of first importance for the Criticality-Safety, as all other quantities of interest (Decay Heat, Spectra, Masses, Reactivity factor…) depend on the composition of the considered material. Isotopic densities are computed by solving the Generalized Bateman equations. MENDEL is the new generation of CEA’s fuel cycle code systems, successor to DARWIN/PEPIN2. Its solvers of the Generalized Bateman Equations are used by the stand-alone code MENDEL itself (for applications such as Decay Heat or particle spectra calculations) but also inside deterministic APOLLO3® and stochastic TRIPOLI-4® transport code systems. They can use any pre-processed nuclear data evaluation. MENDEL aims to compute as precisely and as rapidly as possible the isotopic densities during irradiation or cooling periods. For this purpose, several algorithms are available in MENDEL, and two new approaches have been added in MENDEL version 3.1. The analytical approach, was extended from cooling period computations only to in-flux calculations as long as the depletion matrix can be triangularized. Being naturally faster than any numerical approach, this enables, in adequate configurations, an exact and quick way to compute isotopic densities. Nevertheless, fuel cycle depletion chains are often too complete to be triangularized. When only part of the matrix can be triangularized, only Runge-Kutta 4th order and CRAM methodologies were possible until MENDEL v3.0. From version 3.1, we added a mixed approach between CRAM and analytical method, which can offer for some depletion matrices a high speedup. This paper describes the different solvers used in MENDEL version 3.1.