Accelerator Driven Systems (ADS) are specifically studied for their capacity in transmuting Minor Actinides (MA). Electronuclear scenarios involving MA transmutation in ADS are widely researched. The dynamic fuel cycle simulation code CLASS (Core Library for Advanced Scenarios Simulations) is used for predicting the inventory evolution induced by a complex nuclear fleet. For managing reactors, the code CLASS is based on physic models. A Fuel Loading Model (FLM) provides the fuel composition at Beginning Of Cycle (BOC) according to the storages composition and the reactor requirements. A Cross Section Predictor (CSP) estimates mean cross sections needed for solving evolution equations. Physic models are built from reactors calculation set ahead of the scenario calculation. An ADS standard composition at BOC is a mixture of plutonium and MA oxide. The high number of fissile isotopes present in the sub-critical core leads to an issue for building an ADS FLM. A high number of isotopic vectors at BOC is needed to get an exhaustive simulation set. Also, ADS initial reactivity is adjusted with an inert matrix that induces an additional degree of freedom. The building of an ADS FLM for CLASS requires two steps. For any heavy nuclide composition at beginning of cycle, the core reactivity must be imposed at a sub-critical level. Also, the reactivity coefficient evolution should be maintained during the irradiation. For building a FLM, a simulation set has been built. Reactor simulations are done with the transport code MCNP6 (Monte Carlo N particle transport code). The ADS geometry is based on the EFIT (European Facility for Industrial-Scale Transmutation) concept. The simulation set is composed of more than 8000 randomized. A complete neutronic study is presented that highlight the effect on MgO on neutronic parameters.