NEXT (Neutrino Experiment with a Xenon TPC) is an international collaboration with the objective of searching for neutrinoless double beta decay in xenon. It uses a TPC filled with high pressure xenon, providing energy and tracking capabilities. After an initial R&D phase and successful construction and operation of a small (5 kg of xenon) detector, NEXT-White (2016-2021), the collaboration faces a new phase. The current phase consist of the construction (ongoing) and operation of a larger (100 kg of xenon) detector, NEXT-100. The data taking is expected to start during the last quarter of 2022. Both detectors are placed at Laboratorio Subterráneo de Canfranc, an underground facility in the Spanish Pyrenees, border between Spain and France. Being a low background experiment, detector materials must be carefully selected and measured. In addition, a reliable simulation of the detector and physics processes is needed in order to evaluate the background contribution to the signal region. The aim of this poster is to present the current status of the simulation of NEXT-100 detector, together with the background model and radioactivity measurements of the experiment.