Disruptions, the sudden and uncontrolled losses of the plasma, are among the main threats to ITER: they expose the machine to strong heat loads, electromagnetic forces and the formation of runaway electrons. At the heart of every disruption, during the thermal quench, the 'Global Reconnection Event' (GRE) takes place. It is a violent MHD relaxation in which magnetic islands overlap, the field becomes stochastic and the current is redistributed. This phenomenon is still poorly understood: we do not yet know what triggers it, how it evolves, or how long the stochastic phase lasts. This duration is decisive, because if magnetic surfaces re-form too early, fast electrons are confined again and can form a runaway beam. The effectiveness of Massive Material Injection (MMI), the mitigation strategy chosen for ITER, therefore depends largely on it. The thesis will study the GRE with JOREK, a nonlinear 3D MHD code, using a new procedure that replaces the injection with a local increase in resistivity and makes robust parametric studies possible. The first goal is to identify the mechanisms that trigger the GRE and to quantify how long the stochastic phase lasts. The results will then be validated against measurements from WEST, JET and JT-60SA. Finally, depending on progress, the thesis will return to realistic MMI simulations in order to produce predictions for ITER.