The reestablishment of the hybrid scenario in JET after installation of the ITER-like wall (ILW) was successful in reaching similar global performances as in CFC wall (H98 ~1.3, bN ~ 3). However, these performances have only been maintained for 2­3s, after which W accumulation in the core compromises the plasma performance, both due to radiative cooling and to the onset of MHD activity with negative effects on confinement. This paper aims at studying the dynamics of core W accumulation in hybrid plasmas, understanding its origin and possible ways to counteract it, to allow time extension of the improved confinement phase. The focus of the paper will not be on W sources from the edge, which can be regulated by e.g. gas puffing, ELMs pacing, SOL and divertor density control, which are the object of other studies. The focus here will be on core impurity transport and MHD, and how they evolve during the hybrid discharge eventually leading to accumulation.