  We present the first high resolution MHD simulation of cosmic-ray electron
reacceleration by turbulence in cluster mergers. We use an idealised model for
cluster mergers, combined with a numerical model for the injection, cooling and
reacceleration of cosmic-ray electrons, to investigate the evolution of cluster
scale radio emission in these objects. In line with theoretical expectations,
we for the first time, show in a simulation that reacceleration of CRe has the
potential to reproduce key observables of radio halos. In particular, we show
that clusters evolve being radio loud or radio quiet, depending on their
evolutionary stage during the merger. We thus recover the observed transient
nature of radio halos. In the simulation the diffuse emission traces the
complex interplay between spatial distribution of turbulence injected by the
halo infall and the spatial distribution of the seed electrons to reaccelerate.
During the formation and evolution of the halo the synchrotron emission spectra
show the observed variety: from power-laws with spectral index of 1 to 1.3 to
curved and ultra-steep spectra with index $> 1.5$.
