  At the extreme densities and temperatures typical of the central engine of
GRBs, the accreting torus is cooled mainly by advection and by neutrino
emission. The latter process is dominated by electron and positron capture onto
nucleons ($\beta$ reactions). We calculate the reaction rates and the nuclear
composition of matter, assuming that the torus consists of helium,
eletron-positron pairs, free neutrons and protons. After determining the
equation of state and solving for the disk structure for a given initial
accretion rate, we subsequently follow its time evolution. We find that, for
accretion rates of the order of $10 M_{\odot}$/s, likely typical for the early
stages of the accretion event, the disk becomes unstable, giving rise to
variable energy output. This instability may play an important role for
producing internal shocks.
