  We calculate the structure and evolution of a gamma ray burst central engine
where an accreting torus has formed around the newly born black hole. We study
the general relativistic,MHD models and we self-consistently incorporate the
nuclear equation of state. The latter accounts for the degeneracy of
relativistic electrons, protons, and neutrons, and is used in the dynamical
simulation, instead of a standard polytropic -law. The EOS provides the
conditions for the nuclear pressure in the function of density and temperature,
which evolve with time according to the conservative MHD scheme. We analyze the
structure of the torus and outflowing winds, and compute the neutrino flux
emitted through the nuclear reactions balance in the dense and hot matter. We
also estimate the rate of transfer of the black hole rotational energy to the
bipolar jets. Finally, we elaborate on the nucleosynthesis of heavy elements in
the accretion flow and the wind, through computations of the thermonuclear
reaction network. We discuss the possible signatures of the radioactive
elements decay in the accretion flow. We suggest that further detailed modeling
of the accretion flow in GRB engine, together with its microphysics, may be a
valuable tool to constrain the black hole mass and spin. It can be
complementary to the gravitational wave analysis, if the waves are detected
with an electromagnetic counterpart.
