  We investigate the properties and evolution of accretion tori formed after
the coalescence of two compact objects. At these extreme densities and
temperatures, the accreting torus is cooled mainly by neutrino emission
produced primarily by electron and positron capture on nucleons (beta
reactions). We solve for the disc structure and its time evolution by
introducing a detailed treatment of the equation of state which includes
photodisintegration of helium, the condition of beta-equilibrium, and neutrino
opacities. We self-consistently calculate the chemical equilibrium in the gas
consisting of helium, free protons, neutrons and electron-positron pairs and
compute the chemical potentials of the species, as well as the electron
fraction throughout the disc. We find that, for sufficiently large accretion
rates (> 10 solar masses per second), the inner regions of the disk become
opaque and develop a viscous and thermal instability. The identification of
this instability might be relevant for GRB observations.
