  The essence of the ``Supercritical Pile'' model is a process for converting
the energy stored in the relativistic protons of a Relativistic Blast Wave
(RBW) of Lorentz factor $\Gamma$ into electron -- positron pairs of similar
Lorentz factor, while at the same time emitting most of the GRB luminosity at
an energy $E_p \simeq 1$ MeV. This is achieved by scattering the synchrotron
radiation emitted by the RBW in an upstream located ``mirror'' and then
re-intercepting it by the RBW. The repeated scatterings of radiation between
the RBW and the ``mirror'', along with the threshold of the pair production
reaction $p \gamma \to p e^-e^+$, lead to a maximum in the GRB luminosity at an
energy $E_p \simeq 1$ MeV, {\sl independent of the value of $\Gamma$}.
Furthermore, the same threshold implies that the prompt $\gamma-$ray emission
is only possible for $\Gamma$ larger than a minimum value, thereby providing a
``natural'' account for the termination of this stage of the GRB as the RBW
slows down. Within this model the $\gamma-$ray ($E \sim 100$ keV -- 1 MeV)
emission process is due to Inverse Compton scattering and it is thus expected
to be highly polarized if viewed at angles $\theta \simeq 1/\Gamma$ to the
RBW's direction of motion. Finally, the model also predicts lags in the light
curves of the lower energy photons with respect to those of higher energy;
these are of purely kinematic origin and of magnitude $\Delta t \simeq 10^{-2}$
s, in agreement with observation.
