  We present the general notions and observational consequences of the
"Supercritical Pile" GRB model; the fundamental feature of this model is a
detailed process for the conversion of the energy stored in relativistic
protons in the GRB Relativistic Blast Waves (RBW) into relativistic electrons
and then into radiation. The conversion is effected through the $p \gamma \to p
e^+e^-$ reaction, whose kinematic threshold is imprinted on the GRB spectra to
provide a peak of their emitted luminosity at energy \Ep $\sim 1$ MeV in the
lab frame. We extend this model to include, in addition to the (quasi--)thermal
relativistic post-shock protons an accelerated component of power law form.
This component guarantees the production of $e^+e^- -$pairs even after the RBW
has slowed down to the point that its (quasi--)thermal protons cannot fulfill
the threshold of the above reaction. We suggest that this last condition marks
the transition from the prompt to the afterglow GRB phase. We also discuss
conditions under which this transition is accompanied by a significant drop in
the flux and could thus account for several puzzling, recent observations.
Finally, we indicate that the same mechanism applied to the late stages of the
GRB evolution leads to a decrease in \Ep $\propto \Gamma^2(t)\propto t^{-3/4}$,
a feature amenable to future observational tests.
