  Mitigation of runaway electrons is one of the outstanding issues for the
reliable operation of ITER and other large tokamaks, and accurate estimates for
the expected runaway-electron energies and current are needed. Previously,
linearized tools (which assume the runaway population to be small) have been
used to study the runaway dynamics, but these tools are not valid in the cases
of most interest, i.e. when the runaway population becomes substantial. We
study runaway-electron formation in a post-disruption ITER plasma using the
newly developed non-linear code NORSE, and describe a feedback mechanism by
which a transition to electron slide-away can be induced at field strengths
significantly lower than previously expected. If the electric field is actively
imposed using the control system, the entire electron population is quickly
converted to runaways in the scenario considered. We find the time until the
feedback mechanism sets in to be highly dependent on the details of the
mechanisms removing heat from the thermal electron population.
