  Benign termination of mega-ampere (MA) level runaway current has been
convincingly demonstrated in recent JET and DIII-D experiments, establishing it
as a leading candidate for runaway mitigation on ITER. This comes in the form
of a runaway flush by parallel streaming loss along stochastic magnetic field
lines formed by global magnetohydrodynamic instabilities, which are found to
correlate with a low-Z injection that purges the high-Z impurities from a
post-thermal-quench plasma. Here we show the competing physics that govern the
post-flush reconstitution of the runaway current in a ITER-like reactor where
significantly higher current is expected. The trapped ``runaways'' are found to
dominate the seeding for runaway reconstitution, and the incomplete purge of
high-Z impurities helps drain the seed but produces a more efficient avalanche,
two of which compete to produce a 2-3~MA step in current drop before runaway
reconstitution of the plasma current.
