  The formation of a substantial post-disruption runaway electron current in
ASDEX Upgrade material injection experiments is determined by avalanche
multiplication of a small seed population of runaway electrons. For the
investigation of these scenarios, the runaway electron description of the
coupled 1.5D transport solvers ASTRA-STRAHL is amended by a fluid-model
describing electron runaway caused by the hot-tail mechanism. Applied in
simulations of combined background plasma evolution, material injection, and
runaway electron generation in ASDEX Upgrade discharge #33108, both the Dreicer
and hot-tail mechanism for electron runaway produce only $\sim$ 3$~$kA of
runaway current. In colder plasmas with core electron temperatures
$T_\mathrm{e,c}$ below 9$~$keV, the post-disruption runaway current is
predicted to be insensitive to the initial temperature, in agreement with
experimental observations. Yet in hotter plasmas with $T_\mathrm{e,c} >
10~\mathrm{keV}$, hot-tail runaway can be increased by up to an order of
magnitude, contributing considerably to the total post-disruption runaway
current. In ASDEX Upgrade high temperature runaway experiments, however, no
runaway current is observed at the end of the disruption, despite favourable
conditions for both primary and secondary runaway.
