  Runaway electron populations seeded from the hot-tail generated by the rapid
cooling in plasma-terminating disruptions are a serious concern for next-step
tokamak devices such as ITER. Here, we present the first combined treatment of
crucial terms in the runaway seed process: hot-tail generation and magnetic
perturbation induced loss. We show simulations of the runaway generation in
plasma disruptions mitigated by material injection, including the superthermal
electron dynamics, heat and particle transport, atomic physics, and radial
losses due to magnetic perturbations. We find limits on the impurity density
and magnetic perturbation level for which the runaway seed current is
acceptable without excessive thermal energy being lost to the wall via particle
impact. The consistent modelling of generation and losses allows us to identify
parameter regimes which lead to runaway beams forming near the edge of the
plasma, where they could be deconfined via external perturbations.
