  A new fluid model for runaway electron simulation based on fluid description
is introduced and implemented in the magnetohydrodynamics code M3D-C1, which
includes self-consistent interactions between plasma and runaway electrons. The
model utilizes the method of characteristics to solve the continuity equation
for the runaway electron density with large convection speed, and uses a
modified Boris algorithm for pseudo particle pushing. The model was employed to
simulate magnetohydrodynamics instabilities happening in a runaway electron
final loss event in the DIII-D tokamak. Nonlinear simulation reveals that a
large fraction of runaway electrons get lost to the wall when kink
instabilities are excited and form stochastic field lines in the outer region
of the plasma. Plasma current converts from runaway electron current to Ohmic
current, and get pinched at the magnetic axis. Given the good agreement with
experiment, the simulation model provides a reliable tool to study macroscopic
plasma instabilities in existence of runaway electron current, and can be used
to support future studies of runaway electron mitigation strategies in ITER.
