  For the simulation of disruptions in tokamak fusion plasmas, a fluid model
describing the evolution of relativistic runaway electrons and their
interaction with the background plasma is presented. The overall aim of the
model is to self-consistently describe the nonlinear coupled evolution of
runaway electrons (REs) and plasma instabilities during disruptions. In this
model, the runaway electrons are considered as a separate fluid species in
which the initial seed is generated through the Dreicer source, which
eventually grows by the avalanche mechanism (further relevant source mechanisms
can easily be added). Advection of the runaway electrons is considered
primarily along field lines, but also taking into account the ExB drift. The
model is implemented in the nonlinear magnetohydrodynamic (MHD) code JOREK
based on Bezier finite elements, with current coupling to the thermal plasma.
Benchmarking of the code with the one-dimensional runaway electron code GO is
done using an artificial thermal quench on a circular plasma. As a first
demonstration, the code is applied to the problem of an axisymmetric cold
vertical displacement event in an ITER plasma, revealing significantly
different dynamics between cases computed with and without runaway electrons.
Though it is not yet feasible to achieve fully realistic runaway electron
velocities close to the speed of light in complete simulations of slowly
evolving plasma instabilities, the code is demonstrated to be suitable to study
various kinds of MHD-RE interactions in MHD-active and disruption relevant
plasmas.
