  In order to contribute to the understanding of runaway electron generation
mechanisms during tokamak disruptions, a test particle tracker is introduced in
the JOREK 3D non-linear MHD code, able to compute both full and guiding center
relativistic orbits. Tests of the module show good conservation of the
invariants of motion and consistency between full orbit and guiding center
solutions. A first application is presented where test electron confinement
properties in a JET massive gas injection-triggered disruption simulation are
investigated. It is found that electron populations initialised before the
thermal quench (TQ) are typically not fully deconfined in spite of the global
stochasticity of the magnetic field during the TQ. The fraction of "survivors"
decreases from few tens down to a few tenths of percent as the electron energy
varies from 1keV to 10MeV. The underlying mechanism for electron "survival" is
the prompt reformation of closed magnetic surfaces at the plasma core and, to a
smaller extent, the subsequent reappearance of a magnetic surface at the edge.
It is also found that electrons are less deconfined at 10MeV than at 1MeV which
appears consistent with a phase averaging effect due to orbit shifts at high
energy.
