Understanding relativistic runaway electron (RE) behavior in post-thermal quench plasmas is critical in future tokamak devices such as ITER. A highly relevant aspect is the macroscopic instability of a runaway beam and how this instability in turn affects RE confinement. Direct interactions between the n=1 internal kink instability and REs are numerically investigated, utilizing the MARS-F code for a post-thermal quench toroidal plasma. It is found that, within a MHD-RE hybrid model where REs are approximated as a fluid species, runaway electrons destabilize the resistive internal kink mode, leading to a better recovery (as compared to a pure fluid model) of the mode growth rate scaling of S^{-1/3} at low values of Lundquist number S. Furthermore, REs significantly modify the internal kink eigenfunction inside or near the q=1 surface, by forming a new narrow layer within the resistive layer for the perturbed parallel current. The RE contribution is also found to substantially enhance the coupling among poloidal harmonics. All these changes to the perturbation structure consequently affect the RE drift orbits. Compared to the fluid model, 3D perturbations computed with the more consistent hybrid model lead to less loss of relativistic electrons in the RE beam, when the perturbation level reaches that of the equilibrium field. At lower (but still large) perturbation amplitude (e.g. ~10% of the equilibrium field), the internal kink mode has little effect on the RE loss with either model. These results thus indicate that it may be challenging to purge REs in a post-thermal quench plasma, relying only on the internal kink type of MHD instabilities.

Nonlinear 3D MHD simulations of shattered-pellet injection (SPI) in JET have been performed using the M3D-C1 and NIMROD extended-MHD codes. NIMROD simulations show a radiative thermal quench (TQ) which becomes more rapid as the pellet passes the safety factor q=3 surface. The results are similar for both a single, monolithic pellet and a pencil-beam model for the SPI plume. A scan in viscosity from 200-2000 m^2/s for 2D MHD simulations shows no significant change to the predicted radiative collapse. The effect of viscosity in 3D NIMROD simulations of JET with significant MHD activity still needs to be assessed. Nonlinear 3D M3D-C1 modeling of JET shot 95707 SPI with a single, monolithic pellet show a prototypical SPI-driven disruption. An initially radiation-driven TQ is accelerated by MHD activity as the pellet crosses the q=2 and q=3/2 surfaces, leading to a radiation spike, global stochasticization of the magnetic field, and a complete TQ. Eventually a current quench (CQ), preceded by a current spike is seen as the ohmic heating balances the radiative cooling. A simulation performed with half the viscosity (500 vs 1000 m^2/s) showed qualitatively similar behavior, but with a slightly higher radiation spike and an earlier CQ without a current spike. These simulations lay the ground work for more-sophisticated validative and predictive modeling of SPI in JET using both M3D-C1 and NIMROD in the near future.