Strategic framework for the simulation of explosive graphite failure under runaway electron incidence

Runaway electron (RE) incidence poses a rather critical threat to the integrity of plasma facing components (PFCs) in tokamaks due to the volumetric nature of energy deposition at relativistic speeds, which leads to steep internal temperature gradients that trigger explosive material failure. A computational framework is presented for the modeling of RE-induced explosive damage in graphite PFCs, benchmarked against a controlled RE impact experiment performed at the DIII-D tokamak. Reconstructed volumetric energy deposition profiles are integrated with high-fidelity three-dimensional thermomechanical simulations based on the finite element method (FEM). It is demonstrated that RE-induced damage in graphite is governed by a degraded subsurface layer formed due to intense volumetric heating, which undermines the structural integrity of the surface and leads to inertial detachment. Continuum thermo-elastic FEM approaches reliably predict the total volume that is liable to detach during failure, while continuum damage-based FEM approaches capture the irreversible subsurface degradation. In contrast to element erosion FEM approaches, a hybrid FEM-to-smoothed-particle-hydrodynamics (SPH) framework provides physical fidelity, successfully reproducing the experimental blown-off volume and estimating debris kinematics.