Runaway electron mitigation by shattered pellet fragments simulated with MCNP6

Runaway electrons (REs) generated during a tokamak disruption present a catastrophic threat to reactor integrity. To mitigate the effects of REs, ITER will use shattered pellet injection (SPI) to deliver high-Z impurities to the plasma core [1]. In this work, we use the MCNP6 code [2] to assess mitigation efficacy of REs from 1 to 10 MeV by solid Ne and W pellet fragments, with the latter considered as a last-resort runaway termination scheme. Ne pellet fragments are effective for pitch-angle scattering of REs, scattering 20% of incident 10-MeV electrons through large angles (θ ≥ 5.7°). The scattered fraction increases for lower incident energies. However, Ne pellet fragments are ineffective for reducing RE energy or directly terminating REs. W pellet fragments are highly effective for both pitch-angle scattering and termination of REs, with >99% of incident REs either terminated or scattered through large angles at all incident energies. However, the gamma ray flux from W pellet fragments approaches the magnitude of the incident RE flux, and can reseed the RE beam by Compton scattering of cold electrons. For RE current density of 1 MA/m², estimated pellet lifetimes are ~0.7 μs for Ne pellets and ~100 μs for W pellets. This work motivates future plasma physics studies using a macroparticle collision operator to elucidate the early-stage dynamics of SPI.

[1] Gebhart et al., Nucl. Fusion 61 (2021) 106007, [2] Kulesza et al., LA-UR-22-30006 (2022).