Numerical studies of metallic melt stability during disruptions in fusion reactors

High-energy transients during tokamak discharges can induce surface melting of metallic plasma-facing components. The resulting melt pools are subject to a variety of forces and dynamic instabilities, possibly leading to the ejection of droplets, which may then solidify into metallic dust particles and accumulate in the machine. This constitutes a major safety issue for fusion nuclear reactors since such particles are potentially toxic, radioactive, and chemically reactive. In ITER, beryllium droplets produced during disruptions are foreseen as one of the dominant sources of in-vessel dust. In contemporary fusion devices such as JET and ASDEX Upgrade, melt events are typically observed as the result of disruptions or the exposure of deliberately misaligned wall components to high plasma heat fluxes. Although traces of droplet ejection are a common occurrence, direct in situ observations of the melt flow are unavailable: its dynamics must be inferred from analyses of the re-solidified material.

Here, we present the first simulations of dynamic melt instabilities in fusion conditions, focusing on the case of a liquid beryllium layer flowing over the edge of a solid plasma-facing component. This scenario corresponds to experimentally observed melt events in JET, in which so-called frozen beryllium “waterfalls” are found on upper dump plates, along with droplet splash traces on the nearby main chamber wall. Simulation results elucidate several possible qualitative melt dynamics regimes and are shown to exhibit good quantitative agreement with available experimental data. This establishes numerical simulations as a valuable tool for predictive melt instability studies of ITER-relevant scenarios.