Numerical modelling of metallic melt damage in contemporary tokamaks and future fusion reactors
Abstract
EU recognizes fusion energy as the most promising long-term solution for clean virtually unlimited energy, with the construction of ITER and engineering design of EU-DEMO being short-term and mid-term objectives of the EUROfusion Consortium roadmap. The provision of plasma-facing components (PFCs) with sufficient lifetime constitutes one of the major remaining challenges in the development of magnetic confinement fusion reactors. In ITER, currently under construction, PFC integrity is primarily threatened by fast transient power loading owing to magnetohydrodynamic (MHD) instabilities such as edge-localized modes, vertical displacement events or major disruptions. In DEMO, currently undergoing its conceptual design phase, the PFC heat load capability could be surpassed during regular plasma ramp-up/-down and undesired events such as H-L transitions, major disruptions or vertical displacement events. Extreme heat loads will generate metallic melts that will be subject to strong plasma-induced volumetric forces which will displace the material, causing large-scale surface deformations that may severely compromise power handling. Moreover, off normal events can be accompanied by the production of relativistic intense runaway electron beams that may lead to deep melting and even PFC explosion. Hence, the reliable modelling of melt and explosive events as well as the realistic assessment of their possible consequences constitute a priority topic in contemporary fusion research.
The MEMENTO code (former MEMOS–U), developed and maintained by the Complex Plasmas Group of the Space & Plasma Physics division of KTH, is the only European code capable of simulating the self-consistent motion of induced metallic melts coupled with the thermoelectric response of large-area wetted PFCs, thus providing realistic surface deformation profiles. The general numerical problem falls under the category of free-surface MHD flows with phase change. MEMENTO solves the incompressible resistive thermoelectric MHD set of equations within the shallow water approximation and the magneto-static approximation coupled with the heat convection-diffusion equation. MEMENTO has been near-exclusively employed for PFC melt predictions in ITER & DEMO. The MEMENTO predictive capability has been successfully tested in multiple dedicated experiments that deliberately achieved PFC melting in a controlled manner. Benchmarking experiments concerned different plasma heat loads, tokamaks, materials, exposure geometries, cooling types and electrical connections. These extensive validation activities have been partly funded by EUROfusion, the ITER Organization and the Swedish Research Council.
With the present proposal, we request for the computational resources that are necessary for (a) implementation of volumetric heating due to runaway electrons in the MEMENTO code, (b) further benchmarking of MEMENTO against experiments on PFC damage induced by runaway electrons (DIII-D experiments with graphite domes, JET experiments with beryllium plates), (c) further benchmarking of MEMENTO with experiments on PFC damage induced by stationary loads (DIII-D experiments with aluminum samples), (d) predictive MEMENTO runs for different ITER scenarios (focusing on W divertor damage by edge-localized modes, major disruptions, runaway electrons), (e) predictive MEMENTO runs for different DEMO scenarios (focusing on W upper limiter damage by the thermal/current quench of vertical displacement events).
