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Description
The extreme plasma heat loads arising during disruptions play a major role in determining the lifetime of plasma-facing components (PFCs). In particular, transient surface melting events are of crucial importance, not only because melt displacement constitutes a major PFC erosion mechanism, but also due to the risk of liquid metal filling the gaps between adjacent wall components [1]. In such cases, large eddy currents may short through the gaps during subsequent disruptions, leading to potential mechanical failure [1,2].
Gap filling and bridging have been observed experimentally in several fusion devices and under various conditions [3,4], but extrapolations to future reactors remain highly uncertain, owing to lack of understanding of the main driving parameters. Recent numerical modelling works attempting to address this issue have brought new insights into the temporal dynamics of the bridging process and have been shown to successfully reproduce available experimental data, notably in terms of characteristic melt infiltration depth and overall cross-gap transport [5,6]. Building upon these validation studies, this contribution details the first predictions of gap bridging by liquid tungsten (W) under conditions representative of worst-case, unmitigated vertical displacement events in ITER. Using characteristic heat load and exposure duration ranges available in the literature [1,7], multiphase Navier-Stokes computations are carried out to investigate the formation and mobilization of W melt around a gap. Components on both sides of 0.5-1 mm-wide toroidal gaps, representative of gaps between divertor monoblocks as well as between first-wall panel fingers, are loaded identically. Values of the local magnetic field inclination angle, which affects the intensity of thermionic currents [8], are sampled up to 20° to explore the PFC response at different wall locations, including divertor cassette edges. The results are analyzed through figures of merit such as the time between melting onset and bridge formation, as well as the relationship between bridge thickness and the characteristic liquid layer depth.
[1] R. A. Pitts et al, Nucl. Mater. Energy 42 (2025) 101854
[2] M. Lehnen et al, J. Nucl. Mater. 463 (2015) 39
[3] K. Krieger et al, Nucl. Fusion 58 (2018) 026024
[4] I. Jepu et al, Nucl. Fusion 59 (2019) 086009
[5] L. Vignitchouk et al, Nucl. Fusion 65 (2025) 056013
[6] L. Vignitchouk, Nucl. Mater. Energy 46 (2026) 102048
[7] J. Coburn et al, Nucl. Fusion 62 (2022) 016001
[8] M. Komm et al, Nucl. Fusion 60 (2020) 054002