Edge cracking of WEST tungsten actively cooled plasma facing components after plasma operation

Abstract

The WEST tokamak operates with actively-cooled tungsten ITER-like divertor plasma facing units (PFUs) since 2018. Poloidally-distributed cracks along the leading edges of PFUs were observed after WEST plasma operations. Because cracks may have an impact on the PFUs lifetime, mechanisms related to the cracks initiation and propagation are still under investigation. Previous numerical simulations lead to assess the location of the privileged initiation of the first crack on the PFU and the rationale for its initiation (disruption events causing brittle crack). In this paper, the goal is to identify, the most likely mechanism involved in the PFU damage process during transient. For this, in the first part of this paper, the observed crack topologies (depth and width of cracks) are described based on the recent post-mortem analysis in WEST. Then numerical simulations assuming initial crack(s) at the leading edge lead to assess the evolution of the stress mechanical field and explains the cracking mechanisms. Results of the simulations assuming several initials cracks (primary, secondary) highlight the leading edge periodic cracking under successive disruptions up to a threshold which corresponds to a minimum distance between two cracks and no further cracks are generated. The cracking threshold length has been assessed between 0.4 mm and 0.8 mm which is fully consistent with the WEST post-mortem observations. Finally, the leading edge crack initiation domain is specified as function of the thermal quench disruption impact factor and the component misalignment.