Strain mapping and defects inspection of divertor targets by Bragg edge neutron imaging and neutron tomography

The divertor target of a nuclear fusion reactor is a key in-vessel component with critical operational functions to exhaust particles at associated thermal power. For the European demonstration reactor (EU-DEMO, the maximum heat flux that peaks at the strike point is expected to reach 10 MWm−2 during a long pulse normal operation and up to 20–40 MWm−2 for slow transient events. The reliability and longevity of divertor targets compatible with the harsh and complex loading environment of a fusion power plant is the most crucial requirement to ensure power exhaust and thermal management. Therefore, the mechanical stability of the materials and the structural integrity of the components remain crucial requirements. Since 2014, the design and technology R&D activities for the DEMO divertor have been conducted in the framework of the Work Package Divertor of the EUROfusion Consortium. In late 2020, the preconcept design phase of the DEMO fusion reactor reported that uncertainty of stress states of divertor target components brought by fabrication processes and after high heat flux loads is one critical issue that needs to be addressed soon. Such residual stresses are often unknown from both an experimental and a modelling point of view. This work aims to shed light on this issue by Bragg edge neutron imaging of divertor mock-ups to determine residual strain distributions before and after high-heat flux tests. Moreover, non-destructive evaluations of the structural integrity of such divertor mock-ups have been performed by neutron tomography measurements.