There are many design proposals for a He-cooled first wall and divertor concept for DEMO and ARIES-CS.37,160,221 Among these, the He-cooled modular design with jet cooling (HEMJ)102 is the most developed and qualified in terms of microstruc­tural response,103 having survived at reduced coolant temperatures of 450-550 °C at least 100 cycles at 11 MW m~2 without failure. In contrast to the results obtained for water-cooled components for ITER, no influence of grain orientation on the components performance was observed.102 This might be a result of the higher temperature, which was always above the DBTT.

Nevertheless, some difficulties in the design still have to be resolved. First, there are problems related to temperature with a desired coolant temperature of >600 °C; these include material recrystallization at the top surface and the necessary high temperature joining to the W-based heat sink material. Second,

issues related to the material’s mechanical properties must be solved, in particular for the ductility of W-based structural material and its neutron irradia­tion resistance (see Section 4.17.4.3). Finally, the manufacturing reproducibility has to be at a high level because of the large number of small units (1unit« 3 x 10~4m2) necessary for cladding the DEMO divertor.