Lifetime of Be-, CFC- and W-armoured ITER divertor plates

The thermal performance of divertor plates armoured with beryllium (thickness 2-30 mm), carbon fibre composite (CFC, 2-60 mm) or tungsten (W-3Re, 2-60 mm) was modelled for transients of 10 s at 20 MW/m² - with and without vapour shielding. Due to the high resulting temperatures special attention was paid to the material properties up to and above melting or sublimation. Pure copper was chosen for the flat heat sink of Be and W-3Re, the coolant tube material was DS-Cu in all cases. The melt layer depth, the surface temperature and peak power densities to the coolant were obtained in full 2D geometry using finite element modelling. The code was modified to include melting. Evaporation and vapour shielding at elevated temperatures improve the cooling efficiency and reduce the incident power (converting it to radiation, some of which is radiated elsewhere) and thus reduce the depth of melting. For Be armour 10 mm thick, slow plasma transients at 20 MW/m² after 10 s will induce melting to a depth of ∼2 mm without vapour shielding, and ∼0.15 mm with vapour shielding. For up to 60 mm of CFC the peak temperatures attained stay below sublimation; for up to 20 mm of W-3Re the surface temperatures stay below melting. The lifetime of the divertor plates is determined from the reduction of thickness due to these transients (10% of shots, 10 s), as well as from disruptions (10% of shots) and sputtering. When the maximum or initial armour thickness is given either by the maximum permitted surface temperature during steady state operation at 5 MW/m² (which is 1050 K for Be, permitting 11 mm armour thickness; 1780 K for carbon, permitting 40 mm of CFC after neutron irradiation), or in the case of W3Re by the requirement to stay below melting after 10 s at 20 MW/m² (permitting 20 mm), and assuming a minimum remaining thickness of 2 mm for all three armour materials, the predicted lifetimes without vapour shielding during transients are as follows: 120-225 shots for Be, 5840-8170 shots for CFC with moderate chemical sputtering, and 2370-7740 shots for W-3Re.