Development and Testing of High Heat Flux Components for ITER

M. Merola (Sp), M. Pick, EFDA - European Fusion Development Agreement, Garching (Germany) 
 
The ITER machine is an international effort aimed at demonstrating the scientific and technological feasibility of fusion energy. In the context of the European strategic approach to fusion energy production, ITER represents the “Next Step” after JET.
One of the most technically challenging components of the ITER machine is the divertor, the main function of which is to extract the power conducted in the scrape-off layer whilst maintaining the plasma purity. It includes the cassette body (CB) and three plasma-facing components (PFCs), namely the inner and outer vertical target (VT), and the dome liner (DL).

The VTs must be designed to withstand heat fluxes up to 10 MW/m2 for more than 3000 cycles and up to 20 MW/m2 for a few hundred cycles. In addition, pulsed heat depositions of about 1 MJ/m2 with a frequency of about 1 Hz must also be tolerated (the so-called ELMs). During plasma off-normal events (about 10% of the discharges) energy densities of several tens of MJ/m2 are deposited onto the divertor surfaces causing melting and evaporation or sublimation of the plasma-facing materials. Material damage up to a few tenths of displacements per atom (dpa) due to neutron irradiation will occur during the tritium operation of ITER. Finally, the electromagnetic forces, which will act on the divertor during plasma disruptions and vertical displacement events (VDEs), pose severe constraints on the design of the structural parts of the divertor components.

The EU Participating Team of the ITER project has significantly contributed to the design, development and testing of the divertor components.
The power handling parts of the upper part of the VTs and the dome are to be made of tungsten (W). The main design option of the dome involves W tiles, onto which a thin pure copper interlayer is deposited by casting, and which are then bonded to precipitation hardened copper alloy (CuCrZr) bars, which act as heat sink. The use of W tiles, in the form of monoblocks, is under consideration for the vertical target. The W armour thickness is typically 8 mm. The application of a few millimetres thick layer of W by plasma spray is foreseen to protect the liner, located beneath the dome, as well as the side surfaces of the dome, not directly exposed to the plasma.
Carbon fibre reinforced carbon (CFC) is the reference design solution for the lower part of the VTs. The reference design option foresees CFC monoblocks attached to a CuCrZr tube (12/15 mm ID/OD) through a thin pure copper interlayer to reduce the joint interface stress. The CFC armour thickness is typically 18 mm.

After an overview of the ITER design requirements, this paper presents the results of the R&D program carried out by the EU on the development of the joining technologies, including non-destructive inspection, of the high heat flux components.
The large amount of R&D effort has resulted in the development of suitable technologies, which meet or exceed the design requirements and form a solid basis for the construction of the ITER machine.