The Association EURATOM-ENEA (Frascati, Italy), in collaboration with the industrial company CSM S.p.A. (Rome, Italy), has recently completed the manufacturing of a medium scale prototype of the ITER divertor vertical target. This achievement culminates several years of R&D efforts aimed at developing alternative manufacturing technologies for the high heat flux components.

The prototype includes a high heat flux unit and a steel supporting structure. The high heat flux unit is based on the “monoblock” concept, which consists of tiles with a drilled hole. Then, a cooling tube is inserted into these holes and intimately joined to the tiles. The component has a straight and a curved part covered with carbon fibre reinforced carbon (CFC, grade NB31 supplied by SNECMA Propulsion Solide, France) and tungsten (W), respectively. The holes in the CFC and W tiles are lined by a 1 mm thick pure copper interlayer. The aim of this interlayer is to reduce the joint interface stress between the CFC or W material and the cooling tube. The pure copper is joined into the CFC tiles by an ENEA proprietary method named “Prebrazed Casting”, whereas it is joined into the W tiles by simple casting. The cooling tube is made of copper alloy (grade CuCrZr). The high heat flux unit is mounted onto the supporting structure, made of 316L austenitic steel, by means of pins made of “aluminum- bronze”. The pins are inserted in pads obtained in the rear side of the monoblocks and in grooves obtained in the supporting structure.The grooves allow the sliding of the CFC part thus alleviating the thermal stress during the operation under high heat flux loading. The joining of the cooling tube into the monoblock holes is obtained by means of the innovative technique named “hot radial pressing” (HRP). This technology consists in heating the small vacuum chamber that contains the high heat flux unit by an air furnace and then applying an internal pressure to the cooling tube at the prescribed temperature and time to get the joining between the tube and the pure copper interlayer.

With respect to the wellknown “Hot Isostatic Pressing “ (HIP) technology, the “Hot Radial Pressing” (HRP) process has no need of a HIP can. This feature has the following advantages:
• The manufacturing of the can is avoided
• The machining of the can after the HIP process is avoided
• One of the most common failure mode of the HIP technology, that is the failure of the can, cannot occur
• The joining process is carried out in an active vacuum rather then in a sealed can. Therefore in case of outgassing of the material during the temperature cycle, the required vacuum conditions are kept
• The required ~0.5 mm gap between each armour tile can be obtained in a much easier way since no axial forces are applied
• The distortion of the component after the joining process is minimised

The basic feasibility of the HRP technique was already demonstrated in the past by the Fusion Technology Section of ENEA Frascati via the manufacturing of small-scale mockups with W and CFC armour, which have been successfully tested in excess of 20 MW/m2. These encouraging results enabled the manufacturing of this component, which includes all the main features of the corresponding ITER divertor design. “This is a major achievement”, said M. Merola, Divertor Responsible Officer at EFDA CSU Garching, “and demonstrates that EU has various technologies, which are available for the manufacturing of the ITER divertor”. This prototype will be subject to an extensive high heat flux testing campaign at FE200 electron beam test facility located at Le Creusot, France, and operated by AREVA – Framatome ANP and CEA Cadarache.

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