M. Grattarola

Joining of C/C composites to copper

Abstract High heat flux components in International Thermonuclear Experimental Reactor (ITER) are designed as layered structures. In particular, in the heat sink, the assembling of the different parts foresees several joints between C/C composites and Cu alloy. In this paper, two methods to join silicon doped C/C (CFC NS31) to pure copper are described. One method concerns the use of a commercial brazing alloy (70Ti–15Cu–15Ni). The brazing process was optimised and the shear strength of the joined samples resulted to be comparable to the interlaminar shear strength of the C/C composite. The second technique is based on the casting of copper on C/C. The C/C surface was modified by direct reaction with a transition metal. The obtained modified C/C resulted very well wettable by molten copper. The morphological analysis of the C/C–Cu samples was performed.

Manufacturing and testing of a prototypical divertor vertical target for ITER

Abstract After an extensive R&D activity, a medium-scale divertor vertical target prototype has been manufactured by the EU Home Team. This component contains all the main features of the corresponding ITER divertor design and consists of two units with one cooling channel each, assembled together and having an overall length and width of about 600 and 50 mm, respectively. The upper part of the prototype has a tungsten macro-brush armour, whereas the lower part is covered by CFC monoblocks. A number of joining techniques were required to manufacture this component as well as an appreciable effort in the development of suitable non-destructive testing methods. The component was high heat flux tested in FE200 electron beam facility at Le Creusot, France. It endured 100 cycles at 5 MW/m 2 , 1000 cycles at 10 MW/m 2 and more then 1000 cycles at 15–20 MW/m 2 . The final critical heat flux test reached a value in excess of 30 MW/m 2 .

Infrared characterization and high heat flux testing of plasma sprayed layers

Abstract Four series of plasma sprayed actively cooled mock-ups have been evaluated by infrared measurements and heat flux testing. Infrared characterization showed heat transfer capability of the plasma sprayed layer bonded to the substrate. Even if the thermal conductivity of the B 4 C plasma sprayed coating is only 5% of the bulk material, the coating can easily survive 1000 cycles at 7.5 MW/m 2 if the thickness is less than 150 μm. Thick tungsten coatings (3–5 mm) were more fragile, depending on the plasma spray technology. The highest heat flux acceptable for 1000 cycles is 4 MW/m 2 with a vacuum plasma spray coating and a Ni–Al–Si–W precoating, accounting for a reduction in the thermal conductivity by a factor of 3.

Brazing Technology for Plasma Facing Components in Nuclear Fusion Applications Using Low and Graded CTE Interlayers

In Plasma Facing Components (PFCs) for nuclear fusion reactors, the protective material, carbon based or tungsten, has to be joined to the copper alloy heat sink for optimum heat transfer. High temperature vacuum brazing is a possible joining process as long as a proper interlayer is introduced to mitigate the residual stresses due to the mismatch of thermal expansion coefficient (CTE). Pure copper can act as plastic compliant layer, however for carbon based materials a proper structuring of the joining surface is necessary to meet the thermal fatigue lifetime requirements. In this work pure molybdenum and tungsten/copper Metal Matrix Composites (W-wires in Cu-matrix) interlayers have been studied as alternative to pure copper for carbon based protective materials in flat tile configuration. Finite element simulations of the brazing process have been performed to evaluate the expected residual stress reduction near the metal-carbon interface. In fact it has been demonstrated that stiff low CTE interlayers can shift the peak stresses from the weak carbon-metal interface to the strongest metal-metal one. Relevant samples have been manufactured and subjected to preliminary metallographic and thermal shock tests. Results obtained so far are encouraging and active cooled mock-ups are being prepared for high heat flux testing. Research work is in progress as regards monoblock configuration with both Wf/Cu MMC and graded Cu/W plasma sprayed and HIPped layers.

Residual Stress Measurements in Mo/CuCrZr Tiles Using Neutron Diffraction

In plasma facing components (PFC) for nuclear fusion reactors tungsten or carbon based tiles need to be cooled through a heat sink. The joint between the PFC and the heat sink can be realized using a brazing process through the employment of compliant layer of either a low yield material, like copper, or a high yield material, like molybdenum. Experimental verification of the induced stresses during the brazing process is of vital importance. Strains and residual stresses have been measured in Mo/CuCrZr brazed tiles using neutron diffraction. The strains and stresses were measured in Mo tile along the weld direction and at different distances from it. The experimental results are compared with Finite Element Simulations.