EFDA-JET-CP(09)02/06

Clamping of Solid Tungsten Components for the Bulk W Divertor Row in JET – Precautionary Design for a Brittle Material

For the bulk tungsten divertor row, a development for the ITER-like Wall in JET, utmost care was taken to limit the stresses in the solid tungsten plasma-facing components. The bulk W tile is located at the position of the outer strike point for most plasma configurations as they are considered for the ILW operation. The absence of active cooling makes temperature cycling of the refractory tungsten material close to or through the DBTT (ductile-to-brittle transition temperature, about 200-300oC) and above the re-crystallisation threshold around 1250oC hardly avoidable. Each tile is segmented in 4 stacks of 24 solid tungsten lamellae. The clamping scheme minimises the vertical forces applied on the lamellae and provides a compressive pre-load for the integrity of the stack, corresponding to uniform pressures below 6.0N/mm2. The higher loads were deliberately moved to the supporting structure : a wedge-shaped carrier, the clamping, and an adaptor to the base plate of the torus. The thermo-mechanical analysis of the lamellae gives a picture of the distribution of temperature and stresses for nominal exposures 7MW/m2 (10s). The possible onset of the DBT and of recrystallisation were the main reasons for selecting the design with due consideration of the mechanical properties of the material.
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