The performance of bulk W for ITER has been investigated using water-cooled divertor designs, that is, flat tile, macrobrush, and, most relevant, monoblock options. One important factor in these design solutions is the maximum allowable distance between the front surface and coolant to accommodate the heat without melting96 and, if possible, to avoid recrystallization during normal operating conditions. The ability to estimate this parameter requires not only the thermal conductivity of the materials but also the amount of allowable damage at the interface. This requires knowing not only the damage produced during
operation but also understanding the manufacturing accuracy and reproducibility because tens of thousands of armor/heat sink joints will be produced. Studies on this issue have shown that the current W monoblock design with a defect extension up to 50° appears to be suitable for the upper part of the vertical target (P = 10MWm~), but is not well adapted to a heat flux of 20 MW m~ , which is neces­sary for application at the strike point of the vertical target, as systematic defect propagation was observed. A tungsten flat tile design with 6-mm long defects in the material interface was studied and proved to be compatible with fluxes of 5 MW m~ but was unable to sustain cyclic fluxes of 10 MW m~2.212

These results confirm that the monoblock geome­try generally proves to have superior behavior under high heat flux testing when compared with flat tile geometry. However, it is worthwhile to continue the investigation of the flat W tile design for low-flux regions despite the hazard of cascade failure of the flat tiles106 for two reasons: cost and weight.

Besides this characterization, a number of high heat flux tests have been carried out on mock-ups and prototypes without artificial defects representing the different design options to assess the ‘fitness for pur­pose’ of the developed technologies.33’90’161’213-218 The results obtained for small test mock-ups of the flat-tile and monoblock design can be transferred to large-scale prototypes for the divertor vertical target. Independent of the type of pure W or W-La2O3 armor material used in these prototypes’ the W parts survived in the nonneutron-irradiated condition up to 1000 cycles at 20MWm~2 in the monoblock design21 ‘ and up to 1000 cycles at 18 MW m~2 in the flat tile
design (see Figure 7).213 This is far beyond the design requirements for use in the upper part of the vertical target (P = 5 MW m~ ) and, in case of the monoblock design, even meets the design requirements for the strike point area of the vertical target.

Alternative concepts such as explosive bonding of tungsten to a heat sink material,2 PS on a

Cu-alloy216 or on EUROFER steel44 could probably be of use in the divertor but even more for first wall applications for fusion machines beyond ITER. How­ever, these concepts often suffer from high interfacial stresses as a result of the CTE difference between the W coating and the substrate.