At the present time, the ITER first wall and shielding blanket is still undergoing a major redesign to over­come some of the main design shortcomings that were identified in the context of design review
conducted in 2007; for example, the thermal load requirements were updated, in light of experimental experience.16 Most important in this respect was the recognition that the upper X-point region would see much higher loads during burn than 0.5 MW m~2; long transients (approximately up to 5-10 s) of plasma contact with the wall would have to be with­stood. In addition, NB shine-through at low densities would necessitate high heat flux first-wall protection, and a new requirement has been introduced to pro­vide remote maintainability of the first-wall panel to be done in situ and independently of the shield mod­ule (which would also have to be maintainable). The rationale for the ongoing effort is described by Lowry et a/.156 Proposed design modifications are being developed while trying to avoid and minimize changes to other components which are on the criti­cal fabrication path, especially the vacuum vessel, which is under fabrication.

The main features of the proposed design are the following: (1) to abandon the port-limiters and to exploit the first wall for plasma startup by relying on more benign plasma start-up scenarios, including an early X-point formation; (2) to use suitably shaped plasma-facing surfaces to hide edges such that there is no illumination of component surfaces by

Support pins

Halo current path
through the tile

Halo current from plasma

Figure 18 Inner wall guard limiter tile (exploded view, top, and prototype, bottom). The five castellated Be slices have interslice and outer slice internal toroidal edges ski-slope shadowed. The slices are held on an inconel carrier by pins which allow bowing under thermal load. The RH bolts are designed to be shadowed by the next installed tile. Reproduced with permission from Riccardo, V. J. Nucl. Mater. 2009, 390-391, 895-899.

normal or near-normal field lines, emanating in the near SOL; (3) provide power load capability of 4-5 MW m~2, in order to be able to use the first wall as a limiter for startup and termination; and (4) with­standing transients is still subject to discussion. In particular, it must be noted that the vulnerability to damage induced by thermal transients is recognized and linked to the feasibility and efficiency of all processes required for full remote maintenance of the first-wall panels, which is yet to be demonstrated.

In practical terms, the approach adopted is to pro­vide a shadowed poloidal band in the center of the first-wall panel, the two sides being shaped in a form typical for limiters both to provide the shadowing of the band and to ensure that the toroidally facing edge of the first-wall panel is shadowed. Because of the port regions on the low-field side, which contain a variety of structures with varying power handling capabilities, and because the toroidal field ripple is variable with the toroidal field, it is not possible to exploit the entire
wall surface in this location. For this reason, the first — wall panels on the low-field side have the poloidal bands between the ports advanced with respect to those in line with the ports (see Figure 19). The amount of set back required at the edges of the first wall is determined by the penetration angle ofthe field lines and the power scrape-offlength, with the optimi­zation taking into account the differing power handling capability of the front face and the edge of the first wall.

Considerations discussed here are limited to some problems associated with the design of the beryllium tiles and prediction of PWI effects during operation in ITER.

An important design driver for the first wall in the past was the specification of the thermal load during off-normal transient events.3 In particular, the thick­ness of the beryllium tiles had to be such as to prevent overheating of the joints and possible damage of the coolant pipes (see Section 4.19.6.2.2). Also, the thickness of the tile determines the temperature

Be wall

CFC strikepoints W elsewhere

(a)

Figure 19 (a) View of the low field side first-wall surface showing how the panels in line with the port openings are recessed with respect to those between. It also shows the shielded central section of the panels allowing for access to the mechanical and hydraulic connections. Reproduced from Hawryluk, R. J.; etal. Nucl. Fusion 2009, 49, 15, 065012; with permission from IAEA. (b) Allocation of armor materials. Reproduced from Hawryluk, R. J.; etal. Nucl. Fusion 2009, 49, 15, 065012; Federici, G.; Loarte, A.; Strohmayer, G. Plasma Phys. Contr. Fusion 2003, 45, 1523-1547, with permission from IOPP.

gradient and the thermal stress under a prescribed thermal load during steady-state. Limits on the tile temperature during operation arise as a result of many processes including melting, excessive vapori­zation, thermal fatigue, reduced mechanical integrity, and chemical reactions during accidental exposure of armor or structure to air or steam. The last one of the above processes is important as explosion of hydro­gen liberated from the steam-Be reaction is a major concern. In the past, a tile thickness of 10 mm was adopted. This corresponded to a Be maximum tem­perature limit of ^650-750 °C, roughly the level at which the relevant Be material properties (including mechanical, embrittlement, thermal fatigue, and swelling effects) start to degrade considerably.

Because of the differences in the product of the elastic modulus and the coefficient of thermal expan­sion (E) between beryllium and copper or copper alloys (EBe/EcucrZr = 2.4), large thermal stresses are set up around the bond between the beryllium tile and copper allot heat-sink.

The difficulty to successfully join low thermal expansion armor materials such as beryllium and tung­sten to high thermal expansion heat sink materials has been a major problem and has been discussed in

Section 4.19.5. Thermomechanical modeling has shown the desirability of using very small tiles of brush like structure for PFC armor because of the reduction ofthe stress at the armor-heat sink interface. The proper selection of the size of beryllium tile is an important issue which impacts all aspects of compo­nent manufacturing such as increased cost of machin­ing, nondestructive examination features, reliability and repair of unbonded tiles, etc.

In general, the fatigue life issue is difficult to quantify because of a number of factors. The thermal stresses depend on the temperature profile and the degree of constraint in the tiles. Tile castellations must be introduced to further relieve the constraints, and these have been sized following an extensive program of coupled thermal and mechanical analyses using finite elements codes such as ANSYS185 and ABAQUS.186