The erosion mechanisms that affect the erosion/ damage of the first wall in ITER are (1) sputtering erosion by D-T ions and charge-exchange neutrals

during normal operation; and (2) evaporation and loss of melt layers during off-normal transient events such as thermal quench disruptions, ELMs, VDEs, and runaway electrons impact. There are additional localized erosion phenomena such as arcing, over­heating with evaporation, and, possibly, loss of melt layer on exposed edges, but it is very difficult to make predictions of these effects for ITER. Special design attention has been given to avoid the misalignments of PFCs and avoid thermal overloading with possible localized damage.

4.19.6.2.2.1 Erosion of Be wall during normal operation

Calculations have been done to compute erosion of the first wall (due to fuel charge-exchange neutrals and ions, and impurity ions).205,206 It was found that about 20-40 g of Be per 400 s discharge are eroded from the wall with a beryllium peak erosion rate of the order of 0.1 nms~ . These predictions are con­firmed by extrapolation of experimental data from JET.1 0 This erosion rate would be acceptable from a component lifetime standpoint, especially during the low duty-factor operation of ITER. However, the total amount of eroded material may be significant. This material will most likely go to the divertor, and this will affect the composition of the divertor sur­face; therefore, it will affect the divertor performance
and contribute to tritium codeposition and dust inventories. Modeling of the influx of the eroded beryllium on the divertor is in progress to extrapolate from present machines and, in particular, to account for effects arising from material mixing including codeposition as expected in ITER. Several studies have been recently published on this subject (see, e. g., Kirschner et a/.207,208).