Full W ITER: Assessment of expected W erosion and implications of boronization on fuel retention

Substituting Beryllium (Be) with Tungsten (W) as material for the first wall gives rise to new challenges for the ITER project. The additional W at the main chamber could result in a significant influx of high-Z impurities into the plasma owing to the higher particle energies expected at the main chamber wall compared to the divertor. Removing the impurity gettering ability of Be, may require the use of boronization to reduce the partial pressure of low-Z impurities like water and Oxygen, however this potentially introduces the caveat of increased T-retention by co-deposition with Boron (B). Based on existing background plasma solutions with Ne as main radiating impurity the erosion and migration of W and B are modeled using the WallDYN approach. The results on W erosion are strongly affected by transport assumptions and the erosion rates are dominated by Ne- and W-self-sputtering by highly charged W and Ne ions whereas erosion by charge exchange D neutrals is negligible in comparison. The B migration calculations allow to estimate the lifetime of boronization layers which is very short in the heavily loaded plasma wetted regions but can be up to $>10^{4}s$ in remote areas where it is only limited by D-CX erosion. Test simulations of a B-dropper during diverted plasma operation suggest that it mainly deposits B in the heavily loaded areas and thus is not effective in replenishing boronization layers in remote areas which are meant for impurity gettering. In summary this work suggests that based on existing data, a transition to W does not hamper the ITER project from the PWI perspective. However, it also shows that large uncertainties remain with respect to the W-plasma transport, boronization layer sputter yields and T/B ratios for retention calculations.