Tungsten Versus Steel in Inboard Shield of ITER: Impact on Magnet Damage, Reactor Size, and Cost

The shield design of ITER is required to meet both magnet protection requirements and safety-related criteria. Although the W provides excellent magnet protection, its high specific decay heat caused some concern in case of an accident. A trade study was carried out in which W is replaced by steel in the high neutron flux zones of the inboard shield and the sensitivity of the machine size, cost, and magnet damage to such change was determined. Satisfying the 10¹⁹ n/cm² fast fluence limit for the magnet, the direct cost is essentially the same for the steel and W shields, although the steel shield is 0.1 m thicker. The 0.55 m thick inboard shield of ITER is configured in 3 main layers: a 0.05 m Be layer, followed by a 0.18 m steel layer, then a 0.18 m W layer. Five coolant channels, each 0.01 m wide, are properly distributed across the shield. About 0.1 m thick layer of aqueous Li salt solution at the back of the shield was found necessary to minimize the damage in the magnet. This design meets the neutronics, safety, and thermal hydraulics requirements and there appears to be no feasible problems associated with it.