In addition to normal structural considerations, it is necessary to evaluate neutron-induced radiation damage and radioactivation effects in the selection of both structural components and coolants. As discussed previously, radiation damage occurs by atom displacement and by nuclear transmutation involving primarily those producing 4He; as expected the damage is most severe in the first — wall and associated structures on the side facing the fusion plasma. Atomic displacement rates and gas production rates are summarized in Table 13.3 for various materials placed in a neutron flux typical of the first-wall in a tokamak with a 1 MWm’2 neutron wall loading. The displacement rate is not strongly dependent on the type of material whereas the gas production rate is very sensitive to material choices. Nickel bearing alloys generally have a large ratio of gas-production and displacement rate. Lithium also possess a significant gas — production capacity but if this occurs in the liquid, pressure buildup and swelling are not a problem as it can be in solids. Neutron-induced transmutations in blanket materials also result in radioactivation which is most important with respect to reactor maintenance, and storage of reactor components. The level of radioactivation, along with other radioactivity aspects such as the tritium inventory, will be a key factor in determining the environmental impact of fusion reactors.

An illustration of the residual radioactivity of selected materials after a 2-year exposure is shown in Fig.13.10. The large variation in radioactive level and its effects with time for the various materials is a notable characteristic that must be considered. Inertial confinement fusion blanket designs using a thick liquid-metal

first-wall have a built-in advantage relative to minimizing activation of the chamber wall and structure. The falling liquid can reduce the neutron flux hitting this structure so that radioactivity levels are lowered by an order of magnitude or more relative to a dry wall.

Problems

13.1 Consider a neutron wall loading limit of 5 MWm’2 in a torus configuration with minor radius a = 2 m. If the plasma fuel ion densities are given by Na(r) = Nt(r) = N(r) of Eq. (6.54) where N(0) = 1020 m"3, what is the highest plasma temperature allowed?

13.2 Using the sputtering data for D+ at 100 eV bombarding Fe in Fig.13.6, evaluate К in Eq.(13.13). Sketch the sputtering-curve predicted by this equation, and discuss any differences with Fig.13.6. Compute the time it would require for a flux of 106 deuteronscm’^s’1 at 100 eV to sputter away 10% of the thickness of a 1 cm iron wall.

13.3 Consider a deuterium plasma at Te = 10 keV containing 1% oxygen. Estimate the emitted radiation power using a weighted sum of the powers from the individual species.

13.4 Estimate the percentage of iron impurity in a d-t plasma that would cause the ideal ignition temperature to double.

13.5 Evaluate impurity effects on the Lawson criterion.

13.6 Consider a 50:50% MCF device having Tj ~ Te, a plasma beta value of 0.2, |/ ~ 10’3 and a doubly charged-ion impurity concentration of 1% of N,. What is the modified ignition temperature (see problem 5.9)?