In this chapter, the materials for the blanket region have been reviewed, and their permeation para­meters described. In this section, the need for barriers is evaluated. For example, consider the tritium migra­tion processes that might be associated with the liq­uid Pb-17Li systems. In a set of experiments, Maeda et at231 found the solubility of hydrogen in Pb-17Li to be on the order of 10-7 Pa-1/2 atom fraction (^10-6 mol H2 m-3 MPa-1/2). As tritium is pro­duced in the blanket, some of the tritium will be in solution and some will be in the vapor phase. It is the tritium in the vapor phase that will drive the perme­ation through the metal used to contain the liquid. For an 800 MW fusion reactor, Maeda et al. state that 1.5 MCi (150 g) of tritium will have to be bred each day. That means that 1.5 MCi of tritium will be flowing around in stainless steel or similar metal tubes at a temperature >600 K. To estimate tritium permeation in a generic 800 MW plant, we will scale the design parameters proposed by Farabolini eta/.232 for a much larger plant. Approximately 10 000 m2 of surface area will be needed for the tubes passing through the liquid Pb-17Li to extract the heat. We will assume that a sufficient number of detritia — tion cycles per day are performed to keep the amount of tritium in the liquid breeder at 10% of the 150 g listed above. Scaling to 800 MW, the amount of Pb-17Li will be ~-750 000 kg. This leads to a molar fraction of tritium equal to 6.7 x 10-7. Using the solubility of Maeda et a/.231 for Pb-17Li yields a tritium pressure of ^45 Pa. Assuming the contain­ment metal to be 1 mm of MANET with an alumi­nized coating, a temperature of 700 K and an effective PRF of 1000, a permeation rate of 2.7 x 10-10 T2 mols m-2 s-1 will occur.29,118,122,127,128 With the 10 000 m2 surface area, the daily permeation rate is 0.23 mol or 1.4g of tritium per day. To prevent subsequent permeation through the steam generator tube walls, a tritium clean up unit will have to be applied to this helium loop. Because the steam gen­erator tube wall must be thin to permit effective heat transfer, the tritium cleanup loop will have to be extremely effective to limit release of tritium to the environment. This calculation was performed simply to show the extreme need for barriers in the blanket region of fusion reactors. Even with an active detritiation unit and a barrier providing a PRF of 1000, 1.4 g or 14 000 Ci of tritium end up in the cooling system each day. The situation is not much better for the solid breeders. The same amount of tritium will obviously be required for that system. To minimize the tritium inventory in the ceramic breeder materials, temperatures equal to or greater than that of the liquid breeder will be maintained. The tritium will be released into the helium coolant as elemental tritium (T2) and tritiated water (T2O); the relative concentrations of these forms depend on the type of ceramic breeder. The steel or similar containment metal will be exposed to nontrivial pres­sures of tritium gas. We can conclude that effective barriers are needed for the blanket. It is difficult to imagine that, even with double-walled designs, fusion reactor facilities can meet radioactive release requirements for tritium without an effective barrier.

4.16.4 Summary

In this chapter, we have presented tritium permeation characteristics and parameters for materials used in fusion reactors. These materials have included those used to face the plasma in the main chamber as well as materials used as structural materials for the main chamber and blanket. A description of the conditions that exist in those locations has also been provided. Reasons were given why direct contact of the plasma with the plasma facing materials would not lead to sizeable quantities of tritium being lost to the envi­ronment or to the cooling system. The same was not concluded for the blanket region. The need for per­meation barriers there was stressed. A number of materials were listed as possible tritium barriers. These materials included a few metals with some­what reduced permeation and a larger number of ceramics with very low tritium permeability. Due to the difficulty of lining large chambers with bulk ceramics, much of the tritium permeation barrier development around the world has been dedicated to thin ceramic layers on metal surfaces. Unfortu­nately, radiation testing219-222 of these materials has shown that these thin layers lose their ability to limit tritium permeation during exposure to radiation damage. It was suggested, but not proved, that this increase in permeation was due to cracking of the ceramics or the increase in defects.

To make this chapter more useful to the reader with a need for permeation data, tables and plots of the permeation coefficients are provided. The coefficients for metals are presented in Table 1 and Figure 21 and for the ceramics in Table 2 and Figure 22.

In summary, effective permeation barriers are needed for fusion reactors to prevent the release of sizeable quantities of tritium. Fusion is touted as a clean form of energy, and releasing tritium into the environment will eliminate any political advantage that fusion has over fission. Research is needed to find ways to place radiation-resistant ceramic permeation barriers on top of structural metals. The fusion com­munity must find a way to make this happen.