Various tritium breeding fusion blanket concepts have been studied with different combinations of structural materials, tritium breeding materials, and cooling materials. Vanadium alloys have been used in most cases with liquid lithium as the breeding and cooling materials (self-cooled V/Li blankets) for advanced concepts of DEMO (fusion demonstra­tion power plant) and commercial fusion reactors.7,8 Because of high atomic density of Li atoms in liquid Li relative to Li-ceramics, Li-Pb, and molten-salt

Flibe, V/Li systems can obtain high tritium breeding ratio (TBR) without using the neutron multiplier Be. A neutronics calculation showed that ‘tritium self sufficiency’ can be satisfied without Be both in Tokamak and Helical reactor systems.9 Without the necessity of using beryllium as a neutron multiplier, the replacement frequency of the blanket will be reduced because the blanket system is free from the periodic replacement due to the lifetime of Be, which can lead to enhanced plant efficiency.

V/Li blankets can be designed with a simple structure as schematically shown in Figure 1. The blanket is composed of Li cooling channels made of vanadium alloys, reflectors, and a shielding area, which is in contrast to more complex solid breeder blankets that need a solid breeder zone, a neutron multiplier beryllium zone, cooling channels using gas or water, and tritium recovery gas flow channels in addition to reflectors and shielding.

A self-cooled Li blanket using neutron multiplier beryllium was also designed in the Russian pro­gram.10 This concept can downsize the blanket area because of efficient tritium generation per zone. However, the blanket structure must be more

Figure 1 Illustration of self-cooled Li blanket with V-4Cr-4Ti structural material.

complex than V/Li and new issues need to be solved such as Li/Be compatibility.

General requirements for structural materials of fusion blankets include dimensional stability, compat­ibility with breeder and coolants, high-temperature strength and low-temperature ductility during irradia­tion. For vanadium alloys, issues concerning industrial maturity such as developing large-scale manufacturing technology need to be resolved.

Vanadium alloys could be a candidate structural material for molten-salt Flibe (LiF-BeF2) blankets. For this application, a concept was proposed to dis­solve WF6 or MoF6 into Flibe for corrosion protec­tion of the wall surfaces by precipitation of W or Mo and reduction of the tritium inventory in vanadium alloys by enhancing reaction from T2 to TF, which is more highly soluble in Flibe than T2.11 The TBR of Flibe/V blankets may be marginal, but the neutron shielding capability for the superconductor magnet systems may be superior relative to V/Li according to neutronics investigation.12 In this system, precipitates of W or Mo formed as a result of reaction from T2 to TF needs to be recovered from the flowing Flibe.

Table 1 summarizes the blanket concepts using vanadium alloys with the advantages and critical issues.