In the fusion materials development strategy, the candidate structural materials are categorized into reference and advanced materials. As the reference

materials, RAFM steels were selected because they have the most matured industrial infrastructure. Development of the reference materials is crucial for the realization of DEMO (fusion demonstration power plant) in a timely manner. On the other hand, vanadium alloys and SiC/SiC were nominated as

the advanced materials, which will contribute to increasing attractiveness of the fusion system in terms of cost of electricity and environmental benign­ness. It is recognized that the development of the advanced materials must also be enhanced now due to the long lead time necessary for their development. It should also be noted that vanadium alloys are the only nonferromagnetic and ductile materials of the three candidates. If the impact of the ferromagnetism ofthe RAFM on plasma operation should be unavoid­able and the brittleness of SiC/SiC should be deter­mined unaccepted by design studies, vanadium alloys could be the only candidate of low activation structural materials for fusion reactors.

As shown in the summary of critical issues, a 14MeV neutron source is highly necessary for the qualification of vanadium alloys. IFMIF (Interna­tional Fusion Materials Irradiation Test Facility, a 14 MeV neutron source) is under design and is recog­nized to be essential for developing structural materials for fusion reactors. The TBM to be installed in ITER is also considered to be an important milestone for technological integration. Figure 24 shows the design of the V/Li TBM in ITER proposed by Russia.50 The development of vanadium alloys is planned to pro­ceed with IFMIF for qualification of the alloy and ITER-TBM for technology integration, in addition to fundamental studies using fission reactors, etc.

4.12.9 Summary

As to the application in nuclear systems, vanadium alloys were once candidate cladding materials for LMFBR, but, at present, are considered mostly as candidate low activation structural materials for fusion reactors.

Vanadium alloys, with the reference composition of V-4Cr-4Ti, are one of the three candidate low activation structural materials with RAFM and SiC/SiC. They are the only nonferromagnetic and ductile materials of the three candidates and thus are promising for advanced structural materials of fusion reactors. The self-cooled liquid lithium blanket using structural materials of vanadium alloys is an attractive concept because of the high heat transfer capability, high-temperature operation, simple structure, high tri­tium breeding capability, and low tritium leakage.

Recent progress, especially in the fabrication technologies, has successfully reduced the number of critical issues enhancing the feasibility of the alloys for fusion application. Major remaining issues of vanadium alloys are thermal and irradiation creep, transmutant helium effects on mechanical properties, and radiation effects on fracture properties. For con­clusive characterization of the helium effects, the use of IFMIF is essential.

Efforts are also being made to develop advanced vanadium alloys to extend the temperature window and lifetime of vanadium alloys in fusion reactor environments.