Vanadium alloys potentially have low-induced acti­vation characteristics, high-temperature strength, and high thermal stress factors. For the optimization of the composition, both major alloying elements and minor impurities need to be controlled. For main­taining the low activation properties, use of Nb and Mo, which used to be the candidate alloying elements for application to LMFBR, need to be avoided.

Cr was known to increase the strength of vanadium at high temperature and Ti was known to enhance ductility of vanadium by absorbing interstitial impu­rities, mostly oxygen. However, excess Cr or Ti can

lead to loss of ductility. Hence, optimization of Cr and Ti levels for V—xCr-yTi has been investigated. It was known that with x + y > 10%, the alloys became brittle6 as shown in Figure 2. With systematic efforts, V-4Cr-4Ti has been regarded as the leading candi­date. For low activation purposes, the level of Nb, Mo, Ag, and Al needs to be strictly controlled.

Large and medium heats of V-4Cr-4Ti have been made in the United States, Japan, and Russia.

Figure 2 DBTT as a function of Cr + Ti (wt%) of V-Cr-Ti alloy for various annealing temperatures. Reproduced from Zinkle, S. J.; Matsui, H.; Smith, D. L.; Rowcliffe, A. L.; van Osch, E.; Abe, K.; Kazakov, V. A. J. Nucl. Mater. 1998, 258-263, 205-214, with permission from Elsevier.

An especially high-purity V-4Cr-4Ti ingot pro­duced by the National Institute for Fusion Science (NIFS) in collaboration with Japanese Universities (NIFS-HEAT-1 and 2) showed superior properties in manufacturing due to their reduced level of oxygen

4

impurities.

Figure 3 compares the contact dose rate after use in the first wall of a fusion commercial reactor for four reference alloys. The full-remote and full-hands-on recycle limits are shown to indicate the guideline for recycling and reuse.1 SS316LN-IG (the reference ITER structural material) will not reach the remote­recycling limit after cooling and hence the recycling is not feasible. F82H (reference RAFM steel) and NIFS-HEAT-2 behave similarly, but NIFS-HEAT-2 shows significantly lower dose rate before the 100- year cooling. The dose rate of F82H and NIFS- HEAT-2 reached a level almost two orders lower than the remote-recycle limit by cooling for 100 and 50 years, respectively. The dose rate of SiC/SiC com­posites (assumed to be free from impurities because of lack of reference composition) is much lower at <1year cooling, but slightly higher at >100 year cooling relative to F82H and NIFS-HEAT-2.