In the blanket, the tritium inventory is not considered to be the issue once liquid Li is used as the breeding and cooling materials owing to the high hydrogen solubility of Li. The behavior of hydrogen and its isotopes in vanadium alloys is a concern for tritium retention in the first wall. Deuterium retention of V-4Cr-4Ti was investigated by deuterium ion implantation followed by thermal desorption, in comparison with other candidate first wall materials. The study showed that the retained amount at 380 K was one and two orders of magnitude larger than graphite and tungsten, respectively. For the irra­diation at 773 K, the retained amount was almost the same as that of graphite and one order larger than tungsten.42 Surface composition was also known to influence the hydrogen transport. For example, the rate of absorption was highly influenced by prior heat treatment, inducing Ti surface segregation.43

Recent progress in detecting tritium by means of imaging plate (IP) enhanced the understanding of the tritium behavior in vanadium alloys. Figure 20 com­pares IP images of cold rolled V-4Cr-4Ti and pure V after tritium charging. Tritium is preferentially absorbed in Ti-rich precipitates that have a band structure to the rolling direction.44

Figure 20 Distribution of tritium measured by imaging plates for cold-rolled V-4Cr-4Ti and pure vanadium. The tritium was charged by gas absorption. After Homma, H.; Hatano, Y.; Daifuku, H.; et al. J. Nucl. Mater. 2007, 367-370, 887-891.