1. Introduction of a Nuclear Fusion Reactor

Most of energy resources on the earth are originated from energy given by the sun in which all energy is produced by nuclear fusion reactions. To build a small sun or to realize controlled fusion as an energy source on the earth has been a dream of human being. Owing to extensive research and development, the fusion reaction of Deuterium (D) and Tritium (T) soon comes in burning phase. Nevertheless, to realize a D–T fusion reactor as an energy source, lots of engineering issues still remain to be solved. Among all, T-relating issues are quite important, because T is hazardous due to its radioactivity and its resources are quite limited. In this chapter, after the introduction of nuclear fusion reactions, issues relating T to establish the D–T reactor as an energy source are summarized.