Abstract
The present chapter is thoroughly dedicated to nuclear power reactors to
understand its fundamental conceptions. It also envelops the ancient background and
constructed nuclear fission reactors, important components of nuclear reactors, thermal
reactors, heavy water, moderator reactors, graphite-gas moderator reactors, and
accelerator-driven subcritical reactors (ADSR). The Oklo ancient nuclear reactor is the
foundation of natural fission reactors.