Small reactors and the modular construction of reactors are not new. Historically, early reactors for commercial production of electricity were of small size, a consequence of the prudent engineering process of constructing plants starting at small ratings to gain the needed construction and operating experience necessary to move confidently to larger ratings. Now, after a half-century of experience, commercial civil reactors are being deployed with ratings up to 1660 MWe. Additionally, small units were built for terrestrial deployment to provide electric power for remote, vulnerable military sites; for ocean deployment for propulsion of submarines, naval and commercial ships and for aircraft propulsion. Modular construction techniques historically have also been used for serial production of selected products. However, what is new is the vision of small rated power reactors composed of a single or multiple modules sized to markets of small — or large-sized electric grids, thereby creating new nuclear generating sites which require significantly reduced capital investments and capital investment rates. The further economic premise is that electric generation cost can be made sufficiently comparable to that of existing large-sized plants by employing a strategy of economy of numbers (manufacture of multiple identical modules) and simplification of design versus the traditional economy of scale.