A main issue for the future of nuclear power plant operation is capital cost of new build. One way to achieve competitive economics is to increase the unit power rating (Oka, 1999), taking advantage of economies of scale. This approach has been adopted by many of the evolutionary water reactors’ designers. The power ratings of some of the proposed evolutionary LWRs may be as high as 1500-1700 MWe, many exceed 1300 MWe. Examples are given in Chapter 10. Large evolutionary designs of LWRs incorporating both proven active safety protection systems and more recently large plants putting more emphasis on passive safety systems, e. g. AP1000 are being proposed.