Nuclear reactor safety analysis is the vehicle to determine if the safety systems discussed in Chapter 3 would perform satisfactorily under all Anticipated Operational Occurrences (AOOs) or transients and Design Basis Accidents (DBAs). Historically, the U.S.A. has been the leader in nuclear safety analysis as a number of nuclear power plants for commercial electricity generation started to operate in the U.S.A. in the 1960s. Two PWRs (Yankee-Rowe and Indian Point Unit 1) and one BWR (Dresden Unit 1) each of net electrical output of around 200 MWe started operation in early 1960s. Larger U.S. nuclear power plants of net electrical output of 500 MWe or more (e.g., Haddam Neck (PWR), Oyster Creek Unit 1 (BWR)) started operation in late 1960s, after which many large nuclear power plants (∼1000 MWe) were ordered, heralding the modern era of nuclear power plant operation using the Generation II reactors described earlier. At that time, the U.S. Atomic Energy Commission (USAEC) had the dual role of promoting and regulating the U.S. nuclear power plants.