 Weinberg replaced Wigner as Director of Research at ORNL in 1948 , and became director of the laboratory in 1955 . Under his direction it worked on the Aircraft Nuclear Propulsion program , and pioneered many innovative reactor designs , including the pressurized water reactors ( PWRs ) and boiling water reactors ( BWRs ) , which have since become the dominant reactor types in commercial nuclear power plants , and Aqueous Homogeneous Reactor designs . 
