 Weinberg had the Materials Testing Reactor converted into a mock @-@ up of a real reactor called the Low Intensity Test Reactor ( <unk> ) or " Poor Man 's Pile " . Experiments at the <unk> led to the design of both pressurized water reactors ( PWRs ) and boiling water reactors ( BWRs ) , which have since become the dominant reactor types in commercial nuclear power plants . Weinberg was attracted to the simplicity and self @-@ controlling features of nuclear reactors that used fluid fuels , such as Harold Urey and Eugene Wigner 's proposed Aqueous Homogeneous Reactor . Therefore , to support the Nuclear Aircraft project in the late 1940s , Weinberg asked ORNL 's reactor engineers to design a reactor using liquid instead of solid fuel . 
