The first pile at Hanford generated 250 million watts—250 megawatts or MW—of thermal power and produced each year about a hundred kilograms of plutonium. A rule of thumb is that a megawatt of fission heat in a natural uranium reactor accompanies the production of about a gram of plutonium-239 per day. About six kilograms were sufficient to make a bomb.
Garwin, Richard L. and Charpak, Georges. Megawatts and Megatons: The Future of Nuclear Power and Nuclear Weapons. University of Chicago Press, 2002. p. 33