The plutonium production process generates vast quantities of toxic and radioactive liquid waste products. The foundations for Hanford's current tank waste problems were laid during World War II, as the contractors and construction crews charged with building immense underground stainless steel storage tanks that would hold the chemical waste from the plutonium separation process experienced difficulties with welding. Fearing the tanks would not be ready when needed, the Army Corps of Engineers site managers reduced the welding inspection standards in order to reduce the amount of rewelding necessary and speed up tank construction. In addition, because stainless steel was in short supply, the stainless steel tanks were connected with cast iron pipes. Corrosion began immediately. (See Michelle Gerber's On the Home Front, 42-43).
One account of Hanford's tank form story is provided in this Department of Energy presentation.
This timeline traces pivotal incidents in the history of tank farms and tank vapors.