The Hanford Site was built in Richland, Washington by the Manhattan Project to produce plutonium.
Plutonium is a radioactive element that can be produced by bombarding uranium with neutrons.
Until November 1942, the Manhattan Project leaders had assumed that the plutonium operation would be located at the Clinton Engineer Works in Oak Ridge, Tennessee. However, they decided it would be safer to build the operation in a more isolated area where an unforeseen accident wouldn't threaten a city or another Manhattan Project plant. This site offered flat terrain, substantial hydroelectric power, isolation, and existing transportation facilities. During the summer of 1943, thousands of workers poured into Hanford, its population growing to 50,000 by the summer of 1944. The first reactor began operation in September 1944.