The H Reactor was the first reactor to be built at Hanford after World War II. It became operational in October of 1949, and represented the fourth nuclear reactor on the Site.
H Reactor was built as tensions grew between the United States and Russia, ultimately ushering in the Cold War. With the continued strain on relations between the two superpowers, H Reactor was followed by DR, C, K-West, and K-East Reactors all being built within a five and a half year period.
The reactor, located between the D and the F Reactors at Hanford, operated for fifteen years. It was shut down in April of 1965. In 2005, H Reactor became the fifth facility on the Site to be cocooned and represents the most recent cocooning of a Hanford nuclear reactor.