The H Reactor was the first reactor to be built at Hanford after World War II. It became operational in October 1949 and was the fourth nuclear reactor on site. It operated for 15 years, ultimately shutting down in April 1965.
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.
In 2005, H Reactor became the fifth facility on the Site to be cocooned.