Its purpose was to convert part of its natural uranium fuel into plutonium-239 by neutron activation, for use in nuclear weapons.

Primarily constructed by DuPont, the operation was assisted by scientists including Enrico Fermi, John Archibald Wheeler, and Chien-Shiung Wu.

The plutonium from the site was used in the Trinity test, the Fat Man bomb detonated above Nagasaki, the demon core, and thousands of US warheads during the Cold War.

It is historically significant as the world's first large-scale reactor, the first to use water cooling, the first large enough to and to experience xenon poisoning, the first employed for thermonuclear weapon tritium production, and the seventh critical assembly in total.

Enriching fissionable 235U from 238U, the dominant natural isotope, was extremely complex; plutonium's distinct chemistry made separation trivial by comparison.

For example, the Y12 uranium enrichment plant in Tennessee required 14,700 tons of silver loaned by the Treasury Department for the windings in its calutrons, employed 22,000 people and consumed more electrical power than most states.

Reactor B on the other hand needed only a few dozen employees and far fewer exotic materials required in much smaller quantities.

The most important special material needed were the 1,200 tons of purified graphite for neutron moderation, and only enough electricity to run the cooling pumps.

It is penetrated horizontally through its entire length by 2,004 aluminum tubes containing fuel and vertically by channels housing the control rods.

[4] The core is surrounded by a thermal shield of cast iron 8 to 10 in (20 to 25 cm) thick weighing 1,000 short tons (910 t).

The bottom of the thermal shield was supported by a 23 ft-thick (7.0 m) concrete pad topped by cast-iron blocks.

Its coolant was pumped from the Hanford Reach of the Columbia River, through the aluminum tubes and around the uranium slugs at a rate of 75,000 US gal (280,000 L) per minute.

Each reactor had its own auxiliary facilities that included a river pump house, large storage and settling basins, a filtration plant, large motor-driven pumps for delivering water to the face of the pile, and facilities for emergency cooling in case of a power failure.

[6][14] As of 2014[update] six of the nine production reactors at Hanford were considered to be in "interim safe storage" status, and two more were to receive similar treatment.