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National Museum of Nuclear Science & History

Atomic Glossary

History Page Type:
Wednesday, June 18, 2014
Manhattan Project Insignia. By Aaron Sauers, Argonne National Laboratory

Some of the terms encountered when perusing the website can be confusing or obscure. This glossary is provided to assist readers and learners of Manhattan Project history.

509th Composite Group

The 509th Composite Group, part of the 20th Air Force of the U.S. Army Air Force, was organized as the weapon delivery arm of the Manhattan Project. After its formation at Fairmont, NB it relocated to Wendover Army Air Field in Wendover, Utah where extensive training took place with the newly modified B-29 Superfortresses. In early 1945, the entire group moved to Tinian Island in the South Pacific for further training and to ready itself for the upcoming missions to Japan.

ALSOS Missions

The purpose of the Manhattan Project was to produce an atomic weapon before Germany could. Very little espionage activities were carried out by the Allies to determine Germany’s nuclear capabilities because to do so would reveal our hand. Therefore, it wasn’t until the Allies began making inroads into Europe that General Leslie Groves initiated the ALSOS missions.

The ALSOS missions were comprised of a team of military security personnel and scientists; their mission was to capture enemy scientists and raw materials such as uranium and, in the process, to determine how far Germany had progressed toward developing an atomic bomb. There were three ALSOS missions: ALSOS I into Italy, ALSOS II into France, and ALSOS III into Germany itself.

B Reactor

The B Reactor at Hanford was built and operated by du Pont and was the world’s first production-scale nuclear reactor. It was the main site of plutonium production for the Manhattan Project.


Bockscar was the name given to the B-29 Superfortress that dropped the second atomic bomb on the Japanese city of Nagasaki on August 9, 1945. It is often misspelled as Boxcar or Bock’s Car.

Box 1663

Box 1663, Santa Fe, New Mexico was the enigmatic address used by all residents of Los Alamos. It was required for all incoming and outgoing mail. In addition, all outgoing mail was read and censored by Manhattan Project security personnel.

British mission

The British mission to Los Alamos was a group of top-notch British physicists who had played a crucial role in encouraging America’s bomb effort and had been invited to Los Alamos to work on the project. Some members also rode along as “observers” on the two atomic bomb missions over Japan.

Clinton Engineer Works

One of the three major facilities of the Manhattan Engineer District (MED), the Clinton Engineer Works was the name given to the huge uranium enrichment facilities and the pilot plant for plutonium production. Located a short distance from Knoxville, Tennessee, it was renamed Oak Ridge after the war.

Corps of Engineers

The “Corps of Engineers” is the construction arm of the U.S. Army. During World War II, the Corps was responsible for all construction of military installations. Domestically, the Corps of Engineers is organized into many “districts” that encompass the continental U.S.

General Leslie R. Groves, who had been in charge of building the Pentagon in Washington, DC, was eventually put in charge of the effort to build an atomic bomb. Since he was affiliated with the Corps of Engineers office in New York City and since he was looking for a name that was not at all suggestive of its true purpose, he chose the name “Manhattan Project” and organized it under the auspices of the “Manhattan Engineer District” (MED) in New York City.

Enola Gay

The Enola Gay was the name given to the B-29 Superfortress that dropped the first atomic bomb on the Japanese city of Hiroshima on August 6, 1945. It was named after Col. Paul Tibbet’s mother, Enola Gay Tibbets.

“Fat Man”

“Fat Man” was the name given to the plutonium implosion-type atomic bomb and was the second bomb to be dropped on the Japanese city of Nagasaki. It replaced the inefficient gun-type bomb “Little Boy”.


Fission is the term given to the process of “splitting” atoms through their bombardment by neutrons. The term was first coined by the physicist Otto Frisch, nephew of Lise Meitner.


The “gadget” was the nickname of the first atomic bomb which was detonated at the Trinity test site outside Alamagordo, New Mexico on July 16, 1945. This plutonium implosion-type bomb was similar to the “Fat Man” bomb dropped on the Japanese city of Nagasaki.

Hanford Engineer Works

One of the three major facilities of the Manhattan Engineer District (MED), the Hanford Engineer Works was home to the giant reactor “piles” and the cavernous plutonium separation and concentration facilities. Built and operated by DuPont, it was one of the largest construction projects ever undertaken, employing at one time nearly 45,000 construction workers.

The “Hill”

Residents of Los Alamos referred to the military installation as “the Hill”, due to its 7,200-foot elevation.


The United States used the first atomic bomb in wartime against the city of Hiroshima, Japan on August 6th, 1945. The bomb dropped on Hiroshima was known as “Little Boy,” a uranium gun-type bomb that exploded with about 13 to 18 kilotons of force. At the time of the bombing, Hiroshima was home to 280,000-290,000 civilians as well as 43,000 soldiers. Between 90,000 and 166,000 people are believed to have died from the bomb in the two- to four-month period following the explosion.


The “implosion-type” atomic bomb utilizes an array of exterior explosive charges placed on a sphere that were focused inward toward the core. When detonated correctly, the charges compress a core of plutonium inward from the size of a grapefruit to that of a tennis ball, thereby creating a critical mass and inducing a nuclear explosion. This procedure was so complicated that it took the best minds of the Los Alamos explosives group many months to perfect it. The process was first suggested by physicist Seth Neddermeyer. Both the “gadget” tested at Trinity and the “Fat Man” bomb dropped on the Japanese city of Nagasaki were implosion-type weapons.


“Jumbo” was the name given to the large steel vessel which was originally designed to contain the first atomic explosion, but was never used.


The K-25 plant was one of four primary production facilities at Oak Ridge. Construction on this plant began on September 27, 1943 and Union Carbide was the contracted operator. Its mission was to provide enriched uranium utilizing the gaseous diffusion process.

“Little Boy”

“Little Boy” was the name given to the uranium gun-type atomic bomb and was the first to be dropped in wartime, exploding with about 13 to 18 kilotons of force on the Japanese city of Hiroshima. It was the first and last of this type of device and was replaced by the more efficient implosion-type plutonium bomb, “Fat Man”.

Mutually Assured Destruction (MAD)

Mutually Assured Destruction (MAD) was a doctrine of military strategy first articulated by US Secretary of Defense Robert McNamara to ensure stability in international relations during the Cold War arms race between the United States and Soviet Union. MAD was premised on the idea that nuclear parity between the US and USSR advanced, rather than compromised, national security. Since neither country possessed a first strike advantage over the other, as each retained a secure second strike capability, the doctrine deterred both from launching a preemptive strike.

Manhattan Project

The Manhattan Project was the code name for America’s atomic bomb development efforts during World War II. Its name originated from the fact that it was part of the U. S. Army Corps of Engineers and organized under the Manhattan Engineer District (MED) in New York City. The MED encompassed all of the far-reaching labs and installations scattered throughout the country.

MAUD Committee

In July of 1941, Vannevar Bush and James Conant, new head of the National Defense Research Committee, received a copy of a draft report from their liaison office in London. The report was prepared by a group codenamed the MAUD Committee and set up by the British in the spring of 1940 to study the possibility of developing a nuclear weapon. The report maintained that a sufficiently purified critical mass of uranium-235 could fission even with fast neutrons, i.e. that an atomic bomb was feasible.

Met Lab

The “Met Lab” was short for the Metallurgical Laboratory at the University of Chicago. Much of the theoretical and experimental work on uranium and plutonium took place here. Directed by Nobel laureate Arthur Holly Compton, it was home to many of the foremost physicists and chemists of the time. It was here that Nobel laureate Enrico Fermi first achieved a sustained chain reaction on December 2, 1942.


Three days after Hiroshima, on August 9th, 1945, the second atomic bomb was dropped on Nagasaki, a 21-kiloton plutonium implosion-type bomb known as “Fat Man”.


Oralloy was the code name often used for the enriched uranium being produced at Oak Ridge.


The term “Pile” refers to a nuclear reactor. Coined by Enrico Fermi at the Met Lab, it was based on the first rudimentary nuclear reactor which was nothing more than a pile of uranium and graphite blocks. Later on the term was carried forward to Hanford where the giant reactors became known as Pile B, Pile D, etc.


Plutonium is a heavy metal that does not exist naturally. It is produced as a by-product of the fission process in a nuclear reactor.

Project Alberta

Project Alberta was the code name for a team of scientific personnel (civilian and military) that was sent from Los Alamos to Tinian Island to assemble and arm the “Little Boy” and “Fat Man” atomic bombs.

Project Silverplate

Project Silverplate was the code name for the top secret mission to develop specially modified B-29 Superfortresses for use by the 509th Composite Group. It carried a Triple A (“AAA”) priority.


Pumpkins was a coined term for the dummy “Fat Man” bombs dropped on practice missions by the 509th Composite Group. They were mustard colored and were of the approximate size and weight of the “Fat Man” bomb.

Queen Mary

“Queen Mary” was the common name given to the cavernous plutonium separation facilities at the Hanford Engineer Works.

Rad Lab

The “Rad Lab” was the short name for the Radiological Laboratory at the University of California, Berkeley. Its director was Nobel laureate Ernest O. Lawrence. He gained recognition for his 60″ cyclotron and was the driving force behind the electromagnetic separation of uranium that formed the basis for the Y-12 complex at Oak Ridge. In addition, Berkeley was the center for theoretical physics in the United States and spawned such notables as J. Robert Oppenheimer, Glenn Seaborg, and Emilio Segr√®.


The S-50 plant was one of four primary production facilities at Oak Ridge. Construction on this plant began on September 27, 1944 and Union Carbide was the contracted operator. Its mission was to provide enriched uranium utilizing the liquid thermal diffusion process which had been pioneered at the Naval Research Lab at the Philadelphia Navy Yard.

Site Y

Site Y was the code name for the Los Alamos laboratory.

“Tickling the dragon’s tail”

“Tickling the dragon’s tail” was a coined term for the criticality experiments to determine the amount of fissionable material needed for a sustained chain reaction. There was always an element of danger involved. Two Los Alamos physicists lost their lives while conducting the experiment: Harry Daghlian and Louis Slotin, Jr.

Tinian Island

Tinian Island is an island in the Marianas chain in the South Pacific. It was one of a series of islands captured by the Americans on their island-hopping mission to Japan. In 1945, it served as the headquarters of the 509th Composite Group. The final assembly of both the Little Boy and Fat Man atomic bombs was performed at Tinian. The island had several long, parallel runways capable of launching heavily laden B-29 bombers. Both atomic missions to Japan originated from Tinian.

Trinity test site

The Trinity Test Site was where the first successful detonation of an atomic bomb took place. Situated about 150 miles from Los Alamos, it is located near Alamagordo, NM and is now known as the White Sands Missile Range. On the early morning of July 16, 1945, the implosion-type weapon known as the “gadget” exploded with the force of several thousand tons of TNT and lit up the Jornada del Muerto valley.


The X-10 graphite reactor plant was one of four primary production facilities at Oak Ridge. Construction on this plant began in February 1943, with DuPont as the contracted operator. Its mission was to act as a semi-works (pilot plant) to work out kinks in the production of plutonium from a “pile”. Based upon experimental data from the Met Lab in Chicago, it formed the basis for the up-scaled plutonium production and concentration facilities at Hanford, Washington.


The Y-12 plant was one of four primary production facilities at Oak Ridge. Construction on this plant began on February 18, 1943 and Tennessee Eastman (a subsidiary of Kodak) was the contracted operator. Its mission was to provide enriched uranium utilizing the electromagnetic separation process, which had been pioneered by Ernest O. Lawrence at Berkeley.