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Manhattan National Historical Park, Los Alamos

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Project Y, Manhattan Project; Los Alamos Scientific Laboratory; Los Alamos National Laboratory
1942–1945, Leslie Groves, J. Robert Oppenheimer, U.S. Army Corps of Engineers, and Willard C. Kruger. Roughly bounded by NM 4, NM 501, and Parajito Rd.
  • Technical Areas Site Plan, 1951 (Courtesy of Los Alamos National Laboratory)

Authorized by Congress in 2014, Manhattan Project National Historical Park was officially established in November 2015. Commemorating the design and production of the world’s first atomic bombs between 1942 and 1945, this park has units at Oak Ridge, Tennessee; Los Alamos, New Mexico; and Hanford, Washington. The Los Alamos unit includes proving grounds where bomb detonation systems were developed and tested, and will eventually incorporate parts of Los Alamos Scientific Laboratory National Historic Landmark District, designated in 1965 on the site where the main laboratory and original community once stood.

The proving grounds are scattered across mesas to the south of Los Alamos and are now located inside the postwar boundary of Los Alamos National Laboratory. They therefore remained classified facilities and were excluded from the 1965 designation. Unlike the main laboratory, many structures at those sites were not demolished after the war and either continued to function or were left to benign neglect. Now being declassified and restored, they are unique documents of the utilitarian architecture that was used to make the nuclear weapons tested at Trinity Site and dropped on Hiroshima and Nagasaki. While the physically impressive structures at Oak Ridge and Hanford seem appropriately scaled to their monumental purposes, the typically modest and sometimes primitive constructions at Los Alamos appear unequal to their highly technical use and require careful attention to functional context to understand their role in changing the world.

Los Alamos Scientific Laboratory was officially instituted in January 1943 as Project Y of the Manhattan Project, under the civilian direction of the physicist J. Robert Oppenheimer and the military control of Brigadier General Leslie Groves of the Army Corps of Engineers. When the laboratory began full operations in April 1943, Oppenheimer instructed the physicist Robert Serber to explain the purpose of the Manhattan Project to the assembled scientists in a series of lectures that were then compiled into a 24-page mimeographed booklet called the Los Alamos Primer: “The object of the project is to produce a practical military weapon in the form of a bomb in which the energy is released by a fast neutron chain reaction in one or more of the materials known to show nuclear fission.”

The two fissile materials being considered were uranium-235 (U-235) and plutonium-239 (Pu-239). The Clinton Engineer Works at Oak Ridge, Site X of the Manhattan Project, produced U-235 and the first kilogram of enriched uranium reached Los Alamos in September 1943. Oak Ridge also supplied Los Alamos with its first samples of Pu-239, but was succeeded by the reactors at the Hanford Engineer Works, Site W of the Manhattan Project; Hanford began delivering batches to Los Alamos in February 1945.

The parallel development of uranium and plutonium bombs responded to a practical dilemma. While U-235 was hard to manufacture, compressing the isotope to a state of critical mass and thus initiating nuclear fission was fairly easy. This meant that a uranium bomb would almost certainly work but few could be built. Conversely, Pu-239 was easier to produce but its more radioactive isotope had a higher rate of spontaneous fission. A plutonium bomb was therefore harder to build even though, by 1945, the material was relatively plentiful.

Two types of bomb were considered. The straightforward but inefficient gun-type bomb achieved nuclear fission by detonating an explosive charge at one end of a cannon or gun barrel to slam together two quantities of fissile material at high velocity. The technically complicated but more efficient implosion bomb achieved nuclear detonation by using a spherical explosive charge to compress a core of fissile material into a state of critical mass. As Serber explained, a bomb’s efficiency was tied to the speed with which the material blew apart and stopped the reaction: since the gun-type bomb blew apart rapidly, its estimated efficiency was only 2 percent, whereas the implosion bomb, because it compressed the core, had an estimated efficiency of 20 percent.

The initial focus was on the simpler gun-type bomb, using either uranium or plutonium. Steady progress was made on testing and building the components of a gun-type bomb that could be contained in a metal housing compact enough to fit inside the bay of a B-29 bomber. Code-named the Thin Man for its long tubular shape, this became the Little Boy uranium bomb after the planned original length of 17 feet was reduced to 10.5 feet. But research by the physicist Emilio Segrè into the chemistry of plutonium led to the discovery that the Pu-239 from Hanford was contaminated by plutonium-240, whose even higher rate of spontaneous fission greatly increased the risk of pre-detonation in a gun-type bomb.

In August 1944, Oppenheimer launched a crash program at Los Alamos to develop a viable plutonium implosion bomb. An outer sphere of shaped explosive charges called lenses would force a subcritical core of plutonium—formed from two hemispheres around a Polonium-Beryllium initiator and housed inside a buffering “tamper” of uranium-238—into a supercritical state by compressing the core to less than half its original size. To work, the lenses needed to be precisely calibrated and their detonation precisely synchronized, so that the focused blast waves exerted uniform pressure on the core. The technical challenges ranged from nuclear physics and metallurgy to hydrodynamics and electrical engineering, and they offered no guarantee that the bomb would work. The uranium gun-type bomb, Little Boy, would be dropped without advance testing on Hiroshima on August 6, 1945, but the plutonium implosion bomb required a trial run before the first of multiple copies, code-named Fat Man, fell on Nagasaki the following August 9. An experimental implosion device, code-named the Gadget, was detonated on July 16 at Trinity Site in southern New Mexico.

For reasons of both safety and security, the proving grounds for the bomb detonation systems were widely distributed in the canyons and mesas on the south side of Los Alamos Canyon, at a safe distance from the main wartime laboratory to the north. These are categorized in numbered technical areas, with the main laboratory identified as Technical Area 1. Eight outlying areas have structures from the Manhattan Project that either have been included in the historical park or are deemed eligible for inclusion.

Most were built to plans drawn up by the architectural firm of Willard C. Kruger, and all are practical constructions whose homely if sometimes unusual forms answer directly to the dictates of their technical functions and industrial materials. Some are bunker-like buildings of board-formed, cast-in-place reinforced concrete behind earth berms. Others are balloon-framed wooden boxes finished with asbestos shingles, heavier constructions with thick timbers and heavy-gauge steel plates, or prefabricated metal Quonset Huts. Putting scientific rationalism at the service of military objectives, these buildings exemplify the utilitarian logic at work in the Manhattan Project.

The Anchor West Site (or Anchor Ranch Proving Ground) in Technical Area 8 was used to test cannon tubes for the Thin Man and Little Boy gun-type bombs. The seven other sites concentrated on the implosion system of detonation. The explosive lenses for both the Gadget device and Fat Man bomb were tested and made at the Sawmill Site (S-Site) in Technical Area 16. Two Mile Mesa in Technical Area 6 was used to test the feasibility of detonating the Gadget inside a water containment vessel, to capture the plutonium if it failed to detonate properly. K-Site in Technical Area 11 studied implosions with a betatron machine and a cloud chamber: gamma rays generated by the betatron machine passed through an implosion and its effects were tracked by ion trails in the cloud chamber. L-Site in Technical Area 12 was used for “terminal,” i.e. end-result or physical, observations of test explosions. Q-Site in Technical Area 14 used flash photography and a rotating prism camera to study small cylinder implosions. The Pajarito Laboratory in Technical Area 18 was used first to conduct radioactivity tests of plutonium (away from the main laboratory’s high levels of background radiation), and then to study the velocity of explosions passing through magnetic fields. The Trap Door Site (TD-Site) in Technical Area 22 was used for detonator research as well as for final testing and assembly of the explosive components of the Fat Man bomb.

None of these areas can be visited as of 2017, though plans are underway to make them accessible to the public.


Conant, Jennet. 109 East Palace: Robert Oppenheimer and the Secret City of Los Alamos. New York: Simon and Shuster, 2005.


Los Alamos National Laboratory. The History and Legacy of the Manhattan Project at Los Alamos National Laboratory. Los Alamos: Los Alamos National Laboratory, 2015.

McGehee, Ellen, Sheila McCarthy, Ken Towery, John Ronquillo, Kari Garcia, and John Isaacson. Sentinels of the Atomic Dawn: A Multiple-Property Evaluation of the Remaining Manhattan Project Properties at Los Alamos (1942–1946). Historic Building Survey Report No. 215. Los Alamos: Los Alamos National Laboratory, 2003.

National Park Service. Manhattan Project Sites. Special Resource Study/Environmental Assessment. Washington, D.C.: Department of the Interior, 2010.

Rhodes, Richard. The Making of the Atomic Bomb. New York: Simon and Schuster, 1986.

Serber, Robert. The Los Alamos Primer: The First Lectures on How to Build An Atomic Bomb. Richard Rhodes, ed. and introduction. Berkeley: University of California, 1992.


Writing Credits

Christopher C. Mead
Christopher C. Mead
Regina N. Emmer

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