Since the Manhattan Project, Los Alamos National Laboratory has been involved in the manufacture of plutonium pits. These pits form the cores of nuclear weapons; a compressed pit generates a nuclear explosion. Los Alamos produced the first plutonium pits in 1945 and has conducted limited pit production over the years. At the direction of the National Nuclear Security Administration (NNSA), the Laboratory is currently ramping up its pit production capability to be able to manufacture at least 30 pits per year by 2030.
Pit manufacturing at Los Alamos has primarily taken place in three buildings since the 1940s: D Building, DP West, and the Plutonium Facility Building 4 (PF-4). Each was designed and constructed to meet the production requirements of the time. PF-4, which is still in use today, is being updated to execute the current pit production requirements.
Here is a brief overview of each facility, as well as two others that support pit manufacturing and plutonium research and development.
February 1944 to August 1945
Perched on the northern rim of Los Alamos Canyon near modern-day downtown Los Alamos, D Building was part of Technical Area 1, a collection of buildings that formed the core of Project Y—the code name for the Los Alamos branch of the Manhattan Project. Some of the world’s first plutonium was processed at D Building, a structure made mostly of wood with a sheet metal roof. The plutonium metal came from Hanford Engineering Works (now the Hanford Site) in Washington state. Processing involved purification, shaping, machining, and assembly into pits for use in implosion weapons, which were detonated at the Trinity site in southern New Mexico and above Nagasaki, Japan.
1945 to early 1980s
After World War II, the U.S. nuclear stockpile increased in size and weapon diversity as the Cold War intensified. D Building was not large or safe enough to meet pit production demands, so in 1945, Technical Area 21 was constructed in just five months on a mesa southeast of downtown Los Alamos. The western area of the site was dubbed Delta Prime (DP) West and was dedicated to plutonium operations and plutonium storage. (At one point, a rumor circulated that “DP” stood for “displaced persons,” perhaps due to the site’s isolation from the main Laboratory.)
In the early 1950s, the Rocky Flats Plant near Denver, Colorado, began producing most plutonium pits for U.S. nuclear weapons. DP West pivoted from pit manufacturing to uranium and plutonium research and development. In addition to studying plutonium for weapons-related purposes, scientists at DP West determined that plutonium could be used to power everything from human heart pacemakers to spacecraft.
The Chemistry and Metallurgy Research building
1952 to present
The Chemistry and Metallurgy (CMR) building was built to house research and experimental activities for analytical chemistry and plutonium and uranium metallurgy. Analyses and research performed at CMR support national security and space exploration programs, including pit production at PF-4. In 1962, President John F. Kennedy visited the building to check on the progress of Project Rover, a program to develop nuclear rocket engines for space travel.
The Plutonium Facility Building 4
1978 to present
In the early 1970s, the Laboratory, in conjunction with Fluor Engineers and Constructors of Los Angeles, began construction on a new facility in Technical Area 55, south of downtown Los Alamos. Completed and operational in 1978, the Plutonium Facility Building 4, or PF-4, was the first Department of Energy facility designed to withstand potential disasters such as tornadoes, wildfires, and earthquakes. The more than 236,000 square-foot facility, which includes a basement, has thick exterior walls, a sturdy roof, and a floor all made of heavily reinforced concrete.
PF-4 is the only plutonium facility in the nation currently capable of producing plutonium pits. Although the facility was initially established for plutonium research and development purposes, in 2003, PF-4 produced the nation’s first war reserve (stockpile quality) plutonium pit since the closure of the Rocky Flats plant in 1992. From 2007 to 2011, 31 pits for W88 warheads were manufactured at PF-4. In 2018, NNSA tasked Los Alamos with producing at least 30 pits per year by 2030. PF-4 is being renovated to handle this increase in production work.
In addition to the facility’s significant role in sustaining America’s nuclear weapons, PF-4 also supports other NNSA defense programs as well as NASA deep space missions. For example, plutonium heat sources manufactured at Los Alamos currently power the Mars Curiosity and Perseverance rovers.
The Plutonium Facility Building 400
2011 to present
Named the Radiological Laboratory Utility Office Building (RLUOB) when it opened in Technical Area 55 in 2011, the Plutonium Facility Building 400, or PF-400, took on analytical chemistry and materials characterization capabilities from the aging CMR building starting in 2014. A state-of-the-art actinide lab, PF-400 provides 19,000 square feet of laboratory space for chemical and materials analysis. In February 2023, the building was designated a hazard category 3 nuclear facility, allowing it to handle larger quantities of plutonium. Pit manufacturing does not occur in PF-400, but the work happening there directly supports the Laboratory’s pit production mission. ★
Jeremy Brunette, Drew Kornreich, John Moore, and Steve Schreiber contributed to this article.