America’s New Nukes: Plutonium Pits at Los Alamos  


By Andy Oppenheimer

“Eight decades after its inception, Los Alamos National Laboratory is being regenerated for its prime role in modernizing the U.S. nuclear warheads inventory,” writes Andy Oppenheimer. “However, pits for today’s weapons have not substantially been made since the late 1980s. The new effort includes updated missiles, a new weapon design, alterations to existing designs, and new pits.”

During the Cold War, the U.S. nuclear stockpile increased in both size and variety of weapons. In the current century, the U.S. Department of Defense (DoD) and House of Representatives became increasingly concerned there was no reliable capability to produce enough plutonium cores, also known as “pits”, the central component of all the thermonuclear two-stage warheads that form the USA’s nuclear deterrent.

New Nuclear Cores in the U.S.  

Put simply, pits are hollow plutonium cores of the ‘primaries’ (triggers) for the fission device that – surrounded by conventional explosives – is incorporated within the thermonuclear weapon to set it off. 

In 2018, the National Nuclear Security Administration (NNSA) – the agency responsible for the production and maintenance of the nuclear stockpile – formulated a crash plan to build pit production lines at Los Alamos National Laboratory (LANL) in New Mexico and the Savannah River Site (SRS) in South Carolina, with a combined annual output of 80 pits. 

The First Nuclear Efforts 

On the remote Pajarito plateau in northern New Mexico sprawls the world’s most extensive nuclear weapons laboratory, built during World War II around the small high-desert town of Los Alamos.  

As the scientific research heart of the wartime Manhattan Project, the rapidly constructed facility progressed from building the first atomic bombs that were dropped on Hiroshima and Nagasaki at WWII’s conclusion to developing and refashioning warheads alongside the Lawrence Livermore NL in California for the U.S. nuclear arsenal throughout the Cold War to the present day. The first nuclear devices developed at LANL worked by the fission of plutonium or uranium. In modern weapons, plutonium fission ignites a second, more powerful stage in which hydrogen atoms undergo nuclear fusion, releasing several magnitudes more energy.  

Making plutonium (which, as element 94, is very rare) for nuclear weapons is faster, easier, and cheaper than producing highly enriched uranium (HEU). There is no shortage of plutonium as enough has been made over the years for the new pits at LANL and SRS. However, pits for today’s weapons have not substantially been made since the late 1980s. The new effort includes updated missiles, a new weapon design, alterations to existing designs, and new pits.

The LANL PF-4 facility, where the U.S. plans to produce new plutonium pits for its renewed nuclear weapons program, ©Los Alamos National Laboratory

New Role for Los Alamos  

Eight decades after its inception, LANL is being regenerated for its prime role in modernizing the U.S. nuclear warheads inventory.  

The biggest and most ambitious program since the Manhattan Project, the new pit production project has required the hiring of some 3,300 workers since 2021, upping the total workforce to over 17,270.  

The lab’s main role over many years has been maintenance of the nuclear stockpile. In this new project, under the direction of the NNSA, lab workers are preparing for renewed production of at least 30 plutonium pits per year by 2030. 

Producing Plutonium 

After the closure of the country’s prime plutonium manufacturing plant at Rocky Flats in 1992, where 1,000 to 2,000 pits were produced every year, a highly reinforced 236,000 sq-ft facility built at LANL earlier in 1978 became the first Department of Energy (DoE) facility capable of producing plutonium cores.  

Although initially established for plutonium research and development, in 2003 the Plutonium Facility Building 4 (PF-4) within Tech Area 55 at Los Alamos produced the nation’s stockpile quality (first war reserve) plutonium. In 2006, Congress instructed the DoE to focus on producing pits at this facility.

Aerial view of Los Alamos National Laboratory, birthplace of the atomic bomb, and now at the heart of new nuclear pit production, ©Los Alamos National Laboratory

Pit Production 

Already being produced at LANL are ‘development pits’, which are pending the requisite quality finalization for them to be incorporated in warheads. Plutonium from the old material must be recovered and purified, then cast into pieces, which are fitted together into a whole pit. The pits are then transported to the Pantex facility in Texas to be incorporated into nuclear warheads. 

The FP-4 facility has been built with heavily reinforced concrete exterior walls, floor, and roof able to withstand extreme weather and earthquakes. However, as recently as 2022, the Defense Nuclear Facilities Safety Board claimed that PF-4 would not withstand an earthquake at Los Alamos.

Why build more pits? 

The NNSA plan also includes replacing all 1,900 U.S. submarine-launched ballistic missiles (SLBMs) warheads with new ones incorporating shock-resistant, or ‘insensitive’, high explosive. This will make the warheads less prone to accidental detonation, which would disperse highly radioactive plutonium. 

The NNSA proposes to first build 800 pits for new U.S. intercontinental ballistic missile (ICBM) warheads deployed in 400 underground silos in various locations across the northern U.S. Great Plains.  

The current Minuteman III ICBM currently carries two warhead types, the W78 and W87. This is to be replaced by a new ICBM. The first pits will be designed for the W87-1 warhead to fit atop the new Sentinel ICBM. The older W78 will be replaced with an updated version of the W87, which has improved explosive sensitivity safety features.  

The first 800 pits would equip the W87-1 warheads. Production of these pits by the two NNSA proposed pit production facilities operating at full design capacity is expected to take ten years. The U.S. reportedly already has 540 W87 warheads which means one warhead on each of its 400 ICBMs.

The US ‘Baker’ nuclear test as part of Operation Crossroads at Bikini Atoll, Micronesia on July 25, 1946, released highly radioactive water that contaminated many of the ships set up close to the detonation. Above-ground tests by the main NWS ended with the 1963 Partial Nuclear Test Ban Treaty, ©US DOD

 Arms Control Objections 

Arms control advocates argue these efforts would only be necessary to increase the number of warheads on each ICBM from one to three. Previous administrations regarded this increase as destabilizing, partly because increasing the number of warheads on silo-based ICBMs would make them more vulnerable targets. It would also prevent compliance with the New Strategic Arms Reduction Treaty (START) with Russia, assuming that this agreement is extended in 2026. 

There is also the inevitable question of cost. The original NNSA estimate in 2017 for a production capacity at the Savannah River facility to manufacture 80 warhead pits per year was $3.6 billion. This has since risen to $11.1 billion in 2023 – but to make only 50 pits annually.

Pit Life Expectancy 

It is not exactly known for how long the original pits will work. Some estimate that the currently installed pits in the warheads inventory could last 60 years or more. NNSA research indicates that the pits will last at least 150 years, but their degradation could produce sudden defects.  

What is known is that as the plutonium atoms in a pit decay over time, their products – helium, americium, uranium, and neptunium – contaminate and damage its crystal structure.  

There are also claims that the pits have not aged enough to be replaced. Arms control advocates at The Bulletin of Atomic Scientists recommend that LANL prove it can produce 10-20 pits in a year before committing to building more production lines. As well as the cost, new pit production is a risky business. 

Based on research at both national nuclear labs, an independent group of science advisors called the Jason Group advised the U.S. administration in 2006 that the plutonium in existing pits would be functional for a hundred years.

Simulation of the perilous experiment dubbed “tickling the dragon’s tail” shows the configuration of beryllium reflector shells prior to the criticality event that killed LANL scientist Louis Slotin in May 1946. Previously, in August 1945, LANL scientist Harry Daghlian was exposed to a fatal radiation dose after attempting to build a neutron reflector manually by stacking a set of 4.4-kg tungsten carbide bricks in an incremental fashion around a plutonium core, ©Los Alamos National Laboratory

A return to nuclear testing? 

Added to these concerns are fears that as the NNSA designs are new, they would need to be tested, possibly leading to demands to resume nuclear tests. This would end the moratorium on all forms of nuclear testing – above and below ground – that has been observed since 1998 by all nuclear-weapon states other than North Korea. The U.S. conducted its last underground test in 1992. 

For some years now, computer modelling of nuclear weapon processes has replaced testing, and the NNSA believes this highly advanced expertise will enable the design of improved warheads without explosive nuclear tests. They also state that differences between old, tested warhead designs and new, untested ones are minimal.  

There will still, however, be calls for renewed U.S. testing if the reliability of the U.S. stockpile is deemed to be at risk. Much will also depend on who occupies the White House from January 2025. 

In the face of China’s rapid growth of its nuclear arsenal, and Russia threatening new missile tests and its own nuclear missile modernization, the U.S. is bound to forge ahead with its own program of nuclear renewal. The Atomic Age is, once again, having a moment. 

Andy Oppenheimer is author of IRA: The Bombs and the Bullets – A History of Deadly Ingenuity (2008) and a former editor of CBNW and Jane’s NBC Defense. He is a Member of the International Association of Bomb Technicians & Investigators and an Associate Member of the Institute of Explosives Engineers, and has written and lectured on CBRNe since 2002.  

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