Concerns about aging plutonium drive need for new weapon cores

Despite multiple scientific studies pointing to existing long lifetimes of the key nuclear weapons components, the US Department of Energy settles on a dual-site modernization plan.

• David Kramer
• Volume 71, Issue 7
• July 1, 2018

The Department of Energy announced in May that it will spend at least $7.6 billion to build new facilities to manufacture plutonium cores, known as pits, for nuclear weapons. Given the shrinking US stockpile, the probable usability of many pits from retired weapons, and the lack of new weapons on the drawing board, why are new pits required at all?

The Trump administration’s nuclear posture review released in February calls for DOE to build 20 new pits a year beginning in 2024 and increase production to 80 pits annually by 2030. The document said that a sustained pit-manufacturing capability is required “to avoid stockpile age-out, support life extension programs, and prepare for future uncertainty.”

If a US nuclear weapon were ever detonated, high explosives would implode the grapefruit-sized pit as it is flooded with neutrons from a generator. Fissioning of the plutonium would then generate x rays that implode the secondary, fusion–fission stage of the weapon.

The US lost its ability to produce pits in large numbers in 1989, when the Rocky Flats Plant near Denver, Colorado, was shut down after the Federal Bureau of Investigation and Environmental Protection Agency investigated environmental violations at the site. Up to 1200 pits per year were manufactured there—“Dr. Strangelove numbers,” says Los Alamos National Laboratory (LANL) director Terry Wallace. Since then, only 30 pits for weapons have been fabricated—all at LANL, the sole US facility with production capability. Weapons-quality pit production ceased in 2012, when LANL began modernizing its 40-year-old facilities, although several practice pits have since been fabricated. The oldest pits in the stockpile—which now numbers 3882, according to DOE’s National Nuclear Security Administration (NNSA)—date to 1978.

The NNSA plans to split its pit operations between LANL and the Savannah River Site in South Carolina. The controversial plan would include repurposing a plant being built to produce mixed oxide (MOX) fuel from surplus plutonium for commercial reactors to instead fabricate 50 pits per year. South Carolina’s congressional delegation opposes termination of the MOX plant due to the economic benefits it would bring to the region. Under the NNSA plan, LANL will retain its status as the center of plutonium R&D.

Opponents of new pit production, such as the watchdog Los Alamos Study Group, say pit production isn’t necessary to address aging or stockpile needs. Instead, says the group’s executive director Greg Mello, the plans “arise from a combination of ideology, contractor influence, and pork-barrel politics.” And Jay Coghlan, director of the antinuclear group Nuclear Watch New Mexico, says pit-production requirements have been driven by proposals from the labs for new interoperable warhead types that would fit both US Navy and US Air Force missiles. The Trump posture review included no mention of such a warhead. Critics suspect the military wants a pit-manufacturing capability simply because Russia has one.

Long lifetimes predicted

The opponents of pit production point to scientific studies that strongly suggest that aging won’t affect the functioning of pits until at least the 2060s and perhaps much longer. Most notable is a 2006 report by JASON, a group of independent science advisers to the government. Its report said that the pits in most types of weapons will last at least 85 years, so the oldest pits shouldn’t need replacements before 2063.

A 2008 memo signed by the secretaries of energy and defense went further and declared that “depending on warhead type, the best estimate of minimum pit life is 85–100 years.” Even so, the memo stated, “degradation from plutonium aging still introduces uncertainty in overall system performance,” and “as the stockpile continues to age, the United States must plan to replace considerable numbers of pits in stockpiled weapons.”

A 2012 study by Lawrence Livermore National Laboratory identified no unexpected aging issues with plutonium for at least 150 years. Yet in 2014 then secretary of defense Chuck Hagel told lawmakers that the effects of aging plutonium were a factor in establishing the need for new pit production. The basis for his concern, however, was classified.

An NNSA spokesperson says the JASON study “validated the framework used for evaluating pit lifetimes” but notes that the study’s lifetime estimates applied to most, but not all, weapon types. The effects of aging and the lifetimes for a warhead system vary from one type of weapon to another, the spokesperson notes. The 2012 lab study also called for further research to more fully understand the pit-aging process. Although factors other than aging were used to set the production requirement of 80 pits per year, they are classified, the NNSA official says.

“We disagree significantly” with the JASON findings, says LANL director Terry Wallace; he notes that pit aging extends to how plutonium interacts with other components of the pit and weapon. Siegfried Hecker, a plutonium expert who was LANL director from 1986 to 1997, also takes issue with the JASON conclusions, which he says led to a “dramatic decline” in research on pit aging. Hecker maintains that despite differences, scientists at LANL and Lawrence Livermore National Laboratory were pressured to reach a consensus on aging that could be reviewed by JASON, and key aging issues were left unresolved. He also notes that since the JASON review, a reinterpretation by LANL researchers of results from underground experiments suggests the need to revise pit lifetime estimates.

Running the numbers

The nuclear posture review calls for an enduring stockpile of six different weapon types. Former NNSA deputy administrator for defense programs Donald Cook says that even if a 100-year lifetime is assumed for all the pits, new pits will still need to be produced at a rate of roughly 40 per year to refurbish all the weapons within that window. Since the newest pits are about 30 years old, additional pits are required in order to “catch up,” he says. “If you try to get to a steady-state period in 30 years, then you have to build in the range of close to 60 pits a year.” A few more pits a year must be added because full manufacturing capacity isn’t scheduled until 2030. That brings the annual build total to 65–70 pits. Keeping the average age of pits at its current value for the next 30 years would mean that the oldest ones would be about 85 years old by midcentury. That requirement implies an annual build rate of about 80 pits a year.

Mello runs the numbers differently. He says if two shifts were operated, the NNSA could produce 160 pits per year. At that rate, production could be delayed until 2050, and the entire stockpile could be refurbished in 25 years. If the number of weapons continues to decline, the reoutfitting would be completed sooner. Cook says it’s not that simple. Although two-shift operation is standard during production runs, several thermal treatment steps are involved, and some may require up to 72 hours per pit to complete. Gloveboxes, which isolate plutonium from workers during pit fabrication, are fully occupied during that time, and no additional units can be processed. So doubling the production rate could require twice the staff and equipment.

Another factor in the pit-production equation is the defense establishment’s goal of eventually converting all weapon types to use insensitive high explosives (IHEs) as the imploder, says Cook. Three missile warheads, the W88 and W76 submarine-launched ballistic missiles and the W78 intercontinental ballistic missile, contain conventional high explosive (CHE). IHE is far less prone to an accidental detonation that would spew plutonium and injure or kill people in the vicinity—although all weapons are certified not to produce a nuclear yield in such an event. IHE also offers certain security advantages over CHE, Cook says. But because of IHE’s slower burn rate, a simple swap of CHE for IHE isn’t possible; different pit or explosive geometries are required.

An NNSA program is assessing the potential reuse of pits from retired weapons to convert CHE systems to IHE. More than 14 000 pits are estimated to be stored at a weapons assembly plant in Texas. A small number of them, including some of the W76 pits that were produced for the navy and later retired through arms reductions, might be reused in refurbishing other navy or air force warheads, Cook says. Some pit reuse is already factored into the build rate, but if it is found to be infeasible, larger pit-fabrication facilities than the NNSA announced will be required.

Although the JASON report called for continued research on pit aging, the issue has seen little discussion since, at least in the public domain. Unresolved aging issues, Hecker says, include pit degradation caused by surface reactions. Interactions with water vapor could cause cracks in pits, Cook says, although pits in weapons are sealed and those that might be reused are kept in airtight containers. Cook is concerned with the effect of helium buildup in the metal matrix, which changes the properties of plutonium. Annealing of pits could release helium, but the high temperature required causes an increase in grain size, which in turn introduces uncertainty about whether a pit will implode as designed.

Hecker and Wallace also expressed concern about how aged pits will perform in a high-radiation environment. An adversary might flood incoming US warheads with neutrons to disable them, Wallace says. A burst of neutrons can create a shock in a pit. Because plutonium properties change with aging, it’s conceivable that a neutron flux that would leave a fresh pit undamaged could destroy an aged pit. Detailed knowledge of shock propagation through a pit can help determine the severity of the impact, but the simulations of older pits will require significantly better understanding of the detailed effects of aging than is currently available.

“We have Cadillac weapons, or more like Ferraris, compared with the rest of the world,” Wallace says. “We chose to make very small, well-tailored capabilities in our weapons.” In other words, their lightweight and compact designs place them closer to the edge of potential failure modes, compared with earlier, more cumbersome warheads.