Complex safety problems in overhauling USA’s nuclear weapons stockpile, especially plutonium pits

TRUST BUT VERIFY. U.S. labs are overhauling the nuclear stockpile. Can they validate the weapons without bomb tests?

20 APR 2023, BY SARAH SCOLES Science.org

LOS ALAMOS, NEW MEXICO—Behind a guard shack and warning signs on the sprawling campus of Los Alamos National Laboratory is a forested spot where scientists mimic the first moments of a nuclear detonation. Here, in the Dual-Axis Radiographic Hydrodynamic Test (DARHT) facility, they blow up models of the bowling ball–size spheres of plutonium, or “pits,” at the heart of bombs—and take x-ray pictures of the results.

In a real weapon, conventional explosives ringing an actual pit would implode the plutonium to a critical density, triggering an explosive fissile chain reaction. Its energy would drive the fusion of hydrogen isotopes in the weapon’s second stage, generating yet more neutrons that would split additional fission fuel………………………………………………………..

Facilities like DARHT have been important since 1992, when the Department of Energy’s (DOE’s) three weapons labs—Los Alamos, Lawrence Livermore National Laboratory, and Sandia National Laboratory—stopped full-fledged tests of nuclear weapons. By 1996, the United States had signed the Comprehensive Nuclear-Test-Ban Treaty—credited not only with stopping the environmental damage of nuclear testing, but also with disincentivizing new weapons designs.

Without tests, however, the only things ensuring that warheads work are facilities like DARHT, computer simulations from “weapons codes,” and a cache of data from the old days of nuclear testing. For relatively minor changes to old weapons—new fuses, fresh top-ups of the hydrogen isotope tritium—that has been enough. Every year, DOE’s National Nuclear Security Administration (NNSA) and the Department of Defense have certified the stockpile, an assessment that means they are convinced the weapons will work when they’re supposed to, as they’re supposed to—and not do anything when they’re not supposed to. “Because we’ve blown up so many of them, these things are incredibly reliable,” says Geoff Wilson, director of the Center for Defense Information at the Project on Government Oversight, which argues nuclear weapons spending should be reduced.

But now the stockpile is getting an overhaul, the biggest in decades. This fiscal year, NNSA has a record $22.2 billion budget. Much of the money will go to producing new plutonium pits to replace those in the arsenal and to modernizing four warheads. A fifth weapon, dubbed the W93—a submarine-launched warhead—is a new design program. “It’s really the first warhead program we’ve had since the end of the Cold War” that isn’t a life extension or modernization of an existing weapon, says Marvin Adams, NNSA’s deputy administrator for defense programs………………………………

Wilson worries that the international dynamics and the U.S. overhaul could ultimately lead to a revival of bomb tests, bringing back their hazards and stoking a new arms race. “It is not unfathomable to me, which is scary to say.” It’s one thing to tweak weapons with a deep heritage. It’s another to infer functionality for modified weapons that have never been fully tested, he says………………………………….

SIMPLY REPLACING the bombs’ plutonium pits poses a science challenge: understanding how subtle changes affect their behavior. They aren’t easy to make, in part because plutonium, a metal only in existence since 1940, is mysterious and hard to handle. The last time anyone made pits at scale—in the 1980s at Colorado’s Rocky Flats plant—DOE’s contractor was shut down for environmental violations and forced to pay an $18.5 million fine.

This time, NNSA is splitting production between Los Alamos and the Savannah River Site in South Carolina. It has tasked them with making 80 new pits per year by 2030, a deadline NNSA admits it will not meet.

Los Alamos’s pits will be made at a facility called PF-4, a set of high-security buildings surrounded by cyclone fences with razor wire. Inside PF-4 are glovebox enclosures—radiation-shielded workstations where workers use thick gloves and peer through glass windows to manipulate the exotic metal. The lab is hiring thousands of workers, and its first pit is likely to be ready for the stockpile next year.

The gargantuan effort is motivated by a simple fact: many current pits are more than 40 years old, and plutonium behaves in confounding ways as it ages and radioactively decays. A green, fuzzy coating grows on it as its surface oxidizes. Atoms in its metallic lattice are knocked out of place as it spits out uranium isotopes. Its dimensions shift when it slips between six different solid phases. And the pits do not necessarily degrade smoothly. “We know at some point there will be a nonlinear piece,” says David Clark, director of Los Alamos’s National Security Education Center and editor of the Plutonium Handbook. “We just haven’t seen it.”…………….

One might think the new pits would make it easier to certify the stockpile, by avoiding the uncertainties of aging plutonium. But they come with uncertainties of their own. The new pits won’t be twins of their predecessors, so weapons scientists will have to understand how the alterations change pit behavior. They are being manufactured using recycled and purified plutonium from old pits, not fresh material, unlike the originals. Moreover, they will be made with different processes, and in some cases designed to slightly different specifications. “If you look at a new requirement,” Adams says, “you often will find that the old pits we have available to us are really, really suboptimal.”………………………………………. https://www.science.org/content/article/trust-verify-can-u-s-certify-new-nuclear-weapons-without-detonating-them