On January 27, 1996, France conducted Xouthos, its 210th and final live nuclear test, detonating a thermonuclear warhead beneath the Fangataufa lagoon in the southern Pacific Ocean. The test had a yield of between 20 and 120 kilotons of TNT, potentially 8 times as explosive as the bomb the United States dropped on Hiroshima in 1945. Xouthos marked the end of France’s nuclear tests but not the end of its nuclear stockpile maintenance. To ensure that the country’s warheads are in working order, France is working with Lawrence Livermore National Laboratory in the United States to develop a super-cooled laser ignition test facility.

There are eight other nations with nuclear weapons: The United States, Russia, the United Kingdom, France, China, Israel, India, and Pakistan. In the 21st century, only North Korea has conducted live nuclear weapons tests. For the other countries, maintaining and sustaining nuclear warheads without real-world tests is an engineering challenge. One way to manage this is through computer modeling, which lets nuclear laboratories refine warhead design and study new types of warhead refurbishment.

But testing a warhead’s actual fissile material, the gas around which a plutonium pit condenses until it sparks a nuclear fission reaction, means looking at the actual material itself, and seeing how it behaves under high density and intense heat. Lawrence Livermore describes a process for testing nuclear material with lasers at low temperatures as a “cryogenic target system,” which it runs in the National Ignition Facility. This system, in theory, would allow them to understand how nuclear fuels behave in conditions of high energy density, by creating real if temporary fusion energy under laboratory conditions.

The National Ignition Facility started operation in 2010. That year’s budget request from the National Nuclear Security Administration describes the program as providing “the scientific understanding to assess the safety, security, and reliability of the nation’s nuclear weapons without nuclear testing.” To achieve that goal, “[s]cience-based weapons assessments and certification requires that these advanced experimental tools have the capability to create and study matter under extreme conditions that approach the high-energy density (HED) environments found in a nuclear explosion.”

Thermonuclear explosions are hot, with the first-stage fission reaction reaching over 100 million kelvin. They are also dense: the conventional non-nuclear components of a thermonuclear warhead use explosive force to compact plutonium into an even more compact form around a gas made of heavy hydrogen, deuterium and tritium.

Another way to achieve this density is to cool a tiny pellet of deuterium-tritium to 18.5 Kelvin (or -426 Fahrenheit). The cooling is the “cryogenic” part of the “cryogenic target system,” and the pellet is the target. Instead of an exploding inwards plutonium sphere, like in an actual warhead, in these tests the energy for ignition is provided by high-powered laser beams. 

At the National Ignition Facility, this process is done through 192 laser beams, which focus 1.92 megajoules on the single super-cooled pellet of deuterium and tritium. The energy released through fusion is akin to that of a star, though much shorter lived. France’s counterpart of the National Ignition Facility, the Laser Mégajoule, “will use 176 laser beams to focus more than one megajoule of ultraviolet laser energy on tiny targets containing a partially frozen mixture of the hydrogen isotopes deuterium and tritium.”

While the research into nuclear fusion has often been promoted and framed as a path to viable fusion reactors for electrical energy generation, it is fundamentally a weapons evaluation and research tool, with incidental potential for scientific research attached.

[Related: Humans just generated nuclear energy akin to a star]

“NIF was designed to produce extraordinarily high temperatures and pressures—tens of millions of degrees and pressures many billion times greater than Earth’s atmosphere,” reads an explanation from the laboratory. “These conditions currently exist only in the cores of stars and planets and in nuclear weapons.”

When those conditions exist in the lab, they do so only very briefly. “For a few billionths of a second during an ICF experiment, [the National Ignition Facility]’s 192 lasers duplicate the same temperatures, densities, and pressures found within the interiors of stars and planets and detonating nuclear explosives,” says a 2019 report from the laboratory. That’s still enough time for researchers to study the effect of those conditions on relevant materials, crucial in stockpile stewardship. “About 15 experiments per year are aimed at researching materials’ equations of state,” or a mathematical modeling of how the material handles the conditions of a nuclear reaction.

While the National Ignition Facility has been operational for almost 12 years, the closest it has come to creating energy-producing fusion was with a test on August 8, 2021. That shot yielded 1.35 megajoules from igniting the near-frozen pellet, marking an almost 70 percent conversion of laser energy to reaction. 

“For the Stockpile Stewardship Program,” the laboratory said in a statement from February, “the record shot provides access to a new regime of high-energy-density plasmas to test and verify the Laboratory’s nuclear weapons-related simulation codes. [National Ignition Facility] performs experimental studies of fusion ignition and its subsequent thermonuclear burn, which provides the immense energy of modern nuclear weapons.”

Creating the conditions of fusion without outright detonating a nuclear warhead is still a work in progress. By working with France’s Alternative Energies and Atomic Energy Commission, the national laboratories are ensuring both countries can benefit from parallel and collaborative research. When compared to the alternative of resuming live nuclear tests, a hard physics problem in a controlled laboratory setting is a small challenge, relative to the geopolitical risks of a resumed international arms race.