Crushing Plutonium for Nuclear Safety

Deep below the Nevada desert, a machine dubbed Scorpius is under construction that will use high explosives to crush plutonium to states that exist just prior to a nuclear explosion. The aim of the US $1.8 billion project is to scan this plutonium with X-rays to help check the accuracy of supercomputer models designed to predict whether the United States’ aging nuclear arsenal works.

In the first roughly 50 years of the U.S. nuclear weapons program, researchers tested whether the bombs worked by actually detonating them. However, in 1992, President George H.W. Bush signed into law a moratorium on nuclear tests.

Scorpius is specifically designed to “tickle the dragon’s tail.”

Currently, supercomputer models help predict whether U.S. nuclear weapons might still work. However, the models’ accuracy remains uncertain. Scientists do blow up explosives to supply data for these models, but these surrogate materials possess significant differences from the plutonium typically used in nuclear bombs. This raises the question of how well these models simulate real nuclear explosions.

Sandia Labs scientists inspect the X-ray technology behind near-explosive tests of plutonium-based nuclear weapons. Craig Fritz/Sandia National Laboratories

“Plutonium is a strange element, displaying six different crystal structures between room temperature and melting, at normal pressure, and a seventh phase at slightly elevated pressure,” says Jon Custer, a technical manager at Sandia National Laboratories in Albuquerque, New Mexico. “Three of these crystal structures are unique to plutonium. This means that there is no surrogate material that will truly mimic plutonium behavior.”

Nuclear bombs use high explosives to force weapons-grade plutonium or uranium-235 to implode, triggering a catastrophic nuclear chain reaction. Scorpius is designed to create nanosecond-long X-ray images of plutonium as it compresses the radioactive element with high explosives. (Scorpius is named for Scorpius X-1, the brightest extrasolar X-ray source. Its name also reflects its subterranean location, just like desert scorpions that burrow underground.)

The aim of Scorpius is to help give supercomputer models the accurate data they need to see if they are generating realistic simulations of nuclear-weapon behavior. The machine, under construction a thousand feet beneath the Nevada National Security Site—a test area bigger than the state of Rhode Island—is expected to be up and running by late 2027.

“We will understand the performance and reliability of the nuclear stockpile, a critical part of our national security,” says Custer, the lead for Sandia’s part of Scorpius. (Scorpius is a project of the Sandia, Los Alamos, and Lawrence Livermore national laboratories, as well as the Nevada National Security Site.)

Scorpius is specifically designed to “tickle the dragon’s tail,” Custer says. In other words, the explosives are designed to bring plutonium to a highly compressed, hot state, but not past the critical point at which it would explode.

“If you bought a car in 1992 and parked it in your garage, doing nothing to it, do you think it would start tomorrow on the first turn of the key?”
—Jon Custer, Sandia National Laboratory

“The conditions in an implosion, even before nuclear yield, are unfathomable to humans,” Custer says. “Well before the device would go critical, the temperature inside is well above the surface of the sun, and the pressure is approaching that of the core of the sun. So while we think the models we use are really, really good across such huge changes, we need to test just how good they are. Scorpius allows us to image the real thing.”

Custer notes there is a long history of experiments bringing plutonium to “subcritical” conditions. “While Scorpius will image late-time behavior, the subcritical experiments are not physically able to assemble into a critical configuration,” he says.

One major question Scorpius will help address is how effective the plutonium in the U.S. nuclear stockpile may prove, given its age.

“The newest systems we have first went into the stockpile in the 1980s,” Custer says. “If you bought a car in 1992 and parked it in your garage, doing nothing to it, do you think it would start tomorrow on the first turn of the key? Not a perfect analogy, but you could do experiments and simulate things all you want, but you still want to turn the key and hear the engine turn over every once in a while to know it will start when you need it.”

Specifically, “we know that plutonium does undergo spontaneous fission,” Custer says. “This means that the material we first made and shaped precisely decades ago now has impurities from the fission in it, and the crystal structure has been damaged by the energetic decays. If you look at steel, very small changes to composition can make large changes to, for example, the mechanical strength.” The X-ray images from Scorpius will help validate that “at full scale, the plutonium still behaves as needed in spite of such changes.”

“There are upgrade paths in the future for even higher performance of the machine if wanted.”
—Jon Custer, Sandia National Laboratory

Scorpius is designed to generate high-energy pulses of electrons and slam them into a heavy metal target to create the bright X-ray flashes that will capture the pictures of the plutonium.

“There are very good models of what happens inside a nuclear weapon,” Custer says. “We will now be able to take detailed X-ray images of what actually happens as the device implodes to see if the models are right. If the models are not right, we will make them right. We can’t afford to be wrong.”

A key part of the Scorpius design is the way in which it can generate giant pulses of electrical energy. Instead of relying on, say, big banks of capacitors for this task, it will use more than 40,000 commercial printed circuit boards designed to make four 25,000-volt pulses, which will energize a 22.4 million electron-volt electron beam.

The Scorpius Injector, pictured here, generates X-ray images of weapons-grade materials under explosive conditions. Craig Fritz/Sandia National Laboratories

“The flexibility and programmability of the units makes it much easier to create precisely the X-ray bursts that the experimenters want for their particular experiment,” Custer says. “Taking advantage of commercially available chips and standard electronics assembly similar to, for example, a desktop computer board, is just a good engineering choice. It also is likely that as commercial parts get more capable—higher voltages, higher currents—there are upgrade paths in the future for even higher performance of the machine if wanted.”

With this year’s release of Christopher Nolan’s movie “Oppenheimer,” the public eye has turned once again to nuclear weapons. “One of the scenes in the movie was several scientists betting whether the Trinity test would ignite the atmosphere, which was disturbing—an understatement—to General Groves,” Custer says. “There was uncertainty there, although the possibility was swiftly debunked in real life. Scorpius aims to eliminate any uncertainty.”