Inside the 40-mm Impact Test Facility, a heavily instrumented gun – 40 millimeters in diameter – employs compressed helium or explosives like gunpowder to lob projectiles into small plutonium targets at impact velocities of up to 1.7 kilometers per second, all inside a protective steel glove box. At the ends of their brief trips, projectiles made of plastic or metals like aluminum or magnesium can generate pressures from thousands to hundreds of thousands of atmospheres as they smash their targets (normal pressure is one atmosphere). After that, a stack of metal plates stops everything cold.
“It’s how plutonium performs when the high explosives in a nuclear weapon go off and start shocking (the plutonium),” says William Anderson, a physicist and principal investigator of the Los Alamos National Laboratory.
The work of Anderson and his NNSA colleagues provides previews of plutonium-fueled nuclear blasts’ first stages without having to set off real bombs.
The facility allows Anderson and other researchers’ instruments to log how plutonium samples ranging from 1 to 1.25 inch in diameter – too small to go critical – respond to the kind of shock waves bomb triggers could induce. Those data include the pressures and temperatures the material encountered, whether it melted or underwent other phase changes, how the process altered its strength and how it was structurally deformed or damaged as a result.
Measuring plutonium’s properties under extreme pressure provides essential ground-truthing for the Stockpile Stewardship Program, under which computer modeling replaces explosive nuclear testing banned by international agreements.
Although used in bombs since the 1940s, toxic radioactive plutonium is a difficult material to evaluate. It can exist in six different phases before melting and has properties that can alter over time due to radiation-induced aging.
“For most of the nuclear weapons program’s history,” Anderson says, “the approach was to use fairly crude models for the way plutonium behaves, build something and then see if it worked. There was no scientific program to study how plutonium behaved at the high pressures we’re talking about. There were almost no such studies before the 1980s.”
Opened in 1996 and located at Los Alamos National Laboratory’s plutonium processing center is TA-55. Its glove box, where air pressure is reduced so no particles can escape, allows scientists and technicians access via rows of windows and protective gloves along its sides. The center of attention is the 11.5-foot smooth-bore gun barrel stretching along its middle.
In what are called “normal impact” experiments, technicians insert projectiles – lightweight for maximum speed – into the breech and targets into a holder near the barrel’s end. Special transparent windows are glued to the targets to ensure that shock waves from the projectile-target collisions aren’t reflected from the target surface, says Anderson, a shockwave expert.
Two kinds of electrical pin detectors are mounted around the targets. One measures velocity by shorting out as projectiles pass. Another uses pressure-sensitive piezoelectric crystals to detect projectile tilt. A laser velocity interferometer measures how shock waves interact with targets. Variations on this normal scenario can detect other information. And new instruments are being installed to further enhance these studies.
Meanwhile, the Joint Actinide Shock Physics Experimental Research (JASPER) Facility at the Nevada National Security Site uses another high-energy shock gun to study plutonium under pressures from hundreds of thousands to millions of atmospheres. Anderson says modelers of advanced weapons simulation codes seek data from both facilities, as well as other sources, to support their work.
Learn more about NNSA’s mission to maintain the stockpile.