Probing plutonium

In 2023 and 2024, scientists at Los Alamos National Laboratory conducted two novel experiments, Martini and Moonshine. Despite their names, the experiments have nothing to do with alcohol. Instead, both are focused on finding new ways to analyze plutonium, a radioactive element that plays a key role in nuclear weapons. By using existing facilities in new ways and reestablishing a dormant testing capability, scientists say they are uncovering valuable information about how plutonium behaves.

The National Nuclear Security Administration, which oversees the Laboratory, honored Martini with a Defense Programs Award of Excellence and a citation for exceptional achievement, recognizing the test as one of the three most important national security science technical achievements of 2023. Martini demonstrated the Lab’s ability to execute a complex experiment that required extensive collaboration between multiple national laboratories in an extremely short time period—only four months.

Martini was conducted at  Sandia National Laboratories’ Annular Core Research Reactor (ACRR). The ACRR allowed researchers to subject plutonium samples to high neutron irradiation, providing insights into how the material behaves under different conditions.

“Plutonium still has surprises, and it’s important to conduct these experiments to understand its material properties better,” says Karen Paige, Weapon Survivability program manager. “Martini and its follow-on experiments allow us to assess the nation’s nuclear stockpile and provide information that will contribute to future weapons designs.”

Paige says the team will build on what they have learned from Martini by testing plutonium at the Transient Reactor Test facility at Idaho National Laboratory. Transient testing involves the application of controlled, short-term bursts of intense neutron flux, which will provide additional information about plutonium’s material properties. “Different neutron sources with different characteristics extend our knowledge of plutonium’s properties,” she says. “These new approaches offer important insights, and I’m excited to work on the forefront of these discoveries.”

Los Alamos is also conducting a groundbreaking series of plutonium experiments at its Dual-Axis Radiographic Hydrodynamic Test (DARHT) facility. The goal of the series is to answer physics questions about plutonium and how it behaves under certain conditions.

The first experiment in the series, Moonshine, took place in May 2024 and used the DARHT accelerator’s electron beam to change the thermodynamic state and phase of a plutonium sample. Physicist Rendell Carver suggested using DARHT for this experiment series because its beam “provides a unique combination of energy density, penetration, and time scale,” he explains. Moonshine marked the first time plutonium had ever been used in a DARHT experiment. The amount of plutonium used in the Moonshine series tests was small—about the size of a quarter, only a tiny fraction of that contained in a nuclear weapon.

Carver notes that being able to predict plutonium’s behavior is a crucial component of assessing the reliability, performance, and safety of the nation’s nuclear weapons, which all contain plutonium that is decades old. 

Scientists are only beginning to scratch the surface when it comes to understanding the data from Martini, Moonshine, and follow-up experiments.  “These are some of the most innovative experiments on plutonium I have seen in decades,” says Los Alamos Deputy Director for Weapons Bob Webster. “They are critical to expanding our understanding.” ★

Article by Jill Gibson, National Security Science magazine writer