LIVERMORE — Scientists at Lawrence Livermore National Laboratory published a 2026 study in Analytical Chemistry revealing that cooling rates in nuclear fireballs affect how radioactive fallout particles form. The research challenges assumptions in existing models by demonstrating that chemical interactions during cooling alter the composition of fallout, particularly for volatile elements like cesium.

The team recreated part of the intense environment inside a nuclear fireball using a plasma flow reactor. They introduced specific combinations of uranium, cerium, and cesium into a high-temperature plasma, where the materials vaporized and then traveled through a tube with carefully controlled cooling conditions. This setup allowed them to expose the vaporized materials to two different thermal histories: one with gradual cooling and another where materials remained hot longer before rapid cooling.

Researchers found that uranium and cerium—both less volatile—condensed early in the process, while cesium condensed much later. When cesium remained at high temperatures for extended periods, it mixed more extensively with uranium and cerium. Both uranium and cerium also showed changes in their chemistry depending on the thermal history they experienced. These results indicate that fallout formation depends not only on when elements condense but also on how they chemically interact as temperatures drop.

"Historical fallout studies indicate that the path materials take as they cool is important," said LLNL scientist and author Rakia Dhaoui. "Cooling rate and time at elevated temperature can alter chemical speciation and particle formation." She added, "Changing how long materials remain at high temperature can alter chemical reactions and how volatile elements like cesium are incorporated into particles."

Dhaoui said, "These particles preserve a record of how they formed. By studying these processes in a controlled system, we can replace assumptions with measurements, improve the models used to interpret nuclear debris, and support decision-making when it matters most."

The findings suggest that many existing fallout models, which often treat materials as if they behave independently, may overlook important chemical interactions. The researchers generated data that can be used to evaluate and refine these models, which have long relied on simplified assumptions. The team plans to expand the work by studying more realistic mixtures of materials to better capture the complex processes that govern fallout formation during real-world nuclear events.