In the aftermath of nuclear accidents, radionuclides mix with the debris and soil in their vicinity. When this dangerous mixture “falls back” to us, the resulting nuclear fallout can cause lasting damage. For practical reasons, current fallout models fall short in fully describing these toxic events—but a clever miniature may finally offer scientists a better way to study them.
Researchers at Lawrence Livermore National Laboratory (LLNL) created a small replica of the fireballs that trigger nuclear fallout inside a plasma flow reactor. The carefully controlled experiment allowed them to investigate how uranium, cerium, and cesium vaporize and behave. As a result, the team was able to identify limitations in current fallout models under more realistic conditions. They published their findings in a recent Analytical Chemistry paper.
“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,” Rakia Dhaoui, the study’s first author and a scientist at LLNL, said in a statement.
A mini fireball
For the experiment, the team customized a plasma flow reactor to freely program different temperatures and oxygen fugacities (i.e., how easily chemicals move and react). The miniature represents a portion of the fireball process, which triggers nuclear fallout by expanding and mixing into air after a nuclear accident, according to LLNL.
Specifically, fallout is when the fireball begins to cool and condense into tiny solid particles, so the experimental setup was designed to replicate these steps. The researchers set up two scenarios: one imposed a consistent temperature decrease along the tube, whereas the other kept the heat at around 2,060 degrees Fahrenheit (1,127 degrees Celsius) before rapid quenching, according to the study.
“Historical fallout studies indicate that the path materials take as they cool is important,” Dhaoui explained. “Cooling rate and time at elevated temperature can alter chemical speciation and particle formation.”
A fork in the fire
The lab tests found that the three elements studied all behaved differently. Uranium condensed early on, with cerium condensing in a similar temperature range. Both elements’ chemistry varied depending on the cooling scenarios. On the other hand, cesium took much longer to condense and interacted more with other elements when kept for longer at a higher temperature.
“These results suggest that fallout formation depends not only on when elements condense, but also on how elements chemically interact during cooling,” LLNL explained. This is in contrast to existing models that tend to consider each element individually, but these complex interactions are likely “essential” for improving predictive models of processes relevant to nuclear fallout, according to the paper.
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