Plutonium is famously unstable, which is of course what makes it both an incredibly powerful source of energy and a potentially-devastating environmental disaster. Some isotopes of plutonium can persist for tens of millions of years, which is bad news if it gets into the groundwater.
Given those stakes, it’s important to learn as much as we can about plutonium, to ensure it’s being created, used, transported, stored and disposed of as safely as possible. Scientists at the Helmholtz Zentrum Dresden-Rossendorf (HZDR) were doing just that when they accidentally discovered a new, stable form of plutonium.
One of the safer forms of the element is plutonium dioxide, which is a relatively stable ceramic material that doesn’t dissolve in water and has a high melting point. Nanoparticles of plutonium dioxide can be made from different precursors, in the form of different plutonium isotopes. And while trying to use a precursor called Plutonium (VI) (or Pu (VI)) dissolved in water, the team saw a strange reaction.
During its transition into plutonium dioxide, the Pu (VI) passed through a transient phase that’s solid and stable. It appeared to be pentavalent plutonium, or Pu (V), but this form is not usually solid nor stable. In fact, the find was so surprising that the team’s initial reaction was to assume that the synthesis had gone wrong.
“Every time we create nanoparticles from the other precursors Pu (III), (IV) or (V) the reaction is very quick, but here we observed a weird phenomenon half way,” says Kristina Kvashnina, lead researcher on the study. “Chemists were in complete disbelief, but the results were quite clear.”
To confirm what they had, the researchers tested it in the Rossendorf Beamline (ROBL), an x-ray absorption spectrometer. This facility can study the energy of radioactive materials by exposing them to x-rays, then measuring their fluorescence.
Sure enough, the experiments confirmed the new Pu (V) phase. The researchers even checked back in three months later, confirming that the material was stable over long periods.
Exactly what the new discovery could lead to is unclear, but the team says that it will need to be considered when trying to predict what happens to nuclear waste over very long periods of time.
“It is a difficult task and only theoretical predictions are possible, but the existence of this new Pu (V) solid phase, which is stable, will have to be taken into account from now on,” says Kvashnina. “It will change, for sure, the theoretical predictions of plutonium behavior in the environment over a period of million years.”
The research was published in the journal Angewandte Chemie.
Source: European Synchrotron Radiation Facility
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