All fifteen previously known forms of the element plutonium are radioactive. This means that they are unstable and decay into lighter elements. Now an international team led by the Helmholtz Zentrum Dresden-Rossendorf (HZDR) has discovered a new compound of plutonium with a surprising pentavalent oxidation state. They made this discovery on the European Synchrotron in Grenoble, France (ESRF). The new phase of plutonium is solid and stable. It may be a transient phase in depositories of radioactive waste. The research was published in an article in the Angewandte Chemie journal.
There is global research going on to improve the safety of radioactive waste repositories. This work is aimed at preventing radioactive nuclides from being released into the environment. Research on movement of elements carried by ground water is part of this work. Plutonium can be carried for miles underground from contaminated sites in colloids.
Colloids are formed when a homogeneous non-crystalline substance is formed. Large molecules or tiny particles are dispersed through a second substance. These include gels, sols and emulsions. Particles in colloids do not settle out and they cannot be separated by ordinary filtering or centrifuging. The plutonium colloids are formed when plutonium combines with clay, iron oxides or natural organic materials.
A group of researchers led by HXDR studied the chemistry of actinides under condition similar to those found in the natural environment. The electronic and structural behavior of synthesized compounds were studied theoretically and with advanced synchrotron X-ray methods. During an experiment which appeared to fail, the new stable form of plutonium was discovered.
Kristina Kvashnina is a physicist with the HZDR based at the ROBL beamline ESRF, a beamline which is owned and operated by the HZDR. Her team were trying to synthesize plutonium dioxide nanoparticles with different precursors to be used in experiments at ROBL. When the Pu(VI) precursor was used, an unexpended reaction occurred during formation of the plutonium dioxide nanoparticles. Kvashnina said, "Every time we create nanoparticles from the other precursors Pu(III) or (IV) the reaction is very quick, but here we observed a weird phenomenon half way." She decided that it must be because of the formation of Pu (V) which is a pentavalent form of plutonium. This form of plutonium has never been seen before.
When a team of scientists from Moscow State University reviewed the data, they said, "A stable phase of Pu(V)! - no, it is impossible, it doesn't exist, the synthesis must have gone wrong." Kvashnina said that "Chemists were in complete disbelief, but the results were quite clear."
The only way to verify that she was dealing with the pentavalent compound would be to confirm it with using the HERFD at the Pu M4 edge. An edge in this context is the point where an absorption spectra has a sharp spike or discontinuity. She explained that, "Our choice of beamline was straightforward: the ESRF-ID26 beamline, as it is the best place, regarding the intensity and energy resolution, where such high-energy resolution X-ray absorption spectroscopy studies at low energies can be done. In fact the Pu M4 edge HERFD experiment was done at ID26 for the first time. To the best of our knowledge, HERFD data at the Pu M4 edge have never been reported in the literature and never been exploited."
The initial experiments confirmed her hypothesis. Three months later, more experiments confirmed that the new form of plutonium was stable. At the same time, theoreticians in Sweden were developing prediction of the features of the Pu M4 edge spectral features, theoreticians at ROBL worked to identify the species of the new phase of plutonium. Ultimately, these lines of research converged and confirmed the existence of the new Pu (V) phase.
Scientists have been working on predicting what happens to radioactive waste over millions of years. Kvashnina said, "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. It will change, for sure, the theoretical predictions of plutonium behavior in the environment over a period of millions of years."