Plutonium: Advancing Our Understanding To Support Sustainable Nuclear Fuel Cycles

With Global energy needs increasing, we need immediate, real energy solutions to meet demands. Fossil fuels are not an ideal candidate because of their negative impact on the environment. And while renewables such as wind and solar have huge potential, they still need major technological advancements (particularly in the area of battery storage) before they can effectively meet growing world needs. The best option for meeting large energy needs without a large carbon footprint is nuclear energy. Of course, nuclear energy has faced a fair amount of opposition and concern. However, today we’re better equipped to address these concerns using modern engineering and science.

Perhaps the biggest area of concern is what to do with the used nuclear fuel after it is removed from the reactor. Currently, the United States (and several other countries) utilize an open fuel cycle, meaning fuel is used only once and then discarded. It’s worth noting that fuel coming out of a reactor has utilized approximately 1% of the total energy that could be produced by the uranium (U) in the fuel rod—so roughly 99% of the useable fuel is wasted.

The answer here is to close the fuel cycle and recycle the nuclear materials. By reprocessing used nuclear fuel, all the U can be repurposed without requiring disposal. The various fission products can be removed and either discarded or utilized in specialty reactors to make more energy or immensely beneficial research/medical isotopes. While reprocessing technology is currently advanced enough to meet energy needs, completing research to improve and better understand these techniques is still immensely beneficial.

Better understanding the behavior of fission products is one area of important research. Despite being discovered over 75 years ago, plutonium (Pu) is still an exciting element to study because of the complex solution chemistry it exhibits. In aqueous solutions, Pu can exist simultaneously in multiple oxidation states, including 3+, 4+, and 6+. It also readily forms a variety of metal-ligand complexes depending on solution pH and available ligands. Understanding of the behavior of Pu in solution remains an important area of research today, with relevance to developing sustainable nuclear fuel cycles, minimizing its impact on the environment, and detecting and preventing the spread of nuclear weapons technology.

Simply quantifying Pu in aqueous solution can be difficult because of the complex behavior it exhibits. While a number of tools are available for characterizing these systems, ultraviolet-visible (UV-vis) absorption spectroscopy, is one of the most useful options. This technique involves shining broad-spectrum light through a sample and determining which wavelengths of light the sample absorbs.

Different analytes will absorb different wavelengths, creating identifiable “fingerprints” for each analyte. UV-vis data can be collected quickly and, with some engineering, systems can be easily designed to measure Pu concentrations in-situ using flow-through sample cells, immersion probes, etc. Most importantly, UV-vis can provide detailed data on oxidation states (e.g. Pu with different numbers of electrons) and metal-ligand species (e.g. Pu complexed with one, two, four or six nitrate molecules) present, which is a capability not shared by many other fast, in-situ techniques.

Unfortunately, UV-vis absorbance can be a difficult technique to utilize on complex (multicomponent) samples where overlapping fingerprints of the different components interfere with each other. An excellent example of this can be seen with Pu(IV), which exhibits different metal-ligand speciation depending on its solution environment; this manifests itself as a drastically changing spectral fingerprint. Figure 1 depicts the UV-vis spectra of a 15 mM Pu(IV) solution in 0.5 to 10 M nitric acid (HNO₃), which is a common acid used in Pu processing.

Figure 1: Left) 15 mM Pu(IV) in 0.5-10 M HNO3; figure taken from Lines, A. M.; Adami, S. R.; Sinkov, S. I.; Lumetta, G. J.; Bryan, S. A. Anal Chem 2017. Top Right) pictures of Pu(IV) in solution, Bottom Right) pictures of glove bag and glove box systems.

The shifting peak positions illustrated in Figure 1 make it prohibitively difficult to use traditional methods to accurately characterize Pu(IV) in a system where the nitric acid concentration varies. Such situations are common in real-world processing of solutions containing Pu. Beer’s Law analysis is a commonly used method of quantifying species based on UV-vis spectra and involves relating the absorbance observed at a single wavelength to analyte concentration.

For Pu(IV), the degree to which light is absorbed at a given wavelength depends upon the nitric acid concentration. If the nitric acid concentration varies, a separate measurement of the nitric acid concentration is required for each measurement which is a time-consuming process and exposes the researcher to more radiation dose from the Pu solutions. Because of this, Beer’s Law or related forms of single-variate analysis are not effective for determining systems like Pu(IV) in nitric acid.

Fortunately, new developments in the areas of multivariate analysis can allow for advanced analysis of complex spectral data. This form of analysis, called chemometrics, utilizes the entire spectrum instead of a single point or wavelength. Chemometric analysis utilizes spectral training sets to build mathematical models that identify the regions of a spectrum that are important for identifying or quantifying the target analyte. Figure 2 provides a graphical example of how these models “look” at data.

Chemometric models are made from spectral training sets that capture anticipated spectral variation within a chemical system. These models extract latent variables (or principal components) that essentially map out spectral regions of importance. In the case of the model for Pu(IV), four latent variables were identified and are plotted in Figure 2, where larger magnitude on the y-axis (loading) indicates how important spectral response is at that wavelength for the identification/quantification of Pu(IV).

What is interesting in this case, is how the latent variables capture the solution chemistry of Pu(IV). As nitric acid concentration increases, increasing numbers of nitrate ions bind to the Pu(IV) cation forming species that have different spectral response. The latent variables plotted above look very much like the theoretical pure spectra of the four Pu species anticipated across this acid range: mononitrato being predominant below 1 M HNO₃, dinitrato being the most common species from 2-5 M HNO₃, tetranitrato dominating at 6-7 M HNO₃, and the hexanitrato species being the most common above 8 M HNO₃.

Figure 2: top) latent variables identified by a multiplicative curve resolution (MCR) model that also correspond to the spectral fingerprints of the various species of Pu(IV), focused on primary Pu band around 480 nm; Bottom) plot of which components (species) are present at the various acid strengths studied; figure taken from Lines, A. M.; Adami, S. R.; Sinkov, S. I.; Lumetta, G. J.; Bryan, S. A. Anal Chem 2017.

Plutonium is typically not limited to the 4+ oxidation state in aqueous solutions. It tends to disproportionate and split between Pu(III), Pu(IV), and Pu(VI) (in the form of PuO₂²⁺). The electrochemical behavior of Pu can make the solution speciation of Pu(IV) described above appear simple by comparison. It has been studied by a number of approaches but more recently spectroelectrochemistry has been used to provide a more complete understanding of Pu behavior.

Spectroelectrochemistry is the combination of spectroscopy and electrochemistry where spectral signatures are monitored while applied potentials are used to control the oxidation state of species in solution. In this way, spectroscopy can be used to determine speciation of analytes in solution (as with the mono, di, tetra, and hexanitrato species of Pu (IV) above), the concentration of species, and other factors of interest while oxidation state is controlled. An example of this is shown in Figure 3 where a solution of Pu was monitored as it was oxidized from Pu(III) to Pu(IV) in a series of steps.

In this experiment, applied potential could be reversed to demonstrate that the Pu (III)/(IV) couple is reversible. This experiment was repeated from 1 to 6 M HNO₃ with results showing the reversibility of the couple decreased as acid concentration increased, meaning that at higher nitric acid concentration it became harder to reduce Pu to the 3+ state and maintain it in that state. This observation was consistent with prior literature that points out how easily Pu(III) is oxidized to Pu(IV) in nitric acid solutions, with Pu(III) being completely unattainable in solutions of 6 M HNO₃ and above.

Figure 3: Top Left) UV-vis spectra as a function of time while applied potential is stepped from reducing to oxidizing (see bottom left plot for potential step); Middle Left) overlay of absorbance of primary Pu(III) and Pu(IV) from the top plot; figure taken from Lines, A. M.; Adami, A.J. Casella; S. R.; Sinkov, S. I.; Lumetta, G. J.; Bryan, S. A. Electroanalysis 2017. Right) vials containing Pu in the 3+ and 4+ oxidation states.

Spectroelectrochemistry was also used to look at the (IV)/(VI) couple of Pu. In Figure 4 below, Pu was stepped up from Pu(III) to Pu(IV), a reversible couple, and then stepped up further to form Pu(VI). The (IV)/(VI) couple is irreversible where Pu(VI) cannot be converted back to P(IV) by applied potential alone. In fact, very low potentials had to be applied to the solution to convert remaining Pu(IV) to Pu(III) which could then react with Pu(VI) to generate Pu(IV).

Figure 4: Top Left) UV-vis spectra of solution as applied potential is stepped from 100 to 1500 mV; Middle Left) overlay of primary Pu bands as applied potential is stepped from reducing to oxidizing according to the potential profile proved in the bottom plot; figure taken from Lines, A. M.; Adami, S.R.; Casella, A.J.; Sinkov, S. I.; Lumetta, G. J.; Bryan, S. A. Electroanalysis 2017. Right) Picture of a solution containing Pu(VI)

Overall, Pu solution chemistry and electrochemistry is a fascinating area of study. The work described here focuses on HNO₃ based systems but solution behavior is highly dynamic in other media as well. This is an area that allows for complex study of metal-ligand interactions, oxidation state effects, and more that can help develop a fundamental understanding of the chemistry of reprocessing nuclear materials and enable improved technologies to support the worlds growing energy needs.

These findings are described in the articles entitled Multivariate Analysis for Quantification of Plutonium(IV) in Nitric Acid Based on Absorption Spectra, recently published in the journal Analytical Chemistry, and Electrochemistry and Spectroelectrochemistry of the Pu (III/IV) and (IV/VI) Couples in Nitric Acid Systems, recently published in the journal Electroanalysis. The lead author was Amanda Lines, and work was completed a by a team of scientist and engineers including S.R. Adami, A.J. Casella, S.I. Sinkov, G.L. Lumetta, and S.A. Bryan and the Pacific Northwest National Laboratory under funding provided by the Nuclear Energy branch of the Department of Energy.