Most of the world’s electricity is produced using a rather simple method. We heat water to make high-pressure steam, and we release that steam to turn a generator. Voila, electricity. A variable part of that process is: How do we create that initial heat? Currently, we mostly burn coal and natural gas, but because we need to drastically reduce greenhouse emissions, those aren’t viable options long term. Alternatively, there is nuclear energy. Nuclear reactors also generate heat (quite a lot of it, in fact), and produce electricity the exact same way. Nuclear generators are a consistent source of electricity, unlike solar panels and wind turbines, and are an important candidate for carbon-neutral power generation.
Unfortunately, as the horrific accidents at Chernobyl and Fukushima have shown, considerable long-lasting dangers come with nuclear power. The radioactive fuel and waste potentially remain hazardous for thousands of years. These extreme pros and cons of nuclear power make it a great candidate for basic scientific research. Researchers in the Materials Science of Actinides (MSA), an Energy Frontier Research Center, are investigating fundamental ways to make nuclear power safer from the fuel-production process all the way to the storage of nuclear waste.
Thousand-year perspective and waste stability. How do we create a waste product that won’t change, so we can store it without fear of harmful compounds leaking into the environment? Storing nuclear waste is not as straightforward as taking used fuel rods and putting them in a barrel. We can do things to the contents of those rods to make them safer. Xiaofeng Guo and Alexandra Navrotsky from the MSA look at one way to do that by storing dangerous waste elements, such as uranium, within a stable crystal compound called garnet.
A crystal is made of many repeating units, aptly named unit cells, stacked in all directions. To represent a large crystal, we don’t have to picture every atom. We only have to show one unit cell. The particular garnet compound Guo and Navrotsky are investigating is yttrium iron garnet, Y3Fe5O12, the unit cell for which is shown below.
In the yttrium iron garnet unit cell, the vertices of the various polyhedrons represent oxygen atoms. The centers of the white and black polyhedrons are iron atoms, and the centers of the blue polyhedrons are yttrium atoms. In their study, Guo and Navrotsky investigated how to replace some of the yttrium atoms with cerium and thorium, which act as surrogates for uranium.
Substituting one atom for another can be complicated because of electric charge. Oxygen is quite good at attracting electrons away from metal atoms such as iron, yttrium, cerium, or thorium. So the metal atoms in the garnet crystal are not actually charge neutral. They are positively charged, and yttrium prefers to have a state of +3 (it gives up three electrons to nearby oxygen atoms), and cerium and thorium like to be +4. If we were simply to replace the yttrium ions with cerium or thorium ions, we would be forcing cerium and thorium to adopt an unfavorable +3 state.
Guo and Navrotsky instead did something called “charge-couple substitution.” They substituted two yttrium +3 ions, for one calcium +2 ion and one cerium/thorium +4 ion. That maintained charge neutrality and let every metal ion remain in its preferred state. By doing this, they replaced up to 35 percent of the yttrium with cerium or thorium. Most importantly, even with substitution levels that high, Guo and Navrotsky found that the garnet structure was still a fundamentally stable compound, meaning it could store a considerable amount of nuclear waste without fear of it changing form.
Guo said their work to find more crystal hosts that can incorporate uranium and other waste elements is still ongoing. “We are continuing to look at other ceramics for even better improvement of uranium storage in the future,” he said.
Fuel stability. The stability of compounds needed for making the nuclear fuel is also a safety focus. A critical ingredient for making fuel rods is uranium concentrate, colloquially called “yellowcake.” Drums of yellowcake have become pressurized and when opened have released small amounts of uranium, endangering industry workers. Peter Burns and his collaborators have looked into the fundamental reasons for this drum pressurization.
Upon first inspecting the contents of these yellowcake drums, Burns' team found a mysterious amorphous compound that reacted with water to release oxygen gas. In contrast to a crystal, the atoms in an amorphous structure tend to be distributed randomly. Amorphous compounds do not have a unit cell, which makes it very difficult to identify the atoms and their positions. As Burns described it, “The scientific problem was to determine the composition and structure of the amorphous phase, and why it was reactive and released oxygen.”
For the team, the first step was to re-create the amorphous compound from the ingredients found in yellowcake. In doing so, they not only could explain why the amorphous compound was there, but they also could learn its composition. In their studies, they found that studtite, [(UO2)(O2)(H2O)2](H2O)2, a critical ingredient used to make yellowcake, reacted when heated to make an amorphous phase of U2O7.
They next turned to figuring out the positions of those uranium and oxygen atoms. As this is very hard to do for amorphous compounds, the scientists creatively combined experimental measurements with computer simulation. First, they scattered high-energy neutrons off the amorphous material to measure the bond lengths between atoms. By measuring how the neutrons deflect off the atoms, they could determine on average how far away the atoms were from one another. The team could not know which atoms in U2O7 were bonded to which, but they could know the distance separating them. They then used computer modeling to come up with energetically favorable structures for U2O7 that contained bonds of only the right lengths! The modeled structure is shown below, above a picture of yellowcake. The yellow atoms are uranium and the red atoms are oxygen.
In looking at this structure, Burns and colleagues saw that there were two oxygen atoms, circled in black, bonded to one another that easily could be released to produce oxygen gas. The story came full circle. The amorphous substance in the yellowcake drums likely contained U2O7, which, because of its structure, reacted with water to produce gaseous oxygen. The oxygen in turn increased the pressure within the drums. Although they cannot know for sure, by using experiments, computer simulations, and impressive detective work, Burns and colleagues developed a compelling explanation for how industrial drums of yellowcake might pressurize.
As Burns said, “Although our efforts fall in the realm of basic science, we were very well prepared to respond to the yellowcake drum pressurization incidents quickly and effectively. It is sort of a story about closing the loop between basic research and much more applied work.”
From the lab to the power plant. Fundamental research on nuclear power often bridges that gap between basic understanding and real-world application. Whether the focus is on substituted garnet crystals or a mysterious amorphous substance, gaining insight into uranium-containing compounds is critical for developing new technologies in nuclear power. Researchers in the MSA are discovering new ways to make nuclear power a safer and better understood carbon-neutral technology.
