Rusty Nanoparticles Learn New Trick

Issue 1 and Volume 2.

By Ken Kingery, science writer, University of Idaho

Developed in 2005 for biomedical applications, iron nanoparticles featuring a thin shell of iron-oxide have found a new application in reprocessing spent nuclear fuel.

One of the most common points of opposition to the American nuclear renaissance is nuclear waste, or more specifically, what to do with it. Critics point to issues with the Yucca Mountain storage facility, for example, the U.S. Department of Energy’s planned repository for spent nuclear fuel rods and solidified high-level radioactive waste.

But there are other solutions. Reprocessing spent fuel, for instance, is one of the most promising.

Besides providing more fuel for power plants, reprocessing could potentially reduce the volume of high level nuclear waste 100 fold. The Department of Energy recognizes the need for reprocessing technology and recently funded numerous projects to close the nuclear fuel cycle.

One project that received DOE funding comes from a collaboration between the University of Idaho and Lawrence Berkeley National Laboratory (LBNL). The scientists plan to use highly magnetic nanoparticles (MNPs) that were originally devised for biomedical purposes to extract radioactive actinides from spent nuclear fuel. If successful, the project will kill three birds with one nanoparticle: recovering usable nuclear fuel, making nuclear waste easier and safer to dispose of and accomplishing both in an environmentally friendly way.

“To achieve energy independence, America will likely build many new nuclear power plants,” said Andrzej Paszczynski, associate professor of microbiology, molecular biology and biochemistry at the University of Idaho and one of three principle investigators on the project. “More plants mean more waste, which must be reduced and recycled. That’s why we’ve believed in this project from the very beginning.”

Several reprocessing technologies already exist and are employed in massive-scale projects. The best example may be France’s La Hague plant, which handles more than 1,000 metric tons of spent nuclear fuel every year from France’s 59 reactors as well as power plants in Germany, Switzerland, Japan, Belgium, Italy and the Netherlands. La Hague uses an optimized version of the PUREX method, which employs nitric acid to dissolve irradiated fuels, followed by organic solvents to extract the uranium and plutonium.

Although this technique has certainly proven itself on a large scale, it does have its problems. Without breeder reactors to break down the most long-lived elements in nuclear waste, the process does not come close to extracting all that is available to recycle. Also, the process creates secondary waste that is hazardous to the environment and expensive to handle.

Other methods such as precipitation and ion exchange have proven to produce impure products and require expensive and complex equipment, respectively.

“Traditional methods like solvent extraction and ion exchange are not convenient as they stand today,” said Linfeng Rao, a senior scientist specializing in radiochemistry at LBNL and another PI for the project. “One of the main aspects of our research is to create a convenient solution to recycling spent nuclear fuel.”

Enter the MNP

The idea is to make a conjugate of MNPs and ligands that grabs the nuclear actinides and pulls both out of solution using a magnetic field. Even though there is a long road to travel to bring the idea to fruition, many steps have already been taken to make the idea a reality.

The first step was to create MNPs with a large magnetic moment, which was originally accomplished with biomedical applications in mind.

“The original idea was to functionalize the MNPs to address a specific disease, such as cancer,” said You Qiang, associate professor of physics at the University of Idaho and the third principal investigator of the new research project. “Then a magnetic sleeve could be applied so all the MNPs would stay in the place where they are needed instead of systematically wandering the body.”

Additional ideas for the MNPs’ biomedical purposes included using them to improve cancer detection with magnetic resonance imaging (MRI) and having them act as miniature heaters to attack malignant cells or as high precision drug delivery devices.

The biggest obstacle was to create an MNP with a sufficiently large magnetic moment so that a prohibitively large magnetic field would not be required to control their movement. Such large magnets would be both cumbersome and expensive. Pure iron was too unstable to use because it easily oxidizes into rust. And while commercial industries had created iron-oxide MNPs, their magnetic moment was much too small.

Qiang and Pasczcynski found a solution in 2005, which was to create a thin layer of iron-oxide around a core of pure iron. The resulting “core shell” MNPs were perfect. They can be created in sizes ranging from 2 to 100 nanometers in diameter and have a magnetic moment four to five times that of the aforementioned existing commercial products.

The next step to using these core-shell MNPs to reprocess nuclear fuel is to chemically bind them to the nuclear actinides to pull them out of solution. This is where the expertise of both Paszczynski and Rao comes into play.

The duo has already identified a way to functionalize the surface of the MNPs so that a chelator can “grip” both the MNP and the nuclear actinide. This research was done prior to the recent National Science Foundation grant in support of medical applications. In fact, two papers have been published on the topic, including one this past year in The Journal of Nanoparticle Research titled, “Novel method for immobilization of enzymes to magnetic nanoparticles.”

The third step in the research process, which has also been completed, is identifying a chemical chelator to selectively bind to the desired radioactive actinides. According to Pasczcynski, the general name for the chemical they will use is alkyl-oxa-diamides. In addition to working well for the project, the chelator is environmentally friendly.

In fact, once a process has been developed to remove the conjugate from solution and separate the MNPs from the actinide, the entire process will be very environmentally friendly. The chemicals used consist of only carbon, hydrogen and oxygen. And since the MNPs can be used over and over, it will also be cost-effective.

Next Phase: A Strong Bond

Despite all of the advancements that have been made, there is still a long way to go. Releasing the actinides is a problem that will likely involve some form of heating to destroy the chemical bonds, but the group has not experimented on this phase of the project yet. And though the chelators used have a firm grasp on the actinides, the strength of the bond between the chelator and MNPs is unknown.

“The success of this project depends on a few key factors,” said Rao. “How strongly can this ligand attach to the MNP and can it stay attached in very harsh chemical conditions? Can we get the loading capacity of the MNPs high enough? And can we dissolve the ligands to reuse the MNPs? These are pretty important questions and we’re conducting experiments to study how we could solve all of these questions.”

With $732,000 over the next year from the Department of Energy, they will be working to make progress in these areas. Though the problems are difficult and span a wide web of scientific fields, each member of the trio has extensive experience in his respective field and they have bonded together as strongly as they hope the MNPs and actinides will by the end of the project.

“There’s a reason we needed a nuclear chemist, physicist and organic biochemist for the project,” said Paszczynski. “Without each, we couldn’t even design the right experiment.”

“To solve today’s problems, not only do you have to have a good idea, you have to be willing to work together with other scientists and institutions,” said Qiang. “We have matured into a cohesive team and hope this project will last for many years.”

Author: Ken Kingery is a science writer currently writing for the University of Idaho. Educated in aerospace engineering at the Ohio State University and journalism at Indiana University, he has written about science for both his alma maters and the Stanford Linear Accelerator Center.