A team of international researchers are investigating a new metal-organic framework (MOF) material that might help in nuclear fuel recycling and waste reduction by capturing volatile radionuclides such as xenon and krypton that evolve into reprocessing facility off-gas in parts per million concentrations. An open-access paper on their work is published in the journal Nature Communications.
Conventional technologies to remove these radioactive gases operate at extremely low, energy-intensive temperatures. By working at ambient temperature, the new material has the potential to save energy, make reprocessing cleaner and less expensive. The reclaimed materials can also be reused commercially.
The work is a collaboration between experimentalists and computer modelers exploring the characteristics of MOFs.
This is a great example of computer-inspired material discovery. Usually the experimental results are more realistic than computational ones. This time, the computer modeling showed us something the experiments weren’t telling us.—Praveen Thallapally of the Pacific Northwest National Laboratory
Waste avoidance. Recycling nuclear fuel can reuse uranium and plutonium—the majority of the used fuel—that would otherwise be destined for waste. Researchers are exploring technologies that enable safe, efficient, and reliable recycling of nuclear fuel for use in the future.
Thallapally, working with Maciej Haranczyk and Berend Smit of Lawrence Berkeley National Laboratory and others, has been studying MOFs that could potentially trap xenon and krypton without having to use cryogenics.
These materials have tiny pores inside, so small that often only a single molecule can fit inside each pore. When one gas species has a higher affinity for the pore walls than other gas species, metal-organic frameworks can be used to separate gaseous mixtures by selectively adsorbing.
To find the best MOF for xenon and krypton separation, computational chemists led by Haranczyk and Smit screened 125,000 possible MOFs for their ability to trap the gases. Although these gases can come in radioactive varieties, they are part of a group of chemically inert elements called noble gases. The team used computing resources at NERSC, the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility at LBNL.
Identifying the optimal material for a given process, out of thousands of possible structures, is a challenge due to the sheer number of materials. Given that the characterization of each material can take up to a few hours of simulations, the entire screening process may fill a supercomputer for weeks. Instead, we developed an approach to assess the performance of materials based on their easily computable characteristics. In this case, seven different characteristics were necessary for predicting how the materials behaved, and our team’s grad student Cory Simon’s application of machine learning techniques greatly sped up the material discovery process by eliminating those that didn’t meet the criteria.—Maciej Haranczyk
The team’s models identified the MOF that trapped xenon most selectively and had a pore size close to the size of a xenon atom—SBMOF-1, which they then tested in the lab at PNNL.
After optimizing the preparation of SBMOF-1, Thallapally and his team at PNNL tested the material by running a mixture of gases through it—including a non-radioactive form of xenon and krypton—and measuring what came out the other end. Oxygen, helium, nitrogen, krypton, and carbon dioxide all beat xenon out. This indicated that xenon becomes trapped within SBMOF-1’s pores until the gas saturates the material.
Other tests also showed that in the absence of xenon, SBMOF-1 captures krypton. During actual separations, then, operators would pass the gas streams through SBMOF-1 twice to capture both gases.
The team also tested SBMOF-1’s ability to hang onto xenon in conditions of high humidity. Humidity interferes with cryogenics, and gases must be dehydrated before putting them through the ultra-cold method, another time-consuming expense. SBMOF-1, however, retained more than 85% of the amount of xenon in high humidity as it did in dry conditions.
The final step in collecting xenon or krypton gas would be to put the MOF material under a vacuum, which sucks the gas out of the molecular cages for safe storage. A last laboratory test examined how stable the material was by repeatedly filling it up with xenon gas and then vacuuming out the xenon. After 10 cycles of this, SBMOF-1 collected just as much xenon as the first cycle, indicating a high degree of stability for long-term use.
Thallapally attributes this stability to the manner in which SBMOF-1 interacts with xenon. Rather than chemical reactions between the molecular cages and the gases, the relationship is purely physical. The material can last a lot longer without constantly going through chemical reactions, he said.
Researchers hailed from several other institutions as well as those listed earlier, including University of California, Berkeley, Ecole Polytechnique Fédérale de Lausanne (EPFL) in Switzerland, Brookhaven National Laboratory, and IMDEA Materials Institute in Spain. This work was supported by the Department of Energy Offices of Nuclear Energy and Science.
Debasis Banerjee, Cory M. Simon, Anna M. Plonka, Radha K. Motkuri, Jian Liu, Xianyin Chen, Berend Smit, John B. Parise, Maciej Haranczyk, & Praveen K. Thallapally (2016) “Metal-Organic Framework with Optimal Adsorption, Separation, and Selectivity towards Xenon”, Nature Communications doi: 10.1038/ncomms11831