A molecular model of SBMOF-1. Image: PNNL.
A molecular model of SBMOF-1. Image: PNNL.

Researchers are investigating a new material that might aid in recycling nuclear fuel by capturing certain gases released during reprocessing. Conventional technologies to remove these radioactive gases operate at extremely low, energy-intensive temperatures. By working at ambient temperatures, the new material has the potential to save energy, and make the reprocessing of nuclear fuel cleaner and less expensive. The reclaimed materials can also be reused commercially.

The work, which is reported in a paper in Nature Communications, is a collaboration between experimentalists and computer modelers exploring the characteristics of materials known as metal-organic frameworks (MOFs).

"This is a great example of computer-inspired material discovery," said materials scientist Praveen Thallapally at the Department of Energy (DOE)'s Pacific Northwest National Laboratory. "Usually the experimental results are more realistic than computational ones. This time, the computer modeling showed us something the experiments weren't telling us."

Recycling nuclear fuel involves extracting uranium and plutonium – the majority of the used fuel – that would otherwise be destined for waste. Researchers are exploring technologies that allow the safe, efficient and reliable recycling of nuclear fuel for use in the future.

A multi-institutional, international collaboration that includes PNNL has been studying materials to replace costly, inefficient recycling steps. One important step is collecting the radioactive gases xenon and krypton, which are produced during reprocessing. To capture xenon and krypton, conventional technologies use cryogenic methods, in which entire gas streams are brought to a temperature far below where water freezes – such methods are energy intensive and expensive.

Working with Maciej Haranczyk and Berend Smit from the Lawrence Berkeley National Laboratory (LBNL) and others, Thallapally has been investigating MOFs that could potentially trap xenon and krypton. MOFs contain tiny pores that can often only house a single molecule. When one gas species has a higher affinity for the pore walls than other gas species, MOFs can be used to separate gaseous mixtures through selective adsorption.

To find the best MOF for separating xenon and krypton, computational chemists led by Haranczyk and Smit screened 125,000 possible MOFs for their ability to trap the gases. The team used computing resources at the National Energy Research Scientific Computing Center (NERSC), 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," said Haranczyk. "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."

The team's models identified a MOF called SBMOF-1 that appeared to trap xenon most selectively and had a pore size close to the size of a xenon atom. Thallapally 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, indicating 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 so gases must be dehydrated before putting them through the ultra-cold method, which is another time-consuming expense. SBMOF-1, however, performed quite admirably, retaining more than 85% of the 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 to suck the gas out of the molecular cages for safe storage. So in a final laboratory test, Thallapally and his colleagues examined how stable the material was by repeatedly filling it up with xenon gas and then vacuuming the gas out. After 10 cycles of this, SBMOF-1 could collect just as much xenon as in 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 occurring between the molecular cages and the gases, the relationship is purely physical. The material can last a lot longer without constantly partaking in chemical reactions, he said.

Although the researchers showed that SBMOF-1 is a good candidate for nuclear fuel reprocessing, getting these results wasn't smooth sailing. In the lab, the researchers had initially followed a previously worked out protocol from Stony Brook University for synthesizing SBMOF-1. Part of that protocol required them to ‘activate’ SBMOF-1 by heating it up to 300°C.

This activation step cleans out any material left in the pores from the synthesis process. Laboratory tests of the activated SBMOF-1, however, showed that the material didn't behave as well as it should, when compared with the computer modeling predictions.

So the researchers at PNNL repeated the lab experiments, but this time they activated SBMOF-1 at 100°C. Subjecting the material to the same lab tests, the researchers found that SBMOF-1 now behaved as expected, and better than when exposed to the higher activation temperature.

But why? To figure out where the discrepancy came from, the researchers modeled what happened to SBMOF-1 at 300°C. Unexpectedly, they found that the pores squeezed in on themselves. "When we heated the crystal that high, atoms within the pore tilted and partially blocked the pores," said Thallapally. "The xenon doesn't fit."

Armed with these new computational and experimental insights, the researchers can now conduct further investigations into the use of SBMOF-1 and other MOFs for nuclear fuel recycling.

This story is adapted from material from the Pacific Northwest National Laboratory, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.