U.S. Researchers Dig Deep to Prevent Spread of Nuclear Waste

Zachary Murphy, a summer intern with LLNL’s Glenn T. Seaborg Institute, analyzes underground samples from an organic-rich rock formation to determine the location’s potential as a long-term nuclear waste repository. Photo courtesy of LLNL.

The groundbreaking discoveries and scientific advancements that take place at Lawrence Livermore National Laboratory (LLNL) (Livermore, California, USA) and across the broader national laboratory system rely on passing information from tenured staff scientists to new interns and early career scientists.

In the summer of 2023, Zachary Murphy, a Ph.D. student studying chemistry at the University of Central Florida, interned with LLNL’s Glenn T. Seaborg Institute under the mentorship of Gauthier Deblonde, a radiochemist for the nuclear and chemical sciences division.

“Despite there being a persistent and global demand for radiochemists, there has been a decline in those entering the field,” Deblonde says. “Providing young and motivated students like Zach the opportunity to conduct research alongside Livermore staff scientists is key to bridging knowledge gaps in radiochemistry.”

Evaluating Organic-Rich Rock

Murphy’s summer research focused on evaluating an organic-rich rock formation. As part of his work, he aimed to characterize which minerals are present and how radioactive elements interact with different rock layers. This work is a part of a larger, ongoing project to evaluate new locations for a potential nuclear waste repository used to store legacy waste deep underground.

According to LLNL, the ideal repository site would have natural barriers to stop the nuclear waste from spreading into the environment since engineered barriers (such as metallic canisters) slowly break down over time due to corrosion and other natural phenomena.

Prior to Murphy’s internship, the research team drilled more than 100 meters underground to collect five core samples at different depths and two representative surface samples that had been exposed to environmental conditions.

Over the course of his 10-week internship, Murphy ran hundreds of leach and sorption tests with these samples—mixing rock powder with water, along with radioactive elements in some cases. From there, he analyzed which organic compounds are released by the rocks (leach tests) and how radioactive elements stick to the rocks (sorption tests).

Underground organic compounds like carbohydrates, lipids, proteins, and nucleotides come from organic matter consisting of plant/animal tissue in various stages of decomposition. The team suspects that the natural compounds released from the rocks may interact with the waste and impact the radioactive elements’ mobility in the environment.

How Sample Testing Works

In the laboratory, Zachary Murphy (front) and his mentor Gauthier Deblonde (back) discuss the results of the chemical analyses conducted on the underground rock samples using Raman spectroscopy (the equipment pictured on the black table). Photo courtesy of LLNL.

According to LLNL, the tests are similar to brewing coffee, where water is run through the coffee grounds, leaving grounds behind but taking the caffeine content with it. Except, in these experiments, researchers want to measure which sample releases the least amount of organic compounds into the surrounding water. In this analogy, a light coffee is better than a strong one.

For the leach tests, Murphy mixed the rock sample with water. He then separated the two layers from each other to measure how much organic matter is being released into the aqueous layer. His analyses revealed that, generally, rocks closest to the surface leave the most organic matter in the water, while deeper rocks leave the least amount. Surprisingly, it also was found that depth and amount of organic matter, as well as the nature of the compounds, do not directly correlate.

For the sorption tests, Murphy spiked the water with a radioactive isotope typically present in nuclear waste (examples include neptunium, plutonium, americium, curium, etc.), and then separated the two layers to see if the isotope sticks to the rock sample or remains in the water.

To avoid contamination, it is preferred for the isotope to stick to the rock so that it is not released into the water.

Ideas for Future Research

From these experiments, the team can make recommendations as to which rock layer is best for a nuclear waste repository. That is, researchers can determine if rock depth and/or rock composition affect whether radioactive elements stick to the rock or mix with the water. They can also determine what role soluble organic matter plays in these results.

“It has been great to get some radioactive material experience and to use the cool analytical techniques that are available at the Livermore Lab,” Murphy says. “It has even given me a lot of ideas and directions to explore further in my Ph.D. work.”

At the University of Central Florida, Murphy’s research is in the same vein as his LLNL work: looking at the mobility of uranium in the environment. By contrast, however, his university research focuses on how certain chemical compounds—known as chelating agents—may make elements like uranium more mobile. This could enhance their migration at disposal sites.

Source: LLNL, www.llnl.gov.

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