麻豆蜜桃AV

If used nuclear fuel waste was stored in a football field, the 80,000 metric tons produced in the last century by the U.S.alone would fill the stadium up to 8 yards high. Of that waste, 96 percent of it is radioactive uranium. Which means scientists have a hefty task ahead of themselves — figuring out a safe and effective way to dispose of the materials.

Thanks to a three-year $785,000 research grant from the Department of Energy’s Nuclear Engineering University Program, Cal State 麻豆蜜桃AV Professor Ruth Tinnacher and her environmental geochemistry students are playing a role in finding a solution. Or at least a portion of it.

Currently, radioactive waste — a byproduct of nuclear power generation — is stored in containers at temporary storage sites or at one of the 80 plants nationwide where it is produced. However, time is running out, since the containers currently in use were only designed to last up to approximately 60 years.

“We don’t know how much longer we can get away with using what we have, but we also don’t have a good sense of what the fuel looks like inside, whether we’ll have to take it out, if we do that, where we’re going to do that,” Tinnacher said. “There are still a lot of questions to answer, and a huge body of research being done right now and we [at Cal State 麻豆蜜桃AV] are just focused on one small part.”

Undergraduate Nicolas Hall, who is one of the students working with Tinnacher in the Department of Chemistry and Biochemistry, said he was excited to hear about her research because it would give him a chance to practice what he's learning in the classroom and a field he’s passionate about — environmental science.

“It is science that is directed toward solving a real-world issue,” Hall said. “I feel that it is not too common to see the science being done in labs actually applied to real-world situations and questions, and storage of radioactive contaminants is a huge question.”

A HISTORY OF HURDLES

According to the United States Nuclear Regulatory Commission, the U.S. commercial power industry has generated more waste than any other country, and it is expected to increase to about 140,000 metric tons over the next several decades.

Tinnacher said when the U.S. storage program currently in place was designed in the 1970s and 1980s, the assumption was that the hurdles —  political, social and scientific — would be overcome and a long-term, deep geologic nuclear waste repository would be built. For about 40 years before then, everything from lifting waste into outer space or the sun by rocket to submerging it into deep seabeds was proposed.

Eventually, scientists proposed a solution of storing the waste in large containers deep in the subsurface at a site known as Yucca Mountain about 65 miles from Las Vegas, on the edge of the Nevada Test Site (now Nevada National Security Site), which had been used for nuclear weapons testing starting in 1951. But according to the U.S. Government Accountability Office, the Department of Energy terminated its license of Yucca Mountain in 2010 before it even opened. Since then, there has been no consensus between the administration and Congress on how to move forward.

So while politicians are sorting out the question of where to store the waste, Tinnacher and hundreds of other scientists nationwide are attempting to answer the question of “how.”

“The Department of Energy is strongly linked to the current political priorities in DC,” she said. “For a long time it seemed like this was a problem of high priority, then the emphasis shifted to climate change related questions, and now it seems like we may be going back [to finding a solution.]”

TESTING A BARRIER  

In the lab at Cal State 麻豆蜜桃AV, Tinnacher and her students are focusing on the role of the engineered barrier that would surround a container of radioactive waste and protect the surrounding area from contamination after the canisters erode (a process that is currently estimated to take about 4,000 years.)

The barrier is a layer of material known as bentonite, the same material used on highways and other waste disposal sites. Bentonite contains a clay material known as montmorillonite which is highly effective at binding radioactive contaminants and has a very low permeability, which means any radioactive contaminants escaping the proposed containers would move incredibly slow through the barrier layer. However, how much of these contaminants can move through this barrier is largely determined by their binding (sorption) to the clay matrix in the bentonite, which is again dependent on the specific chemical form (or species) the contaminants are found at in these systems.

How the binding of uranium to the clay is affected by the presence of mineral impurities in bentonite and the heat produced by the radioactive decay of spent nuclear fuel is the question that Tinnacher and her students, including graduate student Jonathan Pistorino, are seeking to answer.

“The overall goal of the project is how best to store uranium … we need to find out what species of uranium is in the waste material based on the chemical conditions of that material and how the intended barrier materials might impact the story,” Pistorino said. “It’s problem-solving, this type of science is about figuring out the puzzle.”

To test uranium sorption to these materials as a function of pH, a small amount of bentonite or montmorillonite is placed in a tube and adjusted to a specific pH. From there, uranium is added, pH is adjusted again, and the entire tube is placed on a shaking table for about 48 hours to allow the sorption to occur.

Afterward, the pH is measured again, and with a bit of simple math, the sorption level determined. The data gathered will determine how effective the bentonite barrier will be at various chemical solution conditions, in the presence of mineral impurities and after bentonite had been exposed to heat in the subsurface.

“We know how much uranium we put in, we check how much is left, and the difference is how much has been sorbed,” Tinnacher said. “From there, we change the conditions and do it again.”

LOOKING FORWARD

In the coming months, Tinnacher and the students will continue their current experiments while simultaneously watching the national dialogue about where a site may be built. They’ll also begin working with scientists at two national laboratories — Dr. Peter Nico and Patricia Fox at Lawrence Berkeley and Dr. Florie Caporuscio at Los Alamos, which the students say is a great opportunity for them to work with the scientists they hope to one day become.

“A major benefit I’m getting out of this is the experience and hands-on time with the instruments and techniques,” Hall said. “[I think] having this experience will help me greatly as I move on and work in labs professionally or as I continue my academic career.”

Since the project is funded through the DOE’s Nuclear Engineering University program, the department encourages recipients to collaborate with industry and governmental partners. Before working at Cal State 麻豆蜜桃AV, Tinnacher was a scientist at Lawrence Berkeley National Lab. So when she planned the project she knew the lab, and former collaborators at other institutions were natural partners.

Also, this summer a French scientist named Christophe Tournassat will work alongside Tinnacher, Pistorino and others to help turn the data they've gathered into uranium sorption and transport models that can predict uranium mobility in bentonite barriers in the future. The models will be further supported by spectroscopic analysis of uranium surface species by Department of Earth and Environmental Sciences Professor Michael Massey and the molecular dynamics simulations of uranium solution species in the clay environment will be performed by Department of Chemistry and Biochemistry Professor Patrick Huang.

Next, collaborators at Lawrence Berkeley Lab will test uranium mobility in these systems based on diffusion experiments. Caporuscio at Los Alamos National Lab will expose minerals to heat under controlled conditions to see if and how it affects the bentonite’s ability to sorb uranium.

All in all, more than a dozen of scientists — both students and professionals — will have worked on the project, exemplary of the collaboration Tinnacher believes will be the key to solving the question of where and how the U.S. will store its radioactive waste.