Cesium Removal: Solutions to a Chemically Complex Problem
By Jesenia Hernández
Newswise – Fifty-six million gallons. That’s the amount of radioactive waste left at the Hanford site following the government’s secret mission to provide plutonium for the world’s first atomic weapons and the Cold War that followed. Today, the Hanford site is known as one of the most technically complex environmental challenges in the world.
“The amount of old waste that needs to be treated and the cost of that is astronomical. It represents a huge amount of money and until this problem is resolved we will have to continue monitoring the reservoirs,” said Reid Petersonchemical engineer at Pacific Northwest National Laboratory (PNNL).
Peterson has spent nearly three decades working on tank waste issues for Department of Energy (DOE) Office of Environmental Management sites. It was part of a national response to prevent benzene gas burps in a waste tank at the Savannah River site from reaching flammability limits. He contributed to the development of different chemical separation techniques. But among its many contributions to the challenge of cleaning up chemically complex radioactive waste, one effort stands out above the rest: the capture of cesium-137.
Cesium 137 is mainly of human origin. It is found in large quantities in nuclear waste because it is a by-product of the manufacture of plutonium, a necessary step in the production of nuclear weapons. Scientists have figured out how to safely store this radioactive waste in glass, but before that can happen, some of the liquid waste in the tank must be treated to remove most of the cesium-137. This is because the type of radiation gamma it emits (an energy higher than that of x-rays) can penetrate through the human body and even through steel, making it too dangerous for workers to use and maintain the treatment technology used to manufacture low-level glass waste. That’s been Peterson’s challenge for more than a decade. And to date, thanks to research conducted by PNNL, Hanford staff have removed cesium from more than 697,000 gallons of tank waste, a significant milestone in cleanup progress at Hanford.
The evolution of cesium removal technology
In 2008, Peterson and other PNNL researchers successfully demonstrated in a pilot project that they could remove cesium using a system installed next to a nuclear waste tank. Connecting a drainage system directly to a single tank has proven to be a cost-effective approach.
The demonstration proved important when three years later, an earthquake and resulting tsunami caused a nuclear meltdown at the Fukushima Daiichi nuclear power plant in Japan. Cesium removal technology had to be accelerated and deployed quickly in response to the accident.
“A few days after the event, I went to Washington to review what technology should be used,” Peterson said. “I have been to Fukushima several times to review their cesium removal technologies. We were walking past the reactors that had exploded and my dosimeter was going off as we walked past because there was so much radiation.
The team received a DOE Secretary’s Award in 2011 for this answer.
The Fukushima cleanup efforts served as a catalyst for the deployment of similar systems at the Savannah River site and ultimately at Hanford. In real time, the world saw the effectiveness of the technology.
Cesium Removal, Five Gallons Per Minute
Peterson is the project leader who took cesium removal technology from laboratory scale to full-height demonstration, giving the tank farm operator confidence to carry out large-scale operations. On the Hanford site, it is called the Tank Side Cesium Removal (TSCR) system.
TSCR pretreats waste in a system built inside a shipping container, in which steel columns are placed inside using a forklift. Waste from the tank passes through a filter and flows into a column. Inside the column is an ion exchange medium, consisting of a mixture of silica and titanium as the main ingredients. The ion exchange medium looks like small white beads, and although small, they have a powerful power: to capture cesium.
“This thing loves cesium,” Peterson said of the ion-exchange media. “As the liquid passes through the filter and enters the column, the media absorbs most of it.”
It is a complex balance of getting the liquid flow speed just right so that the media has enough time to absorb the cesium.
Peterson and his PNNL team are mimicking TSCR on a smaller scale in a special laboratory set up at the Hanford site called the Radioactive Waste Test Platform.
“With the radioactive waste testing platform, we are confident that the TSCR is working as it is supposed to because we have all this laboratory data that perfectly matches the performance of the system,” he said.
Once the column is full, the system is paused and the column is replaced by another. The Hanford TSCR system has been in operation since January 2022. It can operate 24/7 at a rate of 5 gallons of pretreated waste per minute. But what happens to the waste once pre-treated?
From radioactive liquid to stable glass
TSCR is the first step in a larger goal to stabilize liquid glass waste, literally integrating it into the structure of the glass, using a process called vitrification. Hanford staff will use vitrification technology to mix pretreated waste with vitrifiable materials, heat it to more than 1,150°C in a high-temperature melter, and pour the molten glass into large steel containers where it will cool and dry. will solidify for long term disposal. .
“Before the Hanford Viticulture Plant starts, 800,000 gallons of tank waste must be pretreated and ready to operate,” Peterson said.
Pretreatment is a vital step for two main reasons: safety and cost.
“We want to be able to do contact maintenance on the equipment rather than having to do everything remotely,” Peterson said. “If the cesium was not removed first, a 6-foot-thick concrete shield wall would be required and the entire design concept would have to change, which would also result in higher costs.”
The Hanford Vit factory, officially called the Waste treatment and immobilization plant, is currently scheduled to enter service in 2025. While more than 697,000 gallons is a major milestone, it is only a small dent in the waste still awaiting pretreatment. A follow-up project could potentially accelerate the pre-processing process by taking the TSRC to a much larger scale.
“I started this career 29 years ago and I’ve stayed here because it’s a major problem to solve,” said Peterson, who was recently honored for his dedication to chemical engineering by the Division of Engineering. nuclear power of the AIChE with the Robert E. Wilson Award.
“I get a note every day of how many gallons TSCR processed,” he said. “Being able to support something that is up and running – and working effectively – makes it feel like we’re making really important progress. »
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About PNNL
Pacific Northwest National Laboratory draws on its distinctive strengths in chemistry, Earth Science, biology And data science advance scientific knowledge and meet challenges sustainable energy And national security. Founded in 1965, PNNL is managed by Battelle on behalf of the Department of Energy’s Office of Science, which is the largest supporter of basic research in the physical sciences in the United States. DOE’s Office of Science is working to address some of the most pressing challenges of our time. For more information, visit https://energy.gov/science. For more information about PNNL, visit PNNL News Center. follow us on Twitter, Facebook, LinkedIn And Instagram.
