Researchers at the U.S. Department of Energy’s (DOE) Argonne National Laboratory are delving into the details of plutonium chemistry with an aim to help clean up the Hanford site in Washington state. Hanford is a 580-square-mile site established during World War II to produce plutonium for America’s defense program. The site generated an enormous amount of waste, some of which is hazardous to humans and the environment.

The Argonne plutonium project — a collaboration with Clemson University — is supported by a $3.9 million award from DOE’s Office of Environmental Management. DOE recently awarded a total of $27 million to 13 projects led by six national laboratories to accelerate cleanup of Hanford waste.

“We want the ability to tell the plutonium particles how to behave. With this ability, we can simplify the tank waste cleanup and make it faster and cheaper.” — Richard Wilson, Argonne chemist

An urgent need for innovative cleanup technologies

From 1944 to 1990, nuclear reactors at Hanford made plutonium to support production of nuclear weapons. In 1990, DOE launched a major initiative to clean up the radioactive and other hazardous wastes created by these efforts. The initiative continues today. DOE recently estimated that the entire site cleanup could cost between $293 and $634 billion and take decades.

The Hanford wastes include 54 million gallons of a mixture of liquids, sludges and salts stored in 177 large underground tanks. Some of the tanks are leaking into the ground. That means the waste could potentially migrate into the nearby Columbia River. As a result, there is an urgent need to treat and dispose of the waste.

DOE’s recent awards to the national laboratories seek to enable scientific breakthroughs that significantly reduce the time and cost of tank cleanup. According to DOE, research, development and deployment of innovative technologies can potentially save up to $150 billion in tank cleanup costs and reduce the cleanup timeline by up to 20 years.

The challenge: separating radioactive materials from the waste mixture

A small fraction of the tank waste constituents are radioactive materials. These radioactive materials must be separated from the mixture, immobilized in glass and permanently buried in an underground repository. The challenge is to design ​“pre-treatment” strategies to selectively remove the radioactive constituents. Such pretreatment could significantly reduce the time and costs of waste disposal.

The good news is that a substance called crystalline silicotinanate (CST) is effective at filtering highly radioactive cesium from the tank waste. Cesium is an element, and a radioactive version of it is often produced in nuclear reactors. The use of CST has the potential to significantly reduce cleanup costs. However, radioactive plutonium tends to stick to CST as well. Plutonium has much longer-lasting radioactivity than cesium and must be treated and disposed of in a different way. That means plutonium needs to be kept separate from cesium and CST.

The Argonne team is figuring out how to solve this challenge. Its approach is to gain a deep understanding of how to influence plutonium’s chemical properties.

“Because Hanford’s plutonium production used many different chemical processes, the tank wastes are extremely complex,” said Richard Wilson, an Argonne chemist and principal investigator on the DOE-funded research project. ​“The chemical properties of the waste are not well understood. We seek to change that through rigorous scientific experiments.”

Getting to know how plutonium behaves

The project builds on previous fundamental Argonne research, which found that the plutonium in the tanks takes the form of plutonium oxide nanoparticles. Importantly, this effort discovered that the nanoparticles’ surface chemistry could be manipulated.

Now, Wilson’s team is investigating how the nanoparticles interact with the surrounding waste and CST — and how to control those interactions.

“We are exploring questions such as, what materials do the nanoparticles react with? Do the nanoparticles gather in clusters? How stable are they?” said Wilson.

An important part of the research is studying the nanoparticles’ chemistry under the unique chemical conditions of the tank wastes. Based on the insights from these experiments, the team can manipulate the particles’ chemistry so that they preferentially stick to some materials but not to others. This approach will enable the team to design a pre-treatment strategy that stops plutonium from sticking to CST.

“We want the ability to tell the plutonium particles how to behave,” said Wilson. ​“With this ability, we can simplify the tank waste cleanup and make it faster and cheaper.”