Finding an Answer to Storing Nuclear Wastes

Thursday, April 25, 2019 @ 05:04 PM gHale

The close-knit CHWM team develops the chemistry necessary to create materials that trap troublemakers from Cold War nuclear wastes.
Source: Center for Hierarchical Waste Form Materials

Back in 1943 a chemical facility called T-Plant housed the first industrial process to pluck plutonium for the nation’s nuclear arsenal from irradiated fuel rods.

Operating T-Plant and similar facilities created radioactive wastes in Washington State, Idaho, and South Carolina. The Cold War waste — approximately 90 million gallons — is in 100-plus interim storage tanks.

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It has been determined, however, the best approach to permanently store the waste, and keep it away from soil and groundwater, is to turn it into a solid material. That is where the Center for Hierarchical Waste Form Materials (CHWM) comes in.

“There won’t be just one material that solves our nation’s nuclear waste problem,” said Kristen Pace, a doctoral student at CHWM.

At CHWM, Pace and her colleagues are learning what it takes to store a highly radioactive subset of defense-related waste. These small batches contain troublemakers, such as cesium-packed liquids that’ll evaporate into the air during the conventional storage process. An alternative is needed to trap the troublemakers to protect air, water, and soil.

The CHWM team’s foundational studies focus on questions about materials that could contain the problematic subset.

The conventional storage process turns liquid nuclear waste into a solid that won’t leak like a liquid could. That solid is radiation-resistant glass. To make glass, engineers prepare the waste from the tanks at a massive industrial plant. They remove plutonium and other troublemakers, creating small batches of waste that are held back. The engineers mix the bulk of the waste with glass-making materials and feed it into ultra-hot melters. They pour the resulting molten glass into special drums. In the drums, the glass cools into a form that’ll last for thousands of years.

Different Storage Option
Some components of the waste, such as cesium, technetium, and iodine, need a different storage option. Glass won’t do. The ideal option would pack in several troublemakers, reducing the volume that’s stored. To create such materials, we need science-based recipes.

“I’m hopeful that in several more years, we’ll be able to come up with a nice material for some of the radionuclides we want to sequester that will last as long as the time it takes for the last radionuclide to decay,” said Hans-Conrad zur Loye, CHWM’s director and University of South Carolina professor. “We’re doing great science for a good reason.”

The CHWM team delves into what they call hierarchical materials. Imagine a tower of cream puffs, with row upon row of pastry stuffed with creamy fillings. Similarly, scientists make a porous structure (like empty pastry shells), incorporating a troublemaking radioactive element. Inside the pores, they add another material (like a filling) with a different troublemaker whipped in. How is that a hierarchy? A small structure (filling) rests inside a larger structure (pastry).

While scientists have made great strides with hierarchical materials, there isn’t a cookbook for making materials for these small-batch wastes.

“The goal is not to have an answer in hand,” said zur Loye, “but to understand the underlying science.”

To get at that science, zur Loye took an outside-the-box approach. He set his sights on a diverse team, including three types of chemists who rarely worked together. His goal was a close-knit team that could make radioactive materials, measure their properties, and model the results.

Sum of the Whole
Making materials means working with highly radioactive elements. That’s not easy. Few labs are set up for it. He brought in experts from DoE’s Savannah River National Lab (SRNL) and other places capable of making highly radioactive materials. He added experimental chemists who measure different features, such as pore size, and computational chemists who simulate how the materials behave using complex mathematics.

They leveraged their talents through projects and short-term fellowships. Working with three classes of hierarchical materials, including tiny particles, they design custom structures, create them, run tests, and use the results to build simulations that, in turn, refine future designs and tests.

Their inclusive team roster gives CHWM a soup to nuts view of hierarchical materials. It may lead to new ways to address troublemakers from Cold War wastes. “None of this happens quickly,” said zur Loye. “But I’d like to think that what we’re doing will help us get there in my lifetime.”

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