Hobbs [+]Enlarge Credit: Steve Ashe SRS Photography

David T. Hobbs is an expert in the complicated chemistry of nuclear waste management. He’s studied nuclear waste materials, radiochemical separations, and complex chemical environments for more than three decades. But three years ago, when an accident in a New Mexico repository brought disposal of U.S. defense nuclear wastes to a standstill, he was called to investigate a different kind of material—cat litter.

Hobbs, who doesn’t own a cat, is one of the researchers who studied the nuclear waste mixture that in 2014 led to a drum failure and radiological release at the Waste Isolation Pilot Plant (WIPP) near Carlsbad, N.M. The accident shut down the facility for three years. It was ultimately traced to an unorthodox sorbent, an organic cat litter called sWheat Scoop, that was used in error to prepare the nuclear waste for disposal at WIPP.

After a 33-year tenure, Hobbs retires this month from Savannah River National Laboratory, a South Carolina facility that produced plutonium isotopes for U.S. nuclear weapon and space programs. He recently spoke with C&EN about the accident at WIPP and the enduring thrill of scientific discovery.

Before WIPP opened in 1999, Hobbs and colleagues conducted studies on the radiolytic generation of gases from organic polymers and the hydrogen transport properties of filter vents used with transuranic waste drums. Their efforts fed into the creation of limits on the quantity of α-particle-emitting materials in waste and transportation packages. They also contributed to the testing protocol for qualifying filter vents.

Vitals ▸ Hometown: Charlotte, N.C. ▸ Education: B.S., University of North Carolina, Chapel Hill, 1974; Ph.D., Vanderbilt University, 1979 ▸ Professional highlights: Engelhard Corp., 1980–82; Pfizer, 1982–84; Savannah River National Laboratory, 1984–2017 ▸ Awards: ACS Fellow, 2010; Glenn T. Seaborg Actinide Separations Award, 2014 ▸ Favorite molecule: NaHTi 2 O 5 (monosodium titanate) ▸ Hobbies: running, hiking, reading fiction

When Hobbs heard about the Feb. 14, 2014, radiological release at WIPP, two things sprang to mind. First, Hobbs recalled a fire at the facility a week earlier that triggered an emergency response crew and sent six workers to the hospital for smoke inhalation. And second, because the disposal facility is built in a salt deposit and is designed to encapsulate the waste packages as the salt formation naturally flows and collapses around them, he wondered if a drum had been punctured in a collapse event.

“I could not think of a mechanism under which a drum itself would release contamination,” Hobbs says. “I thought it had to have been an external event.”

But an April 2014 accident investigation report by the Department of Energy came to a different conclusion. It says the radiological release likely stemmed from a single breached drum. Plus, photographs taken in May 2014 show an open container with heat damage to the surrounding area. This suggests that a thermal event inside the drum caused the container to fail.

The drum came from Los Alamos National Laboratory (LANL). It contained a reactive mix of radioactive nitrate salt waste, a neutralizing liquid, and organic cat litter, which had been used as a sorbent.

An October 2014 report from the Department of Energy’s Office of Inspector General points to a change in the packaging procedure at LANL that specified organic cat litter, when an inorganic sorbent was likely intended. Investigators traced a series of internal communications in which the specifications for “kitty litter/zeolite clay” were transformed into “kitty litter (clay),” the report says.

Combined with inadequate technical review, this resulted in LANL workers filling waste containers with a mixture of nitrate salts and sWheat Scoop, a cat litter that is 100% wheat, according to its manufacturer.

“It would have been much clearer if they had said an inorganic zeolite sorbent,” Hobbs says. “It’s been a very expensive mistake, costing at least half a billion dollars.”

Hobbs led a team that investigated the chemical reactivity of the mixture inside the drum as well as the series of events that caused the drum to fail and release ­radioactive material. Trained as an inorganic chemist, he says the experience brought back memories of organic chemistry labs he did as an undergraduate.

“I had no experience with kitty litter before,” he says. “There was a steep learning curve to think about all sorts of organic nitrate reactions.”

Though from a purely scientific perspective, he adds, it was an interesting problem to solve.

We need to tell the story so that hopefully it won’t happen again.

Hobbs’s team of chemists and engineers from several national laboratories had expertise in the characteristics of energetic materials, the reactivity of organic nitrate mixtures, and the handling of transuranic waste. Using what they knew about the history of the drum and what they discovered about the reactivity of the mixtures through experiments, the researchers pieced together an explanation for the thermal event that breached drum 68660.

Inside the drum was “a complex, heterogeneous mixture of materials with the potential for multiple reaction sites and reaction chemistries,” Hobbs and his team said in a March 2015 DOE report.

The contents of the drum, which included metal nitrate salts, the sWheat Scoop litter, and a neutralization reagent, were incompatible, the report says. The mixture likely underwent a series of exothermic reactions, including hydrolysis, oxidation, and nitration of the organic components, the report says. The reactions produced a thermal runaway condition in which increasing internal heat and pressure caused the waste container to break open and release radioactive material.

Hobbs says questions remain about why only one drum failed. LANL generated nearly 700 drums with a similar waste mixture that includes the organic cat litter. Drums with similar mixtures were isolated and are monitored, DOE says.

“Why didn’t more than one drum react as that drum did?” Hobbs asks. “Never to my knowledge did we firmly establish that there is something completely unique about 68660.”

Explaining how a nuclear weapons lab got mixed up in an expensive radioactive release accident because of cat litter is an odd circumstance. Hobbs says, “In hindsight, it certainly makes me uncomfortable having to talk about this occurring when I think of things that we do to evaluate hazards and risk in the laboratory—evaluating the compatibility of chemicals, for example.”