In late November, huddled in a freezing tent, a group of people watched a live video feed of a drone as it descended into a giant vault. There was no visible change, but once past the thick concrete ceiling, the drone was flying through hot air irradiated with the equivalent of about 500 X-rays every second. More than enough to kill a person. No problem for a drone.
The drone was conducting reconnaissance as part of a decades-long project by the US Department of Energy (DOE) to clean up nuclear waste at a former weapons and nuclear energy test site in Idaho. The plan: suck out more than 4000 cubic metres of radioactive powder stored inside and get it out of the state.
This particular vault contains radioactive by-products of a process to reclaim enriched uranium from spent nuclear fuel, which was carried out at the site until 1991. The liquid by-products were converted to a powder called calcine and placed in bins shielded by concrete silos for what was then presumed to be permanent storage.
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“It was never designed for retrieval,” says at the Idaho Environmental Coalition, an engineering firm contracted by the DOE to handle the clean-up.
Retrieving the calcine safely has proved challenging for the DOE, which agreed in a 1995 settlement with the US Navy and Idaho to remove the waste from the state by 2035. The settlement came after the state intended “to prevent Idaho from becoming a dumping ground for the nation’s commercial spent nuclear fuel”, according to the Idaho Department of Environmental Quality.
Eventually, the waste was supposed to be moved to a repository beneath Yucca Mountain in Nevada, but Congress cancelled funding for construction of that repository in 2011; Nevada didn’t want to become a dumping ground for nuclear waste either. In the meantime, the DOE says it is focused on consolidating the calcine in a single vault on the site.
The current plan is to snake a vacuum tube through the top of each bin and suck out the calcine powder, like sugar through a straw. Any residues left in the bins will be cleared with specialised robotic equipment. But first the engineers determined they needed a clearer view of the vault, which is about the size of a grain silo.
“We looked at robotic arms” to lower in cameras, says , the technical lead for the drone mission at the Idaho Environmental Coalition. “We looked at everything, including an inflatable blimp.”
They settled on a drone – specifically, the Elios 3 made by Swiss company Flyability – equipped with lidar to create an updated, more detailed 3D map of the vault’s interior than existing blueprints could provide. Drones and autonomous robots are increasingly being used in clean-ups of nuclear material and other toxic waste.
The Elios 3 had flown in radioactive environments before, but none this intense.
To make sure it would function inside the vault, at Idaho State University to expose the drone to intense radiation and high temperatures. The radioactive bins heat the air in the vault up to 65°C (150°F). The drone pilots on a full-scale model of the vault built nearby.
On 30 November, after a year and a half of planning, a pilot flew the drone into the vault. The first two missions went off without a hitch, collecting all the lidar data needed to map the vault and accurately drill holes in the bins.
But on a third flight to get a better measure of radioactivity, the drone pilot “could not maintain altitude”, Young says, possibly because of the extra weight of a dosimeter attached to measure radiation. The drone descended from the top of the vault to alight on a bin, where it remains. There are no plans to retrieve it.
The team is now preparing to drill the holes in the bins to suck out the powder, a process that should be completed by 2024.
