With new imaging technology, researchers are attempting to look inside a nuclear waste storage cask to verify that all the waste is still inside.
Over the coming weeks, a team from Los Alamos National Laboratories (LANL) will head to a nuclear waste storage facility in Idaho, select one of dozens of concrete storage casks, and attempt the unprecedented task of making sure that all the waste is still inside.
Using an experimental imaging process called muon tomography, the technology should allow researchers to look through the cask's heavy radiation shielding and make the first ever direct, detailed verification of a cask's nuclear contents after it has been filled and sealed.
One the one hand, the test is a step towards improving nuclear security in North America. But the technique could have an ever greater impact on oversight of the world's most volatile nuclear nations, where stolen or redirected nuclear waste could be used to create a dirty bomb—or even a nuclear weapon.
Though hopes were once high for centralized national storage of American nuclear waste—"spent fuel," in industry parlance—today the vast majority of used fuel from nuclear reactors is stored on-site at the power plant that used it. Deep pools of water cool the spent fuel rods in "wet storage" for a decade or more, glowing an eerie blue with Cherenkov radiation.
Eventually, authorities move the cooled spent fuel to enormous steel and concrete containers known as dry storage casks. These casks are welded shut and heavily reinforced, allowing them to safely store up to 15 tons of spent nuclear fuel for at least 40 years. Canadian power plants currently employ a similar cask storage strategy, and while practices differ around the world, dry casks have been used to some extent by many nuclear nations as well.
The Department of Energy estimated that a well trained and prepared team of infiltrators with heavy explosives could obtain fuel from a cask in just 10-20 minutes
When it comes to looking through heavy radiation shielding, the LANL team is among the best in the world; as Motherboard previously reported, the lab's muon imaging technology is also being used to map the damaged Fukushima Daiichi reactors through similarly shielded walls. In both cases, natural particles called muons, which are abnormally good at penetrating dense matter, allow the researchers to confirm (or disconfirm) the contents of a shielded chamber.
LANL Fellow Christopher Morris described growing anxiety among some at the US Department of Energy (DoE) regarding the agency's the inability to confirm records of cask storage amounts. Funding is coming from the Domestic Nuclear Detection Office's NA-22 anti-proliferation initiative, aimed at cutting off a potentially undetectable source of nuclear material.
In North America, the strategy for protecting casks is called "containment and surveillance." All casks are kept under tight security, guarded and often watched by video surveillance to prevent tampering, and checked periodically to monitor radiation levels and other vital signs. Nuclear plants are also intrinsically high-security zones themselves, and often far from civilization.
More importantly, the 20-foot casks are incredibly difficult to crack; one DoE-funded report did estimate that a well trained and prepared team of infiltrators could obtain fuel from a cask in just 10-20 minutes, but the scenarios all involved heavy explosives or other obvious methods. The remarkable document imagines terrorist teams equipped with everything from thermal lances to heavy-lift helicopters, but a loss of waste by any such means would not be remotely secret. Spent fuel is also "self protecting," meaning that its radioactivity would pose a serious threat to the life of any thief who did manage to open a cask.
A cross-section of a dry storage cask. Image: Department of Energy
Harvard University nuclear security expert Dr. Matthew Bunn wrote in an email to Motherboard that "the reason some at DoE do worry about [cask storage uncertainties]... has to do with safeguards in non-nuclear-weapon states." Because spent nuclear material could be sold or put toward a clandestine nuclear weapons program without anybody noticing the loss of enrichable material, there is an incentive to divert nuclear material from a cask.
International anti-proliferation efforts have led to global tracking of as much nuclear material as possible, and muon imaging could allow the first independent verification of these records for dry storage sites at home and abroad. If the upcoming tests in Idaho are successful, muon technology could enter the repertoire of UN and international inspection teams all over the world.
"If, for example, a state managed to swap out a few pins from their spent fuel assembly before it went into the cask," Bunn explained, "and that wasn't picked up on measurements before the cask was sealed, you'd never be the wiser." Such inaccuracies could be attributed to genuine errors in the initial measurement—which do occur—or perhaps a deliberate "diversion" of the spent waste by a collaborating group of spies.
And while outright theft from sealed casks is unlikely at domestic reactors, it might be possible under less robust oversight around the world. "Casks in non-nuclear-weapon states would likely be equipped with seals intended to reveal if the cask had been tampered with," Bunn wrote, "but some worry a state might be able to defeat the seals in a way that would be hard to detect."
Many states have been accused of plotting to obtain nuclear weapons material from their nuclear power programs, and cask-skimming of waste for later enrichment could be an enticing way of doing so. Iran, for example, was pressured to ship all spent fuel from its single operational nuclear plant back to Russia, which built the facility and provides the fuel, for example.
In the US, the National Nuclear Security Administration has also been working to shore up all sources of radioactive material from theft or diversion. Sometimes this means modifying existing techniques, as with the oil industry's use of radioactive americium-beryllium in borehole maintenance. Other times, the only way forward is to find new solutions where there were none before, as with muon tomography.
Seen as a non-proliferation technology, one major advantage of imaging with cosmic muons is that doing so uses only existing radiation and requires no extra nuclear sample to take its readings; muon tomography is safe and avoids compounding the proliferation concerns it's meant to address. Morris thinks this would make muons a perfect candidate for non-destructive inspection of the country's atomic weapons stockpiles, which are currently difficult to monitor without affecting the nuclear samples within.
The upcoming Idaho test run is not expected to reveal any discrepancy in the selected storage cask, but to act as a proof of concept for a technology that could change nuclear practices around the world. At the very least, by introducing the technical ability to check fuel levels in dry storage, this research project could change the strategic landscape to end some plots before they ever begin.