Nuclear Weapons Inspection: Encryption System Could Thwart Spies and Expose Hoaxes

A new nuclear weapons inspection technology could enhance inspectors' ability to verify that a nuclear warhead has been dismantled without compromising state secrets behind the weapon's design.

This new non-proliferation tool, its inventors argue, would greatly assist the often delicate dance of nuclear weapons inspectors-who want to know they haven't been hoaxed but are also sensitive to a military's fear that spies may have infiltrated their ranks.

While nuclear non-proliferation treaties have historically verified the dismantlement of weapons delivery systems like ICBMs and cruise missiles, there have in fact never been any verified dismantlements of nuclear warheads themselves (in part for the reasons described above).

Yet there are 13,000 nuclear warheads in the world, meaning the entire globe is still just a hair trigger away from apocalypse-even as we approach the thirtieth anniversary of the Berlin Wall's collapse.

As UN Secretary-General Antonio Guterres told world leaders last month, "I worry that we are slipping back into bad habits that will once again hold the entire world hostage to the threat of nuclear annihilation."

How, then, to verifiably dismantle a nuclear bomb?

The challenge, says Areg Danagoulian, assistant professor of nuclear science and engineering at MIT, is finding a way for both sides of a nuclear disarmament treaty to arrive at a point of confidence. Any party to such a treaty, he says, must verifiably destroy the warheads they say they will destroy. But their military should also be confident that none of their weapons design secrets have leaked out during the inspection and verification process.

Previous research has put forward systems that have strong cryptography but could conceivably be hoaxed with some sleight-of-hand-or which are sensitive to hoaxes but cannot guarantee the security of information about the design and composition of warheads.

Danagoulian says he and his coauthor Ezra Engel (a former student who is now in the U.S. Army) relied on two innovations to create their new system. The first is the neutron beam that the system emits to study an individual warhead. These neutrons, at energies in the range of 5 to 50 electron volts (eV), are "slower than fast neutrons," Danagoulian says. They sit within a range of neutron energies that probe nuclear resonances for most of the heavy elements found in nuclear warheads-including, of course, the relevant isotopes of plutonium and uranium.

This means the inspection technology can potentially spot the difference between weapons-grade and reactor-grade uranium in a sample, as well as detect other elements like tungsten and molybdenum.

This capability could reduce the chance of being hoaxed: A deceitful party could claim they're dismantling a tranche of warheads-but those supposed warheads could in fact be lookalikes that are loaded instead with lower purity uranium or other heavy elements.

Or the warheads could "look" to a neutron beam like a real warhead but only when viewed in one particular direction. Which is why Danagoulian and Engel's method, described in a recent issue of the journal Nature Communications, also exposes candidate warheads to their neutron beam from multiple viewing angles.

The team's experiment-performed on fake warheads composed of non-weaponized heavy elements via a neutron beam from a linear accelerator at Rensselaer Polytechnic Institute-were able to verify bonafide "warheads" and pick out the hoaxes in both scenarios described above.

There are 13,000 nuclear warheads in the world.

The other innovation behind Danagoulian's technology involved adding what the team calls an "encrypting filter" to the neutron beam's path. The neutron beam passes through the warhead, then through the "filter," and proceeds on to the neutron detectors. The filter in this case is just a slab of various heavy elements whose composition is unknown to the inspectors. The country whose weapons are being inspected, Danagoulian says, could even create the filter themselves. So long as the inspectors cannot perform any separate experiments on the encrypting filter, the ultimate composition and geometry of the warhead will be obscured to the neutron beam.

"Inspectors cannot learn anything useful about the warhead's composition," Danagoulian says.

However, if inspectors are able to take a snapshot of a "gold-standard" warhead whose genuineness has independently been established, they now have an encrypted picture of what a legitimate warhead looks like to their system. Then they can place other warheads of the same design into the encrypted neutron beam and compare the snapshots they take of candidate warheads: If the candidate has the same nuclear resonance spectrum as their "gold-standard" warhead, then the candidate warhead is real. If the candidate warhead's spectrum is different from the gold standard, they may have just detected a hoax.

Between the encrypting filter and the sensitivity of their setup, a hoaxer or spy on either side of the exchange would have to work a lot harder to deceive inspectors.

Danagoulian says that his group hopes to adapt its method to a commercial neutron source and a portable detection system. The total pricetag, he says, should be around US $100,000.

"You wouldn't need this at every ICBM site," he says. "We think we can build instruments that are 5 meters [in size]. You could put it in a truck or a van. The inspectors could bring it themselves-set up at some particular base or some particular facility. They could do these measurements and leave."