To prevent 'lost' nukes, scientists suggest storing them in a hall of mirrors

Radio bouncing off multiple reflectors is a hard-to-defeat method of monitoring a weapons cache

Researchers say they have developed a method to remotely track the movement of objects in a room using mirrors and radio waves, in the hope it could one day help monitor nuclear weapons stockpiles.

According to the non-profit org International Campaign to Abolish Nuclear Weapons, nine countries, including Russia, the United States, China, France, the United Kingdom, Pakistan, India, Israel and North Korea collectively own about 12,700 nuclear warheads.

Meanwhile, over 100 nations have signed the United Nations' Treaty on the Prohibition of Nuclear Weapons, promising to not "develop, test, produce, acquire, possess, stockpile, use or threaten to use" the tools of mass destruction. Tracking signs of secret nuclear weapons development, or changes in existing warhead caches, can help governments identify entities breaking the rules.

A new technique devised by a team of researchers led by the Max Planck Institute for Security and Privacy (MPI-SP) aims to remotely monitor the removal of warheads stored in military bunkers. The scientists installed 20 adjustable mirrors and two antennae to monitor the movement of a blue barrel stored in a shipping container. One antenna emits radio waves that bounce off each mirror to create a unique reflection pattern detected by the other antenna.

The signals provide information on the location of objects in the room. Moving the objects or mirrors will produce a different reflection pattern. Experiments showed that the system was sensitive enough to detect whether the blue barrel had shifted by just a few millimetres. Now, the team reckons that it could be applied to monitor whether nuclear warheads have been removed from stockpiles.

At this point, readers may wonder why this tech is proposed for the job when CCTV, or Wi-Fi location, or any number of other observation techniques could do the same job.

The paper explains that the antenna-and-mirror technique doesn't require secure communication channels or tamper-resistant sensor hardware. The paper’s authors argue it is also "robust against major physical and computational attacks."

"Seventy percent of the world's nuclear weapons are kept in storage for military reserve or awaiting dismantlement," Sebastien Philippe, co-author of a research paper published in Nature Communications. Philippe is an associate research scholar at the School of Public and International Affairs at Princeton University, explained.

"The presence and number of such weapons at any given site cannot be verified easily via satellite imagery or other means that are unable to see into the storage vaults. Because of the difficulties to monitor them, these 9,000 nuclear weapons are not accounted for under existing nuclear arms control agreements. This new verification technology addresses this long-standing challenge and contributes to future diplomatic efforts that would seek to limit all nuclear weapon types," he said in a statement.

In practice, officials from and organisation such as UN-led International Atomic Energy Agency, which promotes peaceful uses of nuclear energy, could install the system in a nuclear bunker and measure the radio waves reflecting off its mirrors. The unique fingerprint signal can then be stored in a database.

They could later ask the government controlling the nuclear stockpile to measure the radio wave signal recorded by its detector antenna and compare it to the initial result to check whether any warheads have been moved.

If both measurements are the same, the nuclear weapon stockpile has not been tampered with. But if they're different, it shows something is afoot. The method is only effective if the initial radio fingerprint detailing the original configuration of the warheads is kept secret, however.

Unfortunately, it's not quite foolproof, considering adversaries could technically use machine learning algorithms to predict how the positions of the mirrors generate the corresponding radio wave signal detected by the antenna.

"With 20 mirrors, it would take eight weeks for an attacker to decode the underlying mathematical function," said Johannes Tobisch, co-author of the study and a researcher at the MPI-SP. "Because of the scalability of the system, it's possible to increase the security factor even more."

To prevent this, the researchers said that the verifier and prover should agree to send back a radio wave measurement within a short time frame, such as within a minute or so. "Beyond nuclear arms control verification, our inspection system could find application in the financial, information technology, energy, and art sectors," they concluded in their paper.

"The ability to remotely and securely monitor activities and assets is likely to become more important in a world that is increasingly networked and where physical travel and on-site access may be unnecessary or even discouraged." ®

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