Dramatic art of nuclear warheads

Nuclear weapon destruction could be verified using physics-based encryption

Image credit: Dreamstime

A team of researchers based at Massachusetts Institute of Technology (MIT) have developed a means for ensuring that real nuclear weapons have been destroyed without revealing highly sensitive design secrets.

Negotiating disarmament is complicated and dangerous enough without the technical headache of verifying that, once the treaties have been signed, real bombs and other nuclear devices are being destroyed rather than replicas.

In the past, treaties have relied on verifying that the delivery systems such as bomber planes and intercontinental ballistic missiles – which are considerably larger than the warheads themselves – were destroyed. For instance, the bilateral START treaty between the US and the Soviet Union required 12 different types of inspection, as well as data exchanges and declarations focused on ensuring the destruction of delivery systems on a massive scale. 365 bombers were flown to the Arizonan desert, sliced into five pieces with a giant steel blade dropped form a crane and left out for Soviet satellites to observe.

However, the MIT group argued that future treaties should focus on destroying the warheads themselves, particularly as these dangerous objects could be acquired by terrorist groups or rogue nations; a reliable means of verifying the destruction of these objects would be an important part of these treaties.

The proposed system could be described as the physics equivalent of cryptographic keys, which are used to encode information securely in digital communications. While only the key holders can decode the information, someone viewing the encoded data can still verify that two datasets are identical. This is the principle that the MIT team applied “not through computation, but through physics.”

“You can hack electronics but you can’t hack physics,” said Professor Areg Danagoulian, senior author of the papers proposing the system.

A nuclear warhead can be characterised by two secret details: its fuel – a mixture of heavy elements and isotopes (atoms containing different numbers of neutrons) – and the size and shape of the pit in which the material is stored.

The difficulty of developing a verification system is that any approach capable of revealing sufficient information – such as using isotope-sensitive resonance to probe the interior of the object – is also going to reveal politically sensitive design details. A simpler system which simply measures a radiation signature could be fooled with a different radioactive source.

The MIT researchers developed a physical version of a cryptographic key, which contains a mix of the same heavy isotopes, but in proportions unknown to inspectors. When a bomb is inspected, the country which produced the bomb would provide a ‘reciprocal’ which is lined up with the bomb. The inspectors would then take measurements using a detector which can register radiation signatures specific to that combination of isotopes. The resultant image will appear blank only if the warhead is real. If the bomb is a replica, some details will appear in the image.

In an alternative approach also suggested by Danagoulian and his colleagues, a different technique is used to inspect the radiation signature, and the output is a spectrum rather than an image.

The MIT team have verified their method using simulations, and next hope to use it on fissile materials provided by a national laboratory.

Danagoulian believes that “everyone will be better off” with a system like this in use: “There will be less of this waiting around, waiting to be stolen, accidentally dropped or smuggled somewhere. We hope to make a dent in the problem.”

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