The US and USSR had more than 60,000 nuclear warheads pointed at each other at the height of the Cold War. While the Non-Proliferation Treaty of 1970 and START in 1994 have shrunk that figure to around 26,000, nobody is really sure how many still exist—because nobody’s ever actually verified the number of warheads, just the delivery systems. But with a new zero-knowledge protocol, arms inspectors will soon know exactly what they’re dealing with.
Nuclear regulatory agencies like the International Atomic Energy Agency toe a difficult line. They must identify and confirm the presence of nuclear warheads without revealing or discovering any information about the delivery system that houses it or the chemical makeup of the nuclear material itself (which would violate any number of state secrets). This leaves inspectors in a bit of a quandary, how do you confirm the presence of a nuclear warhead without directly inspecting the device? A team of researchers from Princeton University Plasma Physics Laboratory (PPPL) and the Department of Energy found the answer in cryptography.
The system consists of a small steel probe, a high energy particle beam and two warheads—the one being verified and a “golden warhead,” an identical model known to contain nuclear material. Inspectors bombard the golden warhead with high energy neutrons and record how many strike a particle detector situated on the other side of the missile. This provides a baseline reading with which the inspectors can compare readings from the unverified warhead. If the numbers match, the second warhead is a nuke. If the numbers differ, it’s a regular warhead.
As John Greenwald of the PPPL explains,
The neutrons that do reach the detectors are counted and added to the number that the host nation whose warheads are being inspected had “preloaded” into the detectors. Inspectors would measure the total number of counts in the detectors without knowing how many had been preloaded. This total count could be straightforwardly tallied with non-electronic neutron counters such as the personal dosimeters used to measure exposure to radiation in nuclear power plants.
If the total number of counts matched the number that the parties had stipulated in advance, the warhead would be found to be a true one. But if the total differed from the stipulated number, the warhead would stand exposed as a spoof. To prevent cheating by preloading detectors in such a way that a spoof would pass the test, the inspector decides on the spot which preloaded detectors will be used on the “golden warhead” and which on the item offered for inspection.
This approach can also be applied to batch testing. Inspectors gather a large number of warheads—including a few deployed missiles that are likely to be nukes—and compare their readings. If all the warheads read the same, they’re all confirmed as nukes.
The project is funded by a $100,000 grant from the U.S. State Department as well as another $162,500 from Global Zero. “The goal is to prove with as high confidence as required that an object is a true nuclear warhead while learning nothing about the materials and design of the warhead itself,” said co-principal investigator, physicist Robert Goldston. The team hopes to begin full phase testing by the end of the year.
If the program proves successful, it will revolutionize non-proliferation efforts by removing one of the key impediments to inspection: the protection of state secrets.