President Obama has made it clear in a statement that the Iran nuclear deal signed yesterday was “built on verification.” Technology built to detect an elusive subatomic particle called the antineutrino could help.
The International Atomic Energy Agency wants a reactor-monitoring tool that is portable, safe, inexpensive, and remotely controllable. Antineutrino detectors, which give a peek into how much uranium and plutonium are in a reactor core, promise all of that.
The technology, which has been in the works since the early 2000s, has improved tremendously in the past five years, and it is now almost ready for practical use, says Patrick Huber, a physics professor at the Center of Neutrino Physics at Virginia Tech in Blacksburg. “Less than two years from now, you should have at least one maybe several types of antineutrino detector technologies that would work as nuclear safeguard detectors.”
Nuclear reactors are the strongest source of antineutrinos on Earth. Conventional detectors catch antineutrinos using hundreds of liters of organic solvents mixed with gadolinium atoms. Liquid scintillator-based detectors are being developed by physicists at Lawrence Livermore and Sandia National Laboratories, and a handful of other teams, mostly in Europe. An antineutrino occasionally collides with protons in the solvent, creating a neutron and a positron (the antimatter sibling to an electron). Photodetectors sense precisely separated light flashes produced when the positron crashes into an electron, and when the neutron is captured by a gadolinium atom.
By measuring the number of antineutrinos produced and their energy spectrum, researchers can calculate reactor power and the amount of uranium and plutonium isotopes in the core. So an antineutrino detector would reveal if plutonium was removed or more uranium was added, even if the monitoring was interrupted for a period and then restarted.
Compared to the cameras, seals, and radiation detectors that the IAEA uses to monitor reactors today, antineutrino detectors offer a safe, foolproof, compact, non-intrusive, and potentially low-cost alternative, Huber says. “Conventional safeguards work well but given high proliferation risks, you want a high level of assurance that nothing is going wrong.”
Until now, detectors could only distinguish reactor antineutrinos from the cosmic-ray background if they were underground. But last year, a Japanese group showed that antineutrinos could be accurately detected above ground using a plastic scintillator array rather than a liquid vat.
Above ground, solid-material detectors could be what’s needed for monitoring sensitive sites such as the Iranian 40 megawatt heavy water reactor in Arak, Huber says. The reactor has design characteristics that make it especially suitable for the production of weapons-grade plutonium. As part of this week’s nuclear deal, Iran has agreed to redesign the Arak reactor so that it produces less plutonium. It will be one of the sites that the IAEA monitors.
In a research published in Physical Review Letters last July, Huber and Thomas Shea, an independent consultant who served for 24 years in the International Atomic Energy Agency’s Department of Safeguards, detailed how, with moderate improvements, a detector that uses 20 tons or less of scintillator material could be fit into a 6-meter shipping container and parked outside the Arak reactor building, roughly 19 meters from the core, for continuous monitoring. The team combined detailed reactor simulations with state-of-the-art reactor neutrino-flux calculations and statistical analysis to show that the system could detect whether as little as 2 kilograms of plutonium had been removed from the reactor.
Huber is part of an international group working on a new kind of solid-material-based detector that he says is more accurate, and would be cheaper to transport, install and maintain. The team is testing the detector at a research reactor in Belgium, and it is preparing for a larger test at a commercial nuclear reactor in Virginia by the end of this year.
Shea says that he is a great believer in antineutrino detector technology for nuclear monitoring, but thinks a few research and engineering issues need to be solved before it is practically feasible. That could happen in a few years. “The way the technology is going, maybe soon there will be new reactors that are small and inexpensive and perhaps allow remote monitoring.”
For now, we need an international demonstration project that brings together the global antineutrino detector community and shows the viability of the technology, he says. The soon-to-be-redesigned Arak reactor in Iran, he believes, could be just the stage for such a demonstration.