Antineutrino detectors can potentially achieve the important goal of tracking fissile materials, such as plutonium, to ensure they are not being diverted into weapons.

Ghostly particles streaming from nuclear reactors can reveal from a distance events within the core. Prototype detectors buried near a commercial nuclear power plant are helping physicists develop new ways to independently peer inside and monitor the activity of nuclear reactors. The idea has captured the interest of the International Atomic Energy Agency (IAEA), which works to prevent diversion of fuel that could be used to make weapons.

The nuclear fission that provides power also generates abundant subatomic particles called antineutrinos, which rarely interact with other forms of matter. Antineutrinos fly right through radiation shields in numbers directly proportional to the operating power of the plant, which makes them potentially valuable indicators of the nuclear reactions inside.

“This is something fundamentally new. We can really tell what’s going on inside the reactor core,” said physicist David Reyna, who leads a project to design better detectors at Sandia National Laboratories in Livermore, California. “That information has been so unavailable that people who monitor activities at nuclear reactors have just started to think about what they would do if they had it.”

Reactors consume uranium and create plutonium. Only a few kilograms of plutonium are enough to make a nuclear weapon, so the IAEA closely tracks the material. Both uranium and plutonium produce antineutrinos, but their appearance in the detectors differs slightly.

“If you measure the energy precisely enough, you should be able to extract...the proportion of uranium and plutonium in the core at that moment,” Reyna said.

Their elusive nature makes antineutrinos notoriously difficult to catch, though the sheer numbers generated by reactors increase the chance of success.

“The antineutrino, itself, is basically impossible to see,” Reyna said. “It’s just passing through.” But antineutrinos will interact with particles called protons to set off a brief chain of events that is easier to see. “You get particles coming out of that interaction that you can identify,” Reyna added.

Each collision creates two quick flashes of bluish light in certain types of materials. The split-second timing of the flicker distinguishes an antineutrino’s trail from traces left by other forms of radiation.

“We can easily distinguish between gamma rays and antineutrinos because antineutrinos have such a unique signature,” said Adam Bernstein, a physicist at Lawrence Livermore National Laboratory, who leads the project to develop detectors that would be useful for safeguarding reactors. “It’s extremely difficult to mimic, so it’s very difficult to spoof the detector.”

Since 2004, teams from both laboratories have tested four experimental detectors at the San Onofre Nuclear Generating Station near San Clemente, California.

The first was a set of tanks filled with proton-rich fluid. Because natural radiation from rocks and space can swamp the signal from the antineutrinos, the engineers blanketed the detection tanks with water, which will slow other signals but let antineutrinos through. Although the combination of liquid tanks and water shield made this detector too large and heavy to easily deploy, it accurately reported operations at the station, demonstrating the usefulness of this approach.

In October 2008, the IAEA invited scientists from several nations who are developing ways to detect antineutrinos to Vienna to exchange ideas with people in the agency who are looking for new ways to monitor reactors.

“One very nice thing about this detection is that it’s very difficult to interfere with an antineutrino. Nothing really stops them,” said Julian Whichello, head of new technologies for IAEA. “It has an inherent sort of protection in the source of information.”

Antineutrinos also offer an autonomy from reactor operations that is absent from other forms of monitoring, such as measuring the flow of coolant. “It requires no connection of cabling to other parts of the reactor,” said Andrew Monteith, a physicist at IAEA. “It’s all self-contained. We’d like something that we could just plunk on the ground outside the reactor.”

The agency doesn’t quite see the need for antineutrino detectors for monitoring stations similar to San Onofre, which shut down periodically to load fresh fuel and remove spent rods. That pause gives inspectors the opportunity to directly count the fuel rods and measure the amount of plutonium.

But newer reactors, yet to be built, will operate continuously by cycling “pebbles” of fuel through the core. Antineutrino detection might be well suited for monitoring these new reactors, but they will first need to be made smaller.

Teams from both Livermore and Sandia are collaborating with university scientists to develop smaller detectors. They recently placed one that uses crystals of germanium in a tunnel outside the containment dome at San Onofre.

“We are already collecting data down there with this germanium detector,” said Juan Collar, a physicist at the University of Chicago who helped to develop the new device.

Instead of waiting for antineutrinos to interact with protons, a rare event, this latest prototype looks for evidence of atoms that have been nudged by antineutrinos, a much more common occurrence. That means the detector can be much smaller. “These germanium crystals would fit inside your fist,” Collar said.

Antineutrino detection projects such as these will not only continue to contribute to basic science but to applications relevant to national security as well. They may well be elusive, ghostly particles, but antineutrinos have a substantive future in many aspects of nuclear research.