"Ghost Particle" Detectors Closer to Preventing Nuclear Proliferation

28 April 2011—The neutrino, or "ghost particle," is strikingly aloof. On Earth, tens of billions of the sun’s neutrinos pass through an area the size of a thumbtack every second. But most of these particles zip straight through Earth without a single interaction with another bit of matter.

Neutrino detectors, which aim to catch such ghosts, are traditionally big affairs, sometimes employing large vats of water or oil, a deep patch of the Mediterranean, or even a cubic kilometer of Antarctic ice to boost the chance of seeing the specters.

But physicists have been working to adapt the technology to make detectors small enough to be installed inside nuclear power plants. If their prototypes are proved, such detectors could continuously monitor nuclear reactors and provide a new way to safeguard against nuclear proliferation.

The detectors in development hunt for the neutrino’s antimatter sibling, the antineutrino, which shares most of its properties. Nuclear reactors are the largest man-made source of antineutrinos. A typical 1-gigawatt nuclear reactor pumps out some 100 billion billion antineutrinos per second. Because these particles pass through any shielding pretty much unimpeded, their signal can’t be masked.

A joint group of physicists based in California at Lawrence Livermore National Laboratory and Sandia National Laboratories, along with researchers at Atomic Energy of Canada Limited's Chalk River Laboratories, aims to capture reactor-born antineutrinos with a detector they plan to install next year at the Point Lepreau Generating Station, a CANDU-type nuclear reactor in New Brunswick, Canada. They will present an update on tests of the detector next week at the American Physical Society April Meeting in Anaheim, Calif.

To catch antineutrinos, the detector employs hundreds of liters of organic solvent mixed with gadolinium atoms. Incoming antineutrinos occasionally collide with protons in the mixture, creating a neutron and a positron, the antimatter partner of the electron. The positron creates a flash of light when it meets with and annihilates an electron, and the neutron releases light when it is captured by a gadolinium atom. The detectors are studded with photosensitive tubes that convert these flashes of light into electronic signals.

The two precisely timed flashes are unique to antineutrinos, but physicists must still contend with confounding signals, particularly those created by charged particles that originate in Earth’s atmosphere. They can insulate detectors from this background noise by placing them underground.

The Point Lepreau detector, which will measure about 3 meters on each side, is the fourth in a series developed by the Livermore-Sandia team. The previous three detectors were installed over the course of the past decade at a 1.1-GW pressurized water reactor at California’s San Onofre Nuclear Generating Station.

The team’s first demonstration detector at San Onofre picked up about 400 antineutrinos per day during a 600-day test period. That was good enough to provide near real-time data on the state of the reactor. The detector could "tell when the reactor has been turned off within a few hours, which is important, because if you wanted to remove any fissile material, you would have to turn the reactor off," says team member Timothy Classen, of Lawrence Livermore.

But the team hopes it can mine antineutrino data to give even more information about what’s going on inside the reactor. An operator that runs a reactor at a higher power or uses fuel with more uranium-238 can boost the production rate of plutonium that could be used for nuclear weapons. Uranium releases more detectable antineutrinos than plutonium does, so monitoring the rate of antineutrino emission could potentially indicate whether a reactor is being run as intended or if material for weapons has been removed.

At the moment, the sensitivity of the detectors to changes in plutonium content is fairly low. But in a paper that will appear in the Journal of Applied Physics, Adam Bernstein, who leads the Lawrence Livermore group, and colleague Vera Bulaevskaya determined that, using 90 days of data, a detector that can register 2000 antineutrinos per day from a pressurized water reactor could be used to detect whether 73 kilograms of plutonium had been removed.

The International Atomic Energy Agency (IAEA) deems just 8 kg of plutonium a "significant quantity," because it is enough to make a nuclear explosive device. On their own, antineutrino detectors may never be able to sense whether such a small amount goes missing from the entire reactor. But the data they collect, combined with reactor simulations, might be able to show whether each grouping of fuel rods contains as much plutonium as expected to within a "significant quantity" at the end of a fuel cycle, Bernstein says.

A handful of other teams are also working on antineutrino reactor monitors. At the French Alternative Energies and Atomic Energy Commission (CEA), in Saclay, France, a team led by Thierry Lasserre and David Lhuillier has developed a detector called Nucifer, set to be installed at the 70-megawatt Osiris research reactor within the next several months. Due to some design improvements, Nucifer is expected to detect half of the antineutrinos that interact with the detector—a fivefold increase over the Livermore-Sandia team’s original San Onofre detector. If Nucifer were installed at a similar plant at the same distance from the core, the team estimates it would register several thousand antineutrinos each day—enough to detect a change in plutonium content.

Both teams are aiming for a detector that is inexpensive enough to be used commercially. Bernstein estimates the cost of the Livermore-Sandia detectors at US $250 000 but expects that could be driven down to $100 000. "Our contention is that in an engineering setting, where you’re not buying custom pieces and you’re not putting it together in a lab, the cost would go down substantially," he says.

One of the appealing aspects of the technology is that it is relatively inexpensive to integrate into a nuclear power station. Because antineutrinos pass easily through matter, the detectors can be installed outside of a containment vessel. "There’s a lot of expense in connecting anything inside a nuclear facility," says Julian Whichello, head of the IAEA’s Novel Technologies Unit, in Vienna. "But with an antineutrino detector, it’s a nice self-contained box that sits next to a reactor, and all you have to do is provide it power."

Whichello says today’s reactors are "pretty well covered" by current IAEA safeguard technology. The agency monitors nuclear reactors by sending inspectors when fuel is being switched in and out, a process that ordinarily occurs every 12 to 18 months. A suite of sensors that include cameras, digital seals on fuel assemblies, and radiation detectors are also used.

But Whichello says these safeguards may not be enough for next-generation reactors—like pebble-bed and liquid-core models—that are designed to run continuously, without ever requiring a shutdown to be refueled.

Continuous fuel loading can make tagging and inspection difficult. "You can no longer individually identify an element to trace it," Whichello says. "This is why we feel we need additional technology for the future."

In 2010, the IAEA invited teams working on the technology to form a working group to discuss its potential uses. The group’s first official meeting is set to take place in September at IAEA headquarters in Vienna.