How Neutrinos Can Help Track Nuclear Bombs

Like his fellow neutrino hunters, Georg Raffelt, a scientist at the Max Planck Institute for Physics in Germany, explores how the mysterious subatomic particle assists the universe in often unsung ways. For instance, neutrinos, which are produced in nuclear reactions, appear to play an important role in helping to power the shockwave of an exploding star. This ensures that, rather than becoming a black hole, the elements of the star get recycled into the universe, and eventually into our bodies. "Since neutrinos are crucial for blowing up stars," he says, "they are important for our very existence."

But researchers who study the neutrinos that travel across the galaxy—and which fly through our bodies, billions every second—are beginning to investigate how detecting them might be useful for keeping tabs on nuclear activity on Earth too. Because they can pick up the traces of nuclear reactions, their machines could help expose rogue reactor operators, catch dangerous plutonium smugglers, and even stop would-be nuclear bomb makers in their tracks. “Something as esoteric as neutrinos may turn out to have a practical use,” Raffelt says.

Nuclear reactors used for power generation offer a potential source of weapons-grade plutonium, which builds up over time as uranium undergoes a fission reaction and splits into lighter elements. In its efforts to prevent the spread of nuclear weapons, the International Atomic Energy Agency monitors and inspects reactors in civilian use, and periodically, the IAEA inspectors compare the records kept by the reactor operators with their own monitoring data to assess whether there has been clandestine activity, such as frequent shutdowns to shuffl
e the fuel rods more often than necessary.

Such "containment and surveillance" measures have been used by the IAEA in Iran, for instance, and will continue to be used under a new deal that begins today. Signed in November by Iran and six other governments, the agreement involves freezing key parts of the Iranian nuclear program in exchange for a decrease in sanctions, to provide time to negotiate a permanent agreement. Iran has also agreed to stop enriching uranium beyond five percent.

To verify compliance, the inspectors' remote monitoring instruments need to tap into the reactor’s plumbing—to keep track of the amount of coolant used, for example. But this equipment is unwieldy, expensive, and susceptible to tampering. Fortunately, the same fission reactions that produce plutonium from uranium also release a by-product: antineutrinos. Detecting these antineutrinos would provide a direct, real-time measure of the nuclear reactions, thus a more reliable probe for the international monitors. As Raffelt put it, “Antineutrinos don’t lie.”

There is no way to prevent these particles from escaping the reactor and revealing its activity if a detector could be placed nearby. Normal power plants burn the fuel rods continuously until the fissile material pretty much runs out, typically in about eighteen months. Over that time, as the fissile material is used up, the plant’s power output—and the antineutrino fl
ux—drops slowly and predictably.

But if someone were interested in stockpiling plutonium, he or she would have to shut down the reactor for at least a day every few weeks to swap the fuel rods. “You need to bake the rods just the right amount to get plutonium,” explains John Learned, a physicist who studies neutrinos at the University of Hawaii. “So if there is a reactor that shuts down once a month, then you know for sure they are cooking bomb material,” he warns.

In theory, measuring antineutrinos coming out of a reactor would be a great way to keep tabs on the reactor’s operations. In practice, however, there are a few complications. One of them is that it’s difficult to build a neutrino detector that is relatively compact but sufficiently sensitive. The other is figuring out a way to shield it from stray particles such as cosmic rays. Besides, as Learned explains, “commercial power plant operators don’t really want goofball physicists messing around their facilities. Who knows what trouble they might cause!”

Even so, researchers from Lawrence Livermore National Laboratory and Sandia National Laboratories have tested a prototype at a nuclear power station in southern California. Their refrigerator-size instrument contained more than 1,300 pounds of mineral oil. The detector, placed 10 meters (33 feet) below the ground, was able to measure neutrinos from the nearby reactor and determine its power output to within a few percentage points of accuracy. It could tell within mere hours when the reactor was shut down.

While this test run produced promising results, it also pointed to the limitations of current technology. For one, mineral oil-based neutrino detectors are difficult to deploy. So researchers in several countries, including Brazil, Canada, France, and the United States, are experimenting with detectors that use water or plastic instead. In addition, placing a detector underground is not possible at all nuclear sites. But that is not essential, at least for monitoring the more powerful reactors. If the detector is close enough, the neutrino
flux from the reactor would shine much more brightly than the random background events caused by cosmic rays.

Some scientists expect that compact, reliable antineutrino detectors will be placed at cooperating nuclear reactor sites in the near future to verify that they adhere to the IAEA safeguards. There may even be an economic benefit to the power plant operators: they can improve the efficiency of their reactors by making appropriate adjustments based on real-time feedback from neutrino measurements. In the unfortunate event of an accident, neutrino detectors could trigger a timely shutdown of the reactor.

Other scientists have grander visions, which may be of interest to intelligence agencies and national security officials. They are investigating ways not only to monitor known nuclear sites from a distance but also to uncover clandestine reactors that have not been reported to the IAEA. Indeed, the most powerful reactors are difficult to hide, and since they emit a lot of heat, infrared satellites can spot them from space. Nuclear monitors are most worried about medium-scale facilities in rogue nations, which could be concealed more easily yet produce enough plutonium to make a bomb in only a year. Based on initial calculations, Learned says that future technology should make it possible to detect nuclear sites remotely.

“Of course the farther away you are, the bigger the detector you need,” he points out. The problem, however, is that bigger detectors suffer from noisier backgrounds, so they need to be buried underwater or underground. He envisions “a mobile underwater detector, perhaps mounted in a giant submarine, which could be towed into place.” Such a contraption could remain in international waters and monitor a suspected country for stealth nuclear reactors from a safe distance. Learned even looked into acquiring an old Russian submarine to test this idea.

In a similar vein, Thierry Lasserre of the French Alternative Energies and Atomic Energy Commission and his colleagues have proposed turning an oil supertanker into a mighty neutrino detector to look for undeclared nuclear sites. They have given their concept a name worthy of a spy novel: SNIF, for Secret Neutrino Interactions Finder.

Both schemes sound far-fetched for now, given the formidable technological and political hurdles that need to be overcome, but they may be worthy of further investigation. Someday we may also use neutrino detectors to watch over covert nuclear bomb tests. Current surveillance depends on techniques such as monitoring tremors in the earth.

"They have missed some and have also had some false alarms, which turned out to be seismic events rather than bombs,” according to Learned. Moreover, the use of cushioning materials and variations in cavity size can make it difficult to work out the exact parameters of a test explosion. “Detecting even one neutrino would tell you a great deal. If its timing matches a seismic event perfectly, then you know it was indeed a nuclear detonation,” he explains. “If we measure ten neutrinos, we could nail down its magnitude pretty well.”

Ray Jayawardhana is a physicist and the author of NEUTRINO HUNTERS: The Thrilling Chase for a Ghostly Particle to Unlock the Secrets of the Universe, published in December 2013 by Scientific American/Farrar, Straus and Giroux, from which this story was adapted. Copyright © 2013 by Ray Jayawardhana. All rights reserved.