VT Science Corner: Neutrinos and nuclear weapons

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Patrick Huber

In addition to the title of a horror movie in the 1980s, Poltergeist was the name of the first experiment trying to prove the existence of neutrinos.

The name was aptly chosen because neutrinos, like ghosts, can go through walls or whole planets or stars. Almost all elements, the building blocks of matter, were either created by nuclear fusion in stars or by the violent death of stars. Both processes rely on the exchange and creation of neutrinos.

Neutrinos have many other baffling properties that make them one of the central research subjects in particle physics.

The U.S. is taking leadership in neutrino physics with the construction of the Deep Underground Neutrino Experiment, and my own research since my Ph.D. days has been about neutrinos.

I grew up in the 1980s in West Germany at the height of the Cold War, and my biggest childhood fear was being incinerated with everyone else in a thermonuclear holocaust. One day I came across an article in a popular science magazine that explained in simple terms how a nuclear bomb works. That day, my fear was transformed into curiosity. This experience put me on my path to become a physicist.

The initial discovery of neutrinos happened at a nuclear reactor at Savannah River, and since then much of what we've learned about neutrinos stems from experiments at reactors. Reactors are the brightest neutrino source on earth: about 3% of total reactor power is emitted as neutrinos.

The crux of nuclear technology is that it is dual use: the same reactor that can make electricity also can make plutonium for bombs. It turns out that not only can we use reactors to learn about neutrinos, but also, we can use what we know about neutrinos to learn about the state of a reactor.

Neutrinos, being the ghosts they are, cannot be shielded or deflected, neither can you fake a neutrino source to look like a reactor.

The idea is simple: put a neutrino detector outside of the reactor building or at the fence of the facility, measure neutrinos, and learn about reactor power and plutonium content.

With a reactor used for electricity, a neutrino detector would not detect many refuelings. Frequent refuelings would indicate it was making weapons-grade plutonium.

An accurate neutrino detector could revolutionize the way nuclear non-proliferation safeguards work: we could be certain that a given reactor is used only for peaceful purposes, that is few refuelings, without relying on the operator telling the truth.

When I started my research in this area around 2010, detectors could not distinguish between a facility generating electricity versus one making plutonium. Everyone told me I would be wasting my time to try to develop one.

My first major task was to understand the number and energy of neutrinos a reactor makes with great accuracy. This led to what now is called the Huber-Mueller reactor neutrino flux model. Using this model, we were able to theoretically study how neutrinos could have been used during the North Korean nuclear crisis of 1994 and also develop parameters to monitor the Iran nuclear deal to verify that reactors were operating for peaceful purposes.

My colleagues at Virginia Tech's Center for Neutrino Physics, Professors Jonathan Link and Camillo Mariani, and I founded the Mobile Neutrino Lab. We developed the type of detector needed for these applications, the kind I was told would be impossible. We successfully detected neutrinos at the North Anna nuclear power station in 2018. Now, we are working with Lawrence Livermore National Laboratory to build the next generation of detectors as an evolution of the work started for the Mobile Neutrino Lab.

Nuclear energy is seeing a resurgence driven by concerns about climate change, and at the same time the threat from nuclear weapons is worse than any time since the end of the Cold War. Better technology to monitor for nuclear non-proliferation will be essential to safely navigate those challenges.

Patrick Huber is a professor of physics at Virginia Tech.

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