Scientists Discover Unexplained Abundance of Rare Nuclear Fusion Fuel on Earth

Scientists have discovered evidence that a key rare resource, called helium-3, is potentially ten times more common on Earth than previously known—though the source of all this extra supply remains mysterious, reports a new study. The finding is important because helium-3 could serve as a foundation of limitless clean power for our civilization, but has been seen as inaccessible since it is largely found in outer space locations, especially the Moon.  

Helium-3 is an isotope of helium, which means it contains the same number of protons as this common element but a different number of neutrons. This isotope is considered a potentially powerful energy source for future fusion reactors, making it a star of science fiction as well as a sought-out resource in the real world. However, while small amounts of the substance are produced by geological processes and from the fallout of nuclear weapons testing, there is thought to be very little helium-3 available on Earth.

Now, scientists led by Benjamin Birner, a postdoctoral scholar in geosciences at the University of California San Diego, have captured evidence for a previously unknown abundance of helium-3 in the atmosphere, which “presents a major puzzle in the helium-3 budget” and “motivates a search for missing helium-3 sources on Earth, especially since helium-3 is considered an important, yet scarce, resource,” according to a study published on Monday in Nature Geoscience. Known sources of helium-3 on Earth only account for 10 percent of the surplus, the researchers said. 

Birner and his colleagues serendipitously uncovered this inferred surplus of helium-3 (3He) while tackling another challenging problem: measuring the overall rise in atmospheric helium as a result of human consumption of fossil fuels. The team pioneered a first-of-its-kind technique for estimating these anthropogenic helium emissions by examining another isotope, helium-4 (4He), which in turn led to the perplexing conclusion that there is some unknown source of helium-3 on our planet. 

“We only measured the change in atmospheric 4He,” Birner said in an email. “However, previous work by other researchers indicates that the helium isotopic ratio of the atmosphere (3He/4He) is roughly stable. Together these observations imply an increase in atmospheric 3He that matches the rise in 4He or we would see a change in the atmospheric isotope ratio.” 

Helium-3 could be the ideal fuel for nuclear fusion, a potential energy source that mimics the same process that powers stars. Though nuclear fusion may not materialize as a practical power source for decades, assuming it is feasible at all, its potential to provide clean and limitless energy to the global human population makes it a tantalizing area of study. To that end, scientists across fields are likely to be interested in locating this unexplained surplus of helium-3 on Earth that has been implied by the new research.

“That increase of 3He is quite puzzling because we don't have a good explanation for the source of this 3He so far,” Birner noted. “It's quite an important puzzle to solve also because 3He is an important and scarce resource for nuclear fusion reactors. Based on the reported uncertainties in previous studies of the atmospheric 3He/4He trend, the buildup of 3He looks significant, but our study clearly motivates a closer look at the atmospheric 3He/4He trend.”

Helium is the second lightest and most abundant element in nature after hydrogen, but it can also be produced by human consumption of fossil fuels, especially natural gas. Helium belongs to a special class of elements, called noble gasses, that are relatively unreactive with other substances. As a result, it is not considered a greenhouse gas or a dangerous pollutant, in contrast to other anthropogenic emissions such as carbon dioxide and methane. But though atmospheric helium does not contribute to human-driven climate change, it is an important tracer of those other, more dangerous emissions.

“The atmospheric change in helium is a decades-old question in atmospheric chemistry and helium should be able to inform our understanding of fossil fuel usage,” Birner explained. “Since the 1980s people have been suspecting that there should be a build up of 4He in the atmosphere but clear observational evidence was lacking.” 

“My colleagues and I have worked on atmospheric noble gasses as indicators of ocean temperature change for a while but hadn't applied the same analytical approach to helium so far,” he added. “It was a logical next step in a way because thanks to its link to fossil fuel use, helium is quite an interesting noble gas to study.”

Previous studies have produced anthropogenic helium estimates by focusing on the ratio between 3He and 4He, but the small amounts of 3He make that “an extremely hard measurement,” Birner noted. 

To get around this problem, the team developed a sophisticated method that measures the ratio of 4He against nitrogen gas (N2), which is both the most abundant element in the atmosphere and one that has relatively stable concentrations over time. 

“Our approach not only avoids measuring the rare isotope”—meaning 3He—“which improves our measurement precision, but also normalizes the abundance of 4He to N2,” said Birner, who also led a study last year in Atmospheric Measurement Techniques that delves deeper into the many conceptual advances and technical innovations of this new method. 

“N2 has been very stable in the atmosphere which makes 4He/N2 an indicator of the helium concentrations. 3He/4He in contrast could change because the numerator or the denominator changes,” he noted.

The team applied the new technique to 46 air samples acquired between 1974 and 2020, producing a new estimate of atmospheric helium-4 changes across a timescale of decades. The results revealed that “helium-4 concentrations have increased significantly over the past five decades” and that consequently the abundance of helium-3 “greatly exceeds estimates of anthropogenic emissions from natural gas, nuclear weapons, and nuclear power generation, suggesting potential problems with previous isotope measurements or an incorrect assessment of known sources,” according to the new study.  

“When we started, we weren't sure at all how large the atmospheric change in helium would be and if we would be able to measure it at all,” Birner said. “It took three years to develop and hone the analytical method for 4He/N2 so when I made the first repeatable measurements we were all really excited to see all that hard work come to fruition. The implications for 3He we only realized later as we were looking at our data and comparing it to previous work, and it did come as a surprise to me.”

“The inferred 3He change is more than 10x the natural geological fluxes,” Birner added. “We know that 3He is produced also by decay of tritium. Tritium was released by humans in nuclear bomb tests, by the current stockpile of nuclear warheads and is probably also made in some nuclear power plants. However, our estimate of these sources suggest they can only account for about 10% of the inferred 3He increase. It is not clear at all where the rest comes from.” 

Birner and his colleagues hope to root out this hidden supply of helium-3, whether it is natural or anthropogenic. More broadly, the researchers plan to apply their new technique toward untangling the various sources of anthropogenic greenhouse gasses on Earth, an effort that can help to inform our response to human-driven climate change.  

“Helium can help us disentangle and verify the proportion of carbon emissions from natural gas vs other sources such as coal or oil,” Birner said. “Scientists often perform so-called ‘inversions’ to infer local to global scale carbon dioxide (CO2) emissions. In an inversion, you use observed concentrations of CO2 in different places and infer how large the emissions of CO2 must have been to yield these concentrations.” 

“Now if you measure CO2 alone, the inversion will tell you the flux of CO2 but by also using helium, we may also be able to say what fraction of that CO2 came from natural gas burning because helium should be associated with natural gas but not as much with other emission sources such as car traffic,” he concluded. “I am now working on further developing the method to detect local changes of helium in San Diego. With some more tweaks, I am confident that precision will be good enough to see daily local variability in helium concentrations.”