Nuclear fusion: UK experiment centre explained by expert

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Isotopes are forms of the same element that have the same number of protons in their nuclei but a different number of neutrons, thereby giving them a different atomic mass.

Helium, for example, has nine known isotopes, although only two — helium-3 and helium-4 — are stable, with the rest being incredibly short-lived radioisotopes.

Of the stable isotopes, helium-3 is particularly sought after, with varied applications including as a fuel for future nuclear fusion power plants, for cryogenics and for medical imaging.

In fact, the gas has such potential that since the late eighties people have been drawing up proposals to go to the Moon — where helium-3 can be found on the lunar surface — to harvest the scarce resource.

In their study, geoscientist Dr Benjamin Birner of the Scripps Institution of Oceanography at the University of California San Diego and his colleagues did not actually set out to look for helium-3.

Instead, the team were focused on the atmospheric abundance of another isotope, helium-4, which is released during the extraction and burning of natural gas.

The team explained: “Natural gas is enriched in helium-4 from radioactive decay of thorium and uranium in Earth’s crust.

“Anthropogenic fossil fuel usage has therefore greatly increased the release of crustal helium-4 to the atmosphere over the natural background rate.”

In theory, as most natural gas sources are less enriched in helium-3, the extraction of fossil fuels has long been expected to slowly increase atmospheric helium-4 levels while decreasing the helium-3/helium-4 ratio.

Despite this, however, past studies have yielded seemingly contradictory results — finding both a hint of a human-driven helium-4 signal but also a consistency in the atmospheric helium-3/helium-4 ratio.

Dr Birner said: “The main motivation was to resolve [this] longstanding controversy in the science community about atmospheric helium concentrations”

Accordingly, the team devised a new high-precision mass spectrometry technique that improves the measurement of helium-4 in the atmosphere by nearly two orders of magnitude.

The new technique works by comparing — with a high degree of precision — atmospheric concentrations of helium-4 with that of nitrogen, a common atmospheric gas.

As nitrogen levels in the atmosphere remain constant, an increase in the helium-4/molecular nitrogen ratio can serve as a proxy for increasing helium-4 buildup in the atmosphere and, by extension, fossil fuel consumption.

After analysing 46 air samples collected between 1974 and 2020, the team found significant increases in helium-4 concentrations over the past five decades.

Specifically, the atmospheric concentration of the isotope appears to have risen at an average rate of around 39 billion mol per year.

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Paper co-author and Scripps Institution geochemist Professor Ralph Keeling — who is famous for maintaining the “Keeling Curve”, a daily record of global atmospheric carbon dioxide concentrations — said the study is a “masterpiece of fundamental geochemistry”.

Although helium is itself relatively easy to detect in air samples, Prof. Keeling explained, no one had previously taken atmospheric measurements that were precise enough to confirm the slight but nevertheless significant atmospheric increase.

If, then, atmospheric helium-4 is indeed increasing with fossil fuel usage but the ratio of helium-3 to helium-4 is remaining constant as past studies have concluded, it must follow that the atmospheric helium-3 concentration is also being boosted.

Exactly where this potentially useful isotope is coming from, however, remains to be determined — although the team believe that it may be as much as ten times as common on Earth as was previously thought.

Dr Birner said: “We don’t know for sure, but I wonder if there is more helium-3 coming out of the Earth than we previously thought.”

If this supply could be located, he added, it “could perhaps be harvested and [used to] fuel our nuclear fusion reactors in the future.”

Prof. Keeling added: “The study lays in starker relief a controversy surrounding the rare helium isotope helium-3.

“The implications are far from clear, but it begs additional work.”

The full findings of the study were published in the journal Nature Geoscience.

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