Science on the Attack: Nuclear Fusion – the Energy Hope of the Future

As one of my series of occasional posts showcasing science on the attack rather than under attack, this post reviews the present status of nuclear fusion as an energy source. Although not yet an engineering reality, fusion is one of two potential long-term technologies for meeting the world’s future energy needs – the other being already commercialized nuclear fission.

Whatever one’s views on fossil fuels, it’s likely this now abundant source of energy will become depleted in a century or so. And despite the promise of renewable energy sources such as wind and solar, the necessary development of large-scale battery storage capability, to store energy for those times when the wind stops blowing or the sun isn’t shining, is decades away.

Fission and fusion are both nuclear processes. Fission is the splitting through bombardment of a heavy nucleus such as uranium into two lighter nuclei. Establishment of a self-sustaining chain reaction unleashes an explosive amount of energy; a controlled chain reaction is the basis for a nuclear reactor, while an uncontrolled reaction is the basis of the atomic bomb.

Fusion, on the other hand, smashes two light nuclei together at high speed to form a heavier nucleus. The light nuclei are typically deuterium and tritium, isotopes of hydrogen containing one and two neutrons, respectively (the hydrogen nucleus consists of just a single proton). This process, which powers our sun and other stars, also releases vast amounts of energy and can result in a self-sustaining chain reaction when enough fusion reactions occur. Uncontrolled fusion is the principle of the so-called hydrogen bomb, while fusion as an energy source involves a controlled reaction.

That fusion hasn’t become commercial after nearly 80 years of research and development is because it’s difficult to sustain the very high temperatures required – 50 to 100 million degrees Celsius – to make the process work. At lower temperatures, the light nuclei can’t be pushed close enough together for them to collide and fuse.

What this means in practice is that the deuterium and tritium fuel must be in the form of either a high-temperature plasma confined by strong magnetic fields, or a small pellet a few millimeters across that is heated and compressed by powerful lasers or particle beams. Typical experimental reactors for the first method, known as magnetic confinement, are shown in the two figures below.

The most common kind of magnetic confinement uses a doughnut-shaped or toroidal magnetic field combined with a perpendicular or poloidal field. The combination produces a spiral or helical field, as illustrated in the following schematic; the central solenoid induces a powerful electric current that both ionizes the deuterium and tritium reactor fuel and heats the resulting plasma. High-energy neutrons from the fusion reaction are absorbed in an outer blanket containing lithium, generating heat that can be converted to electricity.

In inertial confinement, precisely focused laser or ion beams heat the outer layers of the fuel pellet, exploding the fuel outwards and in turn producing an inward-moving shock wave that compresses the core of the pellet. The fusion reaction then spreads through the whole pellet as the chain reaction proceeds; extracting the resulting heat provides electricity.

The promise of fusion is immense. A few teaspoons of seawater, from which deuterium can be extracted, can provide as much energy through fusion as several tons of coal, oil or gas. But up until now there’s been a major problem: to release that much energy, an even greater amount of energy has to be supplied to the lasers or the coils powering the magnets.

Only recently have experimental fusion reactors achieved and ever so slightly surpassed this breakeven point. Researchers at the Joint European Torus, a magnetic confinement machine in Oxfordshire, UK reported in February this year that their reactor had been able to sustain a fusion reaction for five seconds, with a net output of heat energy. The National Ignition Facility at Lawrence Livermore National Laboratory in the U.S. announced last August that their inertial confinement reactor had generated over 10 quadrillion watts of fusion power for all of 100 trillionths of a second.

While these seem like baby steps, they represent a breakthrough for fusion technology. Now that the energy threshold has been exceeded at all, scientists are confident that extending the reaction time from seconds to hours or days is not far away. Those in the industry say they expect the 2020s to see a transition from experimental reactors to commercialization, with the first fusion facilities becoming connected to the grid in the 2030s.

Advantages of fusion energy over fission include a small footprint, no risk of a meltdown and no high-level nuclear waste. The radioactive waste that is generated is short-lived in comparison with fission waste.

Next:  Sea Level Rise Is Partly Anthropogenic – but Due to Subsidence, Not Global Warming