Harnessing the power of the sun—that is the dream of nuclear fusion engineers, but how close is the technology to realization?
The inside of the Joint European Torus tokamak generator. (Image courtesy of EUROfusion.)
In this article, we examine the technology, including the numerous projects currently underway—not the least of which is the massive ITER fusion reactor being constructed in the south of France—to understand just how it is evolving and whether or not it will be deployed any time soon.
Fusion vs. Fission
Whereas nuclear fission is the splitting of the atoms of an atomic nucleus, fusion is the fusing of nuclei. Both processes release energy. In the case of fission, an unstable isotope (most commonly Uranium-235) is bombarded with particles (usually neutrons) until one accelerated particle divides the nucleus of the isotope, resulting in two smaller isotopes, three neutrons, gamma rays, and the heat energy of the reaction.
This energy is then used to heat water to create electricity, while the remaining neutrons perform subsequent reactions with other nuclei. The fission products are used for such applications as nuclear fuel, aircraft counterweights, armor plating, radiation therapy, or anti-tank shells.
In contrast, nuclear fusion occurs with the joining of nuclei from two low-mass isotopes (usually forms of Hydrogen, like Tritium and Deuterium) under extreme heat and pressure (think Sun-level extreme). Using hydrogen atoms, this reaction produces a neutron, a helium isotope and several magnitudes of energy more than that which is gained from nuclear fission.
While we have so many nuclear fission reactors and all of their radioactive wastethat we don’t even know what to do with, fusion is another story—all fusion reactor projects are still in the experimental phases. Progress with the creation of a viable fusion reactor is slow-going, not just due to the enormous pressure and heat required, but also the actual containment of the fusion reaction over time. The resulting neutrons actually degrade materials within their containment chambers. In the end, fusion reactors are unable to output more energy than what has been put into the system.
This hasn’t stopped researchers in the field who believe that nuclear fusion reactors could provide more energy for a given weight of fuel than any other energy source humans currently rely on. The fuel source, hydrogen, exists abundantly in seawater. Moreover, fusion reactions are thought to be less radioactive and produce less high-level waste than fission reactions. For these reasons, nuclear fusion is often touted as a completely clean and limitless source of energy, even though it is neither completely clean nor limitless.
In nature—that is to say the cold depths of space—nuclear fusion occurs over tens of millions of years. In the case of our sun, matter within a giant molecular cloud collapsed as the result of a shockwave from nearby supernova.
Most of that matter gathered in the center, while the rest spread out in a disc shape that has come to be known as the Solar System. From the physics of the collapse, the center mass began rotating and accreting matter from the surrounding disc, increasing pressure and gravity on the central core, causing it to heat to such an extent that a fusion reaction was initiated.
On Earth, researchers have to replicate the massive heat and pressure that occurred with the sun’s formation, but they don’t have tens of millions of years to do so.
For two nuclei to merge, they must overcome the repelling forces of their protons (like when you put two positive sides of a magnet together). This means extreme heat of about 100 million Kelvin (about six times hotter than the Sun’s core), hot enough to strip away the electrons and expose the bare nuclei. The result is a hot cloud of ions and electrons known as plasma.
Hydrogen atoms must also be brought within 1x10-15 meters of one another in order to fuse. To achieve such high temperatures and such high pressure, researchers resort to microwaves, lasers, magnetic fields and ion particles.
To achieve the pressure and heat necessary to generate a hydrogen fusion reaction, researchers use either magnetic or inertial containment. The former applies magnetic and electric fields, while the other relies on laser or ion beams to heat and compress hydrogen plasma. While some experiments have been capable of generating more energy than was used to ignite a nuclear reaction, none has been able to make plasma dense enough or confine it for long enough for the reaction to become self-heating, necessary for a nuclear fusion power plant.
In magnet-based approaches, particle accelerators direct neutral particle beams, along with electricity and microwaves, at a stream of hydrogen gas, converting it to a plasma. The plasma is then constrained by super-conducting magnetics which squeezes the material until fusion occurs. A toroid (donut shape) is the most efficient geometry for magnetic confinement of plasma. A donut-shaped fusion reactor is referred to as a tokamak.
ITER construction progress as of 2018.
Perhaps the most notable project employing the use of magnetic containment is that of ITER (International Thermonuclear Experimental Reactor), an international endeavor unfolding in Saint-Paul-lès-Durance, France with a price tag of over $20 billion. Though the history of ITER dates back to talks between Reagan and Gorbachev in 1985, initial design work began in 2001, site preparation kicked off in 2008, and construction of the reactor began in 2015. The first plasma is scheduled for “achievement” in 2027, and the megaproject is supposed to go into operation starting in 2050 (but who’s counting!).
When operational, the ITER tokamak will heat a stream of deuterium and tritium fuel to create plasma. A neutral beam injector will direct particle beams from the facility’s particle accelerator into the plasma to heat it to the necessary temperature. Powered by transformers of the system’s central solenoid, super-conducting magnets will contain and shape the plasma, while a cryopump keeps the magnets cool.
Blanket modules of lithium will be used to absorb the fusion reaction’s heat, which can heat water in a heat exchanger to make steam, and neutrons, which will be used to make more tritium fuel. As occurs with fossil fuel and nuclear power plants, the team will drive electrical turbines to create electricity. Meanwhile, divertors will be used to exhaust the helium products that result from the reaction.
An outline of the ITER tokamak will be the largest tokamak reactor in the world. (Image courtesy of ITER.)
The first fusion reaction will use about 70 megawatts of power, but, if the eggheads have all of their math right, the power generated from the reaction will be about 500 megawatts. Though the initial test reaction (2025) will last just 300 to 500 seconds, a sustained reaction (2035) to run the power plant is the larger goal.
The National Ignition Facility (NIF) at Lawrence Livermore National Laboratory (LLNL) is one of the most widely known experiments in inertial containment. The site was born out of experiments and plans in the late 1970s before construction began in 1997 and was completed in 2009. At that point, experiments were conducted to test the power of the system, concluding in September 2012 after only attaining 1/10 the power necessary to create a fusion reaction.
A diagram of nuclear fusion technology developed by the National Ignition Facility. (Image courtesy of LLNL.)
The NIF system will rely on 192 laser beams to focus a single point onto a pellet of deuterium-tritium housed within a plastic cylinder within a chamber called a hohlraum. A combination of the 1.8 million joules of heat from the lasers and the X-ray radiation produced by the hohlraum will cause the pellet to collapse into plasma, with the inertial forces squeezing it until fusion takes place.
According to the lab’s estimates, the fusion reaction will last just one-millionth of a second but will create 50 to 100 times more energy than the energy that went into the system. If successful, a reactor can be designed using the same principles with multiple targets hit in succession to create enough heat for sustained energy production. Each target could be made for as little as $0.25.
The heat from the reaction would then be used to power a heat exchanger to generate steam power.
Since 2012, further experiments have been able to reach 1/3 the power necessary for igniting a fusion reaction. A test in 2013 had LLNL researchers boasting of a “break even” record, suggesting that the amount of energy put into the system resulted in an equal amount to that generated by the system; however, critics pointed out that the break-even only occurred in terms of the amount of energy directed at the pellet, not the overall energy used by the experiment.
Since then, the NIF has been redirected toward other scientific and weapons research, with the massive laser apparatus directed at shots of plutonium, instead of hydrogen, to simulate the detonation of nuclear bombs by high explosives.
Though these are two of the largest and most notable nuclear fusion projects underway, they are not the only ones. An MIT endeavor, for instance, is aiming to realize fusion power in about 15 years using magnets made from superconducting materials. These smaller, more powerful magnets made from yttrium-barium-copper oxide are meant to exert even more pressure on the hydrogen plasma fuel, hopefully generating somewhere between 50 and 100 megawatts.
A rendering of the Sparc project under development at MIT. (Image courtesy of MIT.)
China’s Experimental Advanced Superconducting Tokamak reactor may be one of the most successful to date, confining hydrogen plasma in a steady state for 101.2 seconds. Other successful experiments include: a test from the Wendelstein 7-X stellarator that contained plasma for about a quarter of a second and research using MIT’s Alcator C-Mod tokamak that was able to contain plasma for two seconds.
There are also a number of startups racing to be the first to build nuclear fusion reactors, such as Tokamak Energy, aiming to build a miniature tokamak reactor using high-temperature superconducting magnets. Tokamak Energy hopes to generate industrial-scale heat by 2025. Other startups include General Fusion, Helion Energy, TriAlpha Energy, Commonwealth Fusion Systems (a spin-off of the aforementioned MIT project) and First Light Fusion.
While these projects indicate that there is progress in the development of nuclear fusion power generation, we aren’t holding our breath: the timescales are still in the multi-decade range before fusion arrives on your household power bill.
Check out this article on today’s nuclear fission technology here.