Harnessing the power of the sun—that is the dream of nuclear fusion engineers, but how close is the technology to realization?
Fusion vs. Fission
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 waste that 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.
Fusion Reactions
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 an astounding 150 million degrees Celsius (about ten 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.
Fusion Reactors
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.
Magnets
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.
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.
Inertia
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.
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.
Timeframes
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.
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.