[Image above] Credit: Pixabay
The current polar vortex in the United States notwithstanding, the global average temperature is increasing.
The starkest example and consequence of global temperature increase is the shrinking of glaciers and the polar ice caps. This shrinkage leads to higher sea levels, coastal flooding, and changes to weather patterns, such as dramatic increases in the number of storms in the north Atlantic. Even worse, shrinking ice means a smaller albedo (amount of solar energy reflected into space), which creates a positive feedback loop that sees greater solar energy absorption leading to temperatures rising and more ice melting.
Though many causes for the recent temperature rise have been investigated, “Only by adding the human-caused increase in greenhouse gas concentrations are the models able to explain the unprecedented warmth we are currently experiencing,” states the National Centers for Environmental Information on its website.
For those reasons, moving away from carbon-based energy and toward sustainable energy infrastructure is important for long-term world health. The new March 2021 issue of the ACerS Bulletin, now available online, offers an overview of these technologies. But today’s CTT will take an even closer look at one technology in particular: fusion power.
The benefits, challenges, and progress in nuclear fusion power generation was the focus of this year’s Kavli Medal and Lecture, which is awarded annually by The Royal Society to researchers who demonstrate excellence in all fields of science and engineering relevant to the environment. The 2019 Kavli Medal and Lecture was awarded to UK Atomic Energy Authority CEO Ian Chapman “for his scientific insight that has illuminated the complex physics of confined plasmas and prepared the way for fusion burn,” according to the website.
Putting the sun in a bottle: The path to delivering sustainable fusion power
Chapman starts his talk by illustrating the starkness of the carbon problem. He says that to achieve net zero carbon usage by 2050, we would have to build one fission nuclear power plant or one gigawatt-sized off-shore wind farm, at a cost of roughly $10 billion each, every day for the next 30 years. The magnitude of this problem is driving global development of safe and cost-effective solutions.
Chapman discusses how fusion is one potential solution to our sustainable energy future. Fusion is the reaction that powers our sun. It involves the creation of a new element by “smashing” together two atoms of other elements. The most common is the reaction of two forms of hydrogen atoms—deuterium and radioactive tritium—to form helium.
Fusion reactions release massive amounts of energy, so much so that the energy from fuel with the mass of a grain of sand could power your car for 20 minutes.
If we could “put the sun in a bottle,” i.e., control fusion reactions in a contained environment, it would offer us a carbon-free power source with a high energy density, low waste, plenty of fuel (water and lithium), and the ability to run constantly (unlike wind and solar). Additionally, fusion reactors are considered to be inherently safe. Unlike nuclear fission, which gets energy from the splitting of heavy atoms, fusion does not have the potential for runaway reactions because fuel is fed slowly and interruptions will cause the reaction to stop.
A brief history of fusion energy
The fusing of atoms requires input of extreme amounts of energy to overcome coulombic repulsion of atoms. In the sun, gravitational attraction provides the energy. In fusion reactors, the elemental reactants are accelerated into the reaction zone. The resulting reaction plasma is both very hot (millions of degrees) and unstable. Because the plasma cannot be contained by solid walls, it is isolated from the walls and stabilized by either electromagnetic fields or inertial fields.
Inertial isolation is achieved by using high power lasers. Facilities such as the National Ignition Facility at Lawrence Livermore National Labs have delved quite deeply into high energy science, with many substantial advancements. But, to the best of my knowledge, “ignition” has not been achieved using laser confinement. (Ignition is when the plasma reaches net positive energy, i.e., more energy comes out of the reaction than is put into it.)
With electromagnetic containment, charged ions in motion are “steered” by magnetic forces. While a number of different electromagnetic configurations are being explored (see this IEEE article), the most mature technology uses tokamaks, which are toroidal (donut shaped) reactor designs first developed in Russia in the 1950s. As is true of the inertial isolation systems, electromagnetic containment systems have yet to achieve net positive energy.
There are several electromagnetically contained fusion demonstration systems around the world, include JET in the U.K., which achieves 16 megawatts (MW) of thermal energy output. Unfortunately, it uses 25 MW of energy.
The ITER project is being built in France through a global public-private partnership including governments, technology company charitable foundations, and energy companies. The project goal is to demonstrate net positive energy generation with a 500 MW reactor, including a 10x return on the 50 MW needed to accelerate the fuel. The project is about 75% complete, with a projected online date of 2025. However, it has already yielded benefits by creating new supply chain companies and factories to produce parts for ITER and, ideally, for construction of commercial fusion power plants.
In his lecture, Chapman says that the ITER design has some challenges. Most significantly, it is too large and thus too costly. He described ongoing work at the UK Atomic Energy Authority, including more efficient heat exhaust (MAST Upgrade) and a spherical tokamak (STEP). Chapman describes a spherical tokomak as an “apple with the core removed.” The advantage of the spherical tokamak is the more efficient use of magnetic fields, resulting in smaller reactors and less expensive magnets.
Materials challenges
Several materials challenges must be met for the technology to become cost effective and commercially viable. First, fusion releases extremely high energy neutrons that bombard the walls of the reactor. This bombardment results in rearrangement of the atomic structure that embrittles and weakens the walls. The most viable solution is to use sacrificial linings on the reactor walls that can last for a few months or years before needing to be replaced. This solution presents an opportunity for the development of ceramics that are resistant to tritium absorption along with the radiation bombardment. Silicon carbide is a promising material, as are MAX phase materials.
Additional research is being conducted on “aneutronic” fusion reaction systems that use fuels other than tritium and deuterium and do not release the neutron. There is much interest in using these reactors for space flight propulsion.
The second materials challenge comes from heat removal and substantial thermal gradients within and around the reactor. The reaction zone is 150 million degrees (10 times hotter than the sun), and the walls (about 2 meters away) are a few hundred to perhaps a few thousand degrees. Just behind the wall must be close to absolute zero (-270°C) for the magnet wires to become superconducting.
The third materials challenge is development of viable high-temperature superconductors. High temperatures in this case are still quite cold at around 77 Kelvin (-196°C). But liquid nitrogen is substantially less expensive than liquid helium, and the reduced temperature gradient provides more leeway for reactor wall development.
Many technological challenges remain as well, such as development of in-situ robots to replace the reactor linings and perform other repairs in radioactive environments. Also, the efficiency of magnetic field generation must improve in order to reduce the size and cost of the reactors and power plants. The work of Chapman and others in the U.K. show promise in these areas and others.
Conclusion
In summary, the field of fusion energy generation will soon see major advances with the ITER project and the lessons that are being and will be learned from it. The materials and technological challenges provide opportunities for ceramic scientists and engineers now and for many years to come.
You can watch the full 2019 Kavli Medal and Lecture here.