By Robert J. Goldston
April 14, 2021
The National Academies of Sciences, Engineering, and Medicine recently completed two studies that together map out a strategy for the development of fusion energy. The first, issued in 2019, titled “Final Report of the Committee on a Strategic Plan for US Burning Plasma Research,” endorsed a new goal for US fusion energy research and development: a fusion pilot plant. It recommended that the United States should focus its fusion R&D on a minimum-cost device capable of putting electricity on the grid, with the capability of qualifying the technologies required for economically competitive fusion energy.
A second National Academies’ report, just issued, is called “Bringing Fusion to the US Grid.” It delves into the specific requirements for a pilot plant and reflects the strong impetus to accelerate fusion development, so that fusion can better help to battle climate change by contributing soon to the decarbonization of electrical energy sources in the United States. At the same time it notes that the world-wide demand for low-carbon electical power will grow by a further factor of 5 to 6 during the second half of this century, providing a large market for fusion electricity.
The drive for a near-term fusion pilot plant comes from three directions. First, the international ITER project—the world’s largest fusion experiment, a collaboration of 35 nations—is now moving forward smoothly, led by its energetic and savvy director general, Bernard Bigot. Low-power operation is to begin later this decade, and high-power operation with ITER’s planned fusion fuel—deuterium and tritium, the heavy isotopes of hydrogen—is to begin the next. ITER’s goal is to produce 400-to-500 megawatts of fusion power, 10 times the amount of power that will be injected into the hot fuel.
It is important to note, however, that ITER will not convert the fusion power it produces into electricity. Instead, ITER’s goal is to demonstrate that a real-world fusion device can maintain high power production for extended periods of time—and test the integration of fusion physics with key materials and technologies along the way. Both China and the United Kingdom have plans to commission net electricity-producing fusion devices to run in parallel with ITER’s high-power operation. Meanwhile, the European Union has a strong program keyed to obtaining results from ITER. This strategic environment lends a sense of urgency to answering the question of what the United States should do.
The second new item adding momentum to fusion’s development is the engagement of 24 industrial enterprises around the world that have collectively made investments of about $1.5 billion in the competition to put fusion electricity on the grid. Some are pursuing technological innovations and variations based upon the current front-runners for fusion power production, the “tokamak” and the “stellarator”—most notably, the use of new high-temperature superconductors. (More about tokamaks, stellarators, and other configurations for fusion below.) These new superconductors are capable of delivering much higher magnetic fields than were available at the time of the design of ITER—a tokamak—or the major stellarator experiments in Japan and Germany. Other enterprises are pursuing scientific innovations to invigorate less well-developed configurations. The success of public-private partnerships, such as SpaceX, suggests that industrial partnering may be key to accelerating fusion development. And industries, of course, are interested in winning the race to fusion and providing low-carbon electricity to complement renewable energy sources in combating climate change.
The third new driver for the development of fusion is that researchers into fusion have made dramatic progress in understanding the physics of the hot, ionized gas, called plasma, that needs to be contained and controlled for practical fusion power production. Remarkably, fusion scientists can now calculate accurately the turbulence in 100-million-degree plasmas—much hotter than the center of the sun—and the resulting transport of heat and particles from the core of a fusion device to its edge. Scientists also have descriptive models for how the heat and particles escape from the edge of the plasma, and how to mitigate their impact on the material surfaces they encounter. Major challenges surely remain, but the National Academies’ reports argue persuasively that the time has come to focus research on the goal of net electricity production.
One important new insight is that while net electric power production requires significant technological development, this effort is not as great as is required for a first-of-a-kind commercial fusion power plant. For example, existing materials are more than adequate to support the production of continuous net electricity for extended periods of time, even though materials able to withstand a higher fluence of fusion neutrons are needed for a deuterium-tritium fusion power plant. Consequently, a pilot plant can first put electricity on the grid using existing materials, and then test out new materials and technologies for a power plant while gaining practical experience to accelerate the process of learning by doing.
Nonetheless, it will be critical that certain technologies be developed for the pilot plant itself. For example, if the pilot plant runs on deuterium and tritium fuel, it will be necessary to “breed” at least a large fraction of the required tritium in neutron-absorbing “blankets” around the plasma, by converting lithium to tritium.
The “Bringing Fusion to the US Grid” report provides the requirements for a pilot plant to accomplish the two goals of net electricity production and technological development for a first-of-a-kind commercial fusion power plant. To define these goals, the panel consulted with utilities to understand the projected needs for the US grid over the decades ahead. They found that a renewables-only path to very low carbon emissions would be much more expensive than one that included cost-effective, firm, low-carbon energy sources that can be “dispatched” to compensate for the intermittency of renewable energy, such as wind and solar, as well as to support the variation in demand for electricity over time. For example, during summer late afternoons when the demand for electricity to power air-conditioners is very high the wind is less likely to blow and the sun is beginning to set—or the sky could even be overcast.
The panel judged that large-scale energy storage for times up to four hours was likely feasible as a means to balance some variations of supply and demand, but issues of cost and the difficulty of siting near markets could preclude storage for much longer periods of time. As a result, it would be desirable for fusion systems to be able to ramp up in power in less than four hours and also be able to provide electricity to the grid for much longer periods as required.
With such considerations in mind, the panel concluded that a fusion pilot plant should aim to provide net electricity of 50-to-100 megawatts—similar to the Shippingport, Pennsylvania, fission pilot plant that first put power on the grid in 1957 and then functioned as a test-bed for fission technologies. Such a plant should provide the basis for a cost-effective, first-of-a-kind fusion power plant capable of producing firm and dispatchable, economically competitive, low-carbon electricity.
The National Academies’ report was not charged with selecting a configuration for the pilot plant’s plasma. The tokamak configuration, with a shape like a smooth doughnut, is the most advanced and is embodied in ITER. But the tokamak faces some major challenges as a fusion power system, perhaps the most severe of which is that it can be difficult to control, leading to a disruption of the large electrical current required in the plasma that can cause a sudden, potentially damaging release of energy to its internal structures. Methods of addressing this issue are being aggressively pursued both experimentally and theoretically—such as by using machine learning techniques to steer away from disruptions, and rapid densification of the plasma to dissipate the energy should a disruption occur. These will be given a full-scale, real-world test in ITER.
The second-most advanced concept is the stellarator, which uses external magnets to confine a plasma in a shape more reminiscent of a cruller. A stellarator does not require a plasma current and does not suffer from disruptions, but rather uses complex magnets to generate the plasma configuration needed for confinement. More optimization is required, however, to find the easiest configuration to manufacture and operate that will give the required plasma confinement.
There are many other plasma configurations that are well behind the tokamak and stellarator in plasma performance, but for which new ideas and approaches are being implemented by industrial groups, with the hope of overtaking the front runners. In general, they promise much simpler configurations to build and maintain. To select just one example, the plasma “pinch” carries a high current in a simple linear geometry, with its self-created magnetic field pinching the plasma to high pressure. It has been known for many years that such a configuration is prone to kinking-up and tearing itself apart, but researchers recently have found in tokamaks that “sheared” flows, where one region of the plasma flows at a different speed from its neighbor, can be used to quiet instabilities. This concept as applied to the much more unstable pinch configuration has resulted in remarkable advances, including the recently published detection of neutrons from fusion. While this approach is a very long distance from significant fusion power production, there are plans to scale it up rapidly. Another dark-horse concept, the “field reversed configuration,” has recently reported significant progress—again with a long way to go. These results remain to be published.
The National Academies’ panel, looking at the overall landscape, recommended that two to four teams be assembled—including participants from industry, national labs, and universities—to develop conceptual and then preliminary designs for fusion pilot plants that would both produce net electricity and support further technological development for a cost-effective, first-of-a-kind fusion power plant. The panel set an aggressive deadline of 2028 for the preliminary designs to be completed, with the goal of operating a fusion pilot plant in the 2035–2040 timeframe. This would put its operation in parallel with ITER’s high-power phase, but such overlapping steps are the hallmark of aggressive and successful technological development. Both of the recent National Academies’ reports, while endorsing a pilot plant, have also strongly underscored the value of full US participation in ITER.
The National Academies has not been alone; the American Physical Society led a broad effort by the fusion and plasma research community that resulted in a “Community Plan for Fusion Energy and Discovery Plasma Sciences.” This plan also strongly endorsed a pilot plant with the goals of net electricity production and the advancement of fusion technology. Subsequently, the Fusion Energy Sciences Advisory Committee (a federal advisory committee to the US Energy Department) developed “a long-range plan to deliver fusion energy and to advance plasma science” under a range of budget scenarios, in a report entitled “Powering the Future, Fusion & Plasmas.” It provided a more detailed development path, and also set a net electricity-producing pilot plant as a central goal for the US fusion program. Like the National Academies’ reports, these two reports strongly supported full US participation in ITER.
This excitement and sense of urgency for a fusion pilot plant to put electricity on the grid is welcome. Although fusion has its critics, it is widely viewed as an attractive energy source, as also gauged by recent industrial interest. Climate change is upon us. New electrical energy sources that can complement renewable systems such as wind and solar by providing firm, dispatchable, low-carbon electricity will provide practical means to sustain a vibrant US economy while drastically cutting back carbon emissions. And combatting climate change is a central pillar of the Biden administration’s new infrastructure plan. It behooves the United States to develop fusion energy as quickly as possible, and a pilot plant is a very attractive step towards this goal.
Editor’s note: The author, Robert Goldston, worked closely with Richard Hawryluk—chair of the recent National Academies of Sciences, Engineering, and Medicine report on fusion—when Hawryluk was deputy director of the Princeton Plasma Physics Laboratory (1997–2009) and Goldston was director.
Published at thebulletin.org
