This blog post will systematically examine how TWR technology overcomes the structural limitations of existing nuclear power plants and opens new possibilities for energy policy.
In 2018, one of the most contentious topics in South Korea was undoubtedly ‘nuclear power generation’. A public deliberation committee, composed of 471 citizen representatives of all ages and genders, was formed to deliberate for approximately three months (July 24, 2017, to October 20, 2017) on whether to halt construction of the Shin Kori Units 5 and 6 nuclear reactors. Countless citizens voluntarily studied nuclear power and held discussion forums, while the government established the deliberative committee to support this effort and reflect public opinion in policy. The deliberation results showed 59.5% support and 40.5% opposition to resuming construction, leading to the restart of the plant. However, regarding the future direction of nuclear power policy—whether to expand, maintain, or reduce it—53.2% favored reduction, 35.5% favored maintenance, and only 9.7% favored expansion, indicating a stance leaning toward nuclear phase-out.
Even after this public deliberation process, South Korea remained structurally unable to escape its high dependence on energy imports. Recent statistics indicate that as of 2023, South Korea’s energy import dependency rate reached approximately 93.8%, and its energy self-sufficiency level remains low among OECD countries. In other words, the debate surrounding energy policy remains a national challenge even after the 2018 public deliberation. Given this domestic energy production shortage, why has South Korea repeatedly grappled with nuclear power—the most realistic alternative for efficiently producing large quantities of energy?
Nuclear power generation, which produces enormous amounts of energy from a small amount of uranium, may seem perfect at first glance. However, underlying this are significant problems: the unpredictable risk of major accidents, concerns that nuclear fuel could be misused for weapons production, and the fact that no country, including South Korea, has yet fully established facilities capable of safely processing the high-risk radioactive waste—spent nuclear fuel—left over after power generation. For these reasons, nuclear power remains one of the most complex dilemmas in global energy policy.
Nevertheless, the reason humanity must accept the various risks of nuclear power is that, among the diverse power generation methods currently used, it produces the largest amount of energy relative to the amount of fuel consumed. If nuclear power were abandoned, a significant portion of the economic benefits gained from it would also have to be relinquished. For example, when comparing energy output per unit of land area, replacing a nuclear power plant of the same installed capacity with a solar power plant would require approximately 20 times more land. So, is there no way to continue stable nuclear power generation while reducing its critical risks? Unfortunately, a complete solution remains elusive with current technology alone.
However, there is next-generation nuclear technology worth pinning hopes on. It is the Traveling Wave Reactor (TWR), currently under research by energy company TerraPower. While this technology remains in the experimental stage and cannot be commercialized immediately, TerraPower has accelerated development with the goal of installing and operating its first prototype reactor in the late 2020s. Indeed, as of 2024, TerraPower is constructing the sodium-cooled fast reactor Natrium in Wyoming, USA, while also continuing TWR research. To understand how the TWR differs from conventional nuclear power generation, it is necessary to first examine the basic principles of existing nuclear power plants.
Modern nuclear power generation uses nuclear fission reactions, where neutrons collide with uranium-235, causing it to split into fission products like strontium and xenon while emitting 2-3 fast neutrons.
During fission, the total mass of the reactants is slightly greater than that of the products. This mass loss is converted into enormous energy according to Einstein’s equation (E=mc²). At the power plant, this energy heats water to produce steam. This steam turns turbines to generate electricity. The water then returns to liquid form via cooling pipes and repeats the cycle, sustaining power generation. Neutrons released after the reaction are absorbed by another uranium-235 atom, continuing the fission chain reaction. However, without controlling this chain reaction, the reaction rate can dangerously accelerate. Therefore, moderators are used to appropriately slow down the neutrons, maintaining a stable reaction rate.
This reaction method is commonly applied in most nuclear power plants currently in operation. So, how are nuclear power plants classified? Rather than by the type of physicochemical reaction, they are broadly categorized into light water reactors (LWRs) and heavy water reactors (HWRs) based on the type of fuel, moderator, and coolant used. First, looking at the fuel used, LWRs use enriched uranium fuel, made by enriching uranium-235 from natural uranium. Conversely, heavy water reactors use uranium-235 as found in natural uranium. Next, light water reactors use light water as both the moderator and coolant, while heavy water reactors use heavy water. Heavy water has a greater mass than light water, providing superior moderation effects. This allows heavy water reactors using natural uranium to maintain sufficient reaction efficiency.
So, how do light water reactors, heavy water reactors, and TerraPower’s TWR differ, and what advantages do these differences bring? Looking first at the history of TWR technology, the TWR is a reactor design that has been under gradual development since it was first conceived by Saveli Feinberg in the 1950s. Despite ongoing research, no commercial TWR power plants exist yet. However, TerraPower was founded in 2006 to accelerate development, and full-scale design and demonstration preparations are underway for the 2020s and beyond.
The TWR possesses several distinctive characteristics compared to conventional reactors. First, it employs a different physicochemical reaction, leading to distinct fuel and coolant types. Unlike light water reactors (LWRs) or heavy water reactors (HWRs), the TWR’s fundamental reaction uses uranium-238 as fuel and liquid sodium (Na) as coolant. Examining this basic reaction step-by-step: First, neutrons collide with uranium-238, producing the unstable isotope uranium-239. Uranium-239 is highly unstable and soon undergoes beta decay, transforming into neptunium-239. This neptunium-239 then undergoes another beta decay, producing plutonium-239. This process is called ‘plutonium breeding’. Finally, the produced plutonium-239 undergoes nuclear fission, releasing energy in the form of heat. This heat passes through the MCFR (Molten Chloride Fast Reactor), heating water to produce steam. The steam drives turbines to generate electricity.
The interior of the TWR is divided into four zones according to these breeding and fission reaction processes. Specifically, it is divided into the zone where fuel remains, the zone where conversion to plutonium occurs before fission, the zone where plutonium undergoes fission, and the zone where post-fission atoms reside. The zone where fission occurs moves toward the zone where conversion to plutonium takes place, leaving behind fission-produced atoms. It then moves like a ‘wave’ through the reactor core at a speed of approximately 1 cm per year. TerraPower claims this reaction can theoretically sustain itself for hundreds of years.
This unique TWR design addresses two of the three major challenges in nuclear power generation while simultaneously offering higher economic viability and efficiency compared to conventional reactors. First, since TWR fuel contains almost no uranium-235, it cannot be used for nuclear weapons development requiring enriched uranium (containing 2-5% or more uranium-235), significantly reducing concerns about weapons proliferation using nuclear power fuel. Second, TWR offers the potential to solve the spent nuclear fuel disposal problem, the most significant challenge for conventional reactors. Approximately 96% of spent nuclear fuel consists of uranium-238, and spent fuel emits radiation for over 100,000 years. Uranium-238, in particular, emits radiation for about 4.5 billion years, making spent fuel disposal a challenge for all of humanity. However, since TWR uses this uranium-238 as fuel, it can effectively process the vast quantities of uranium-238 currently classified as spent nuclear fuel and stored. The amount of residual material after nuclear reactions (per unit of feedstock) is about 80% less than that of existing light water reactors, significantly contributing to solving the spent nuclear fuel problem. Consequently, the cost of stable spent nuclear fuel processing is also expected to decrease to about 20% of the existing level.
Furthermore, the economic benefit is enhanced by eliminating the need for natural uranium enrichment. A 1.15GW TWR can reduce operating costs by approximately $4 to $5 billion compared to a conventional reactor of the same size. Furthermore, while TWRs cannot completely eliminate the risk of nuclear accidents, they can significantly reduce it. Conventional light-water reactors use enriched uranium, requiring periodic shutdowns for fuel replacement. However, as mentioned earlier, TWRs use uranium-238 as fuel and, theoretically, can operate for hundreds of years without fuel replacement once started, greatly reducing the risk of accidents that could occur during the fuel replacement process. Consequently, the accident rate is projected to decrease to one in 100 million, which is 100 times lower than that of conventional reactors (one in a million). Furthermore, in terms of fuel efficiency, while conventional reactors have a maximum efficiency of about 33%, TWRs are analyzed to achieve up to 41% efficiency.
The nuclear dilemma is not unique to South Korea. Switzerland, after prolonged discussions on nuclear phase-out, opted for a gradual phase-out via a 2017 referendum. In the United States, the operation and new construction of nuclear plants have been repeatedly halted and restarted due to declining economic viability. For example, the Westinghouse nuclear power plant construction project in South Carolina, USA, halted construction in 2016 due to economic viability concerns. Thus, countries worldwide continue to grapple between nuclear phase-out and maintaining or expanding nuclear power generation policies.
However, TerraPower is gradually approaching the possibility of satisfying both choices within this dilemma through its TWR nuclear power technology. The commercialization potential of TWRs is becoming increasingly realistic, not only from a technical perspective but also politically and diplomatically. Prominent science and technology experts like Bill Gates, Nathan Myhrvold, and Lowell Wood are participating in TWR technology development. Compared to existing nuclear plants, TWRs can operate with simpler auxiliary systems, making them easier for multiple countries to adopt without significant burden, and broad commercialization is anticipated. Indeed, in the late 2020s, the United States and China discussed plans to adopt next-generation nuclear power technology in cooperation with TerraPower. As of 2024, the construction of a demonstration plant in the United States is being actively pursued.
Thus, TWR is evaluated as a technology that solves the problems of existing nuclear plants while maintaining the advantages of nuclear power, bringing commercialization one step closer. If TWR successfully moves beyond the demonstration phase to commercialization as planned by TerraPower, global energy policy will reach a new turning point, overcoming the existing dilemmas.
