Advanced Nuclear Reactors 101

Nuclear energy is generated by splitting uranium atoms in a controlled operation called fission. Traditionally, nuclear power is generated using light water nuclear reactors to heat water and create steam to drive a turbine; however, several new reactor technologies are in development. These advanced nuclear reactors extend beyond traditional reactors, offering the opportunity of safer, cheaper, and more efficient generation of emissions-free electricity, as well as heat for industrial processes. Currently, almost all nuclear energy across the globe is generated by non-advanced reactors, with few advanced reactors in active use for energy generation. By examining the technological characteristics, economic hurdles to implementing advanced nuclear options, and policy options for encouraging implementation, this explainer details the outlook for future development of advanced nuclear energy sources.

The World Nuclear Association provides a comprehensive overview of how nuclear reactors work in this "essentials" page. Here are a few key terms describing how a nuclear reactor works that are particularly useful for this explainer.

Fission: Nuclear fission reactions generate all nuclear energy today. Through fission, a neutron hits a uranium atom, splitting it into two smaller atoms and producing more neutrons. This starts a chain reaction where the released neutrons collide with other uranium atoms, splitting them in another fission reaction. Each collision releases energy in the form of heat along the way.

Fusion: Nuclear fusion reactions involve two smaller atoms colliding to create one larger atom. This process releases a large amount of energy but has not advanced to the point of commercial use.

Coolant: A coolant removes heat from the reactor core and helps to ensure that the reactor doesn’t overheat. The coolant captures the energy released by nuclear reactions and carries it to a generation facility where it is used to generate electricity.

Moderator: A moderator is a material used in a nuclear reactor to slow down the neutrons that cause fission reactions to happen, allowing the nuclear reaction to proceed with a relatively small amount of fuel.

Thermal Reactors: In thermal reactors, neutrons are slowed down by a moderator (traditionally water), increasing the likelihood that each neutron will cause a fission reaction.

Fast Reactors: Fast reactors do not moderate the speed of neutrons; unlike thermal reactors, which slow neutrons down, fast reactors allow neutrons to stay at the speed at which they emerge from the fission reactions. In some cases, they can use uranium fuel more efficiently than thermal reactors.

Breeder reactors: While reactors all consume fuel, some reactors also produce fuel, leaving extra fuel that can be used to generate energy. These “breeder” reactors could be either fast or thermal reactors.

The types of advanced nuclear reactors are categorized in various ways. The categorization presented here is from the Congressional Research Service, which identifies three primary categories: advanced water-cooled reactors, non-water-cooled reactors, and fusion reactors.

Advanced Water-Cooled Reactors

Advanced water-cooled reactors work similarly to traditional nuclear reactors: they generate energy using fission reactions and use water as coolants and moderators. However, they can be simpler, smaller, and more efficient than traditional reactors. For example, supercritical water reactors use water heated and pressurized to a supercritical state as a coolant. Many advanced water-cooled reactors differ from traditional reactors only in size; these are called small modular reactors (SMRs) and micro-reactors. SMRs are lower risk than larger light water projects, due to benefits described in Table 2. SMRs and micro-reactors can fit into the advanced water-cooled reactor category, though they can also fit into other categories as many technologies can be utilized for each of them.

Non-Water-Cooled Reactors

Non-water-cooled reactors still generate energy using fission reactions, but (as the name implies) they use coolants other than water to control the reactor temperature. The different coolants create opportunities for the reactors to operate with higher safety. For example, while water must be kept at a high pressure to work as a coolant, other coolants may be kept at lower pressures and higher temperatures; this reduces the risk of a sudden release of chemicals, which can be caused by high-pressure coolant.

Molten salt reactors (MSRs) are an example of non-water-cooled reactors; they use molten salt as a coolant. Recently, China has led global research into MSR technology, focusing on thorium-based versions as a fuel source. The high temperature gas-cooled reactor (HTGR) is another type of non-water-cooled reactor. HTGRs use graphite as a moderator and helium as a coolant to operate at high temperatures. Sodium-, lead-, and gas-cooled fast reactors fit into the non-water-cooled category, and SMRs and micro-reactors can be made using non-water coolant technology as well.

Fusion Reactors

The final category of advanced nuclear reactors is the fusion reactor. While all the other reactors discussed so far have generated energy using fission reactions, fusion reactors would theoretically use a process known as nuclear fusion (the reaction that powers the sun and stars). Instead of splitting an atom, this reaction involves two smaller atoms colliding to create one larger atom, releasing a large amount of energy. However, this technology is much further from being ready for deployment than the other reactor types.

The advanced nuclear reactors described here improve upon traditional nuclear reactors in several ways, partially overcoming many of the barriers that nuclear power has faced. As shown in Table 2, different types of reactors have different benefits and face different challenges due to their size, materials used for coolants and moderators, operational temperature, and other factors. For more detail, see reports from the Clean Air Task Force, the Congressional Research Service, and Third Way.

Benefits of Advanced Nuclear Reactors

Safety Benefits: Advanced reactors can operate with significantly enhanced safety compared to traditional light-water nuclear reactors. Advanced reactors often run at lower, safer pressures because of the special coolants they use. In many cases, they can also take advantage of passive safety measures, such as pressure relief valves, rather than relying on active safety features that require a backup power supply or human intervention to work. These passive safety measures allow reactors to withstand a broader set of accident conditions without causing damage.

Lower Costs: There is ongoing debate about whether the capital cost of an advanced nuclear reactor (the up-front, one-time-only costs to construct a reactor) would be lower than those of a contemporary light-water reactor. Recent research points to several opportunities for reducing capital costs, including the following:

• design improvements that may lead to decreased safety infrastructure costs;
• the ability to manufacture many modular units of the same reactor type off-site; and
• improved construction management practices.
  

However, this research is careful to note that these opportunities for cost reduction likely apply to all nuclear plant types, not just advanced reactors.

Industrial Decarbonization: Some advanced nuclear reactors produce high temperatures that can be used for industrial processes. Many industrial processes currently rely on fossil fuels to produce necessary heat levels, and advanced reactors could substitute for fossil fuels in processes that would be difficult to electrify. In this way, advanced reactors have the potential to help decarbonize industries that are currently heavily reliant on fossil fuels.

Versatility and Flexibility: Due to factory construction and varying reactor sizes, many advanced reactors can be much more flexible and versatile than traditional reactors. They can be installed at sites where traditional reactors cannot, such as underground caverns where radiation risk and national security risks are mitigated. In addition, some advanced nuclear reactors can vary the amount of power they produce more easily than traditional reactors can, enabling them to play a greater role in balancing electricity loads. Finally, many advanced reactors can go much longer without refueling, requiring less infrastructure and allowing them to stay online for long periods of time with no interruption in their power output.

Increased Efficiency: Some advanced reactors use fuel much more efficiently than traditional reactors, converting up to 95 percent of the energy in the fuel to usable electricity (traditional reactors convert less than 5 percent). Therefore, they have the potential to provide energy using much less fuel.

Less Danger from Waste: The increased energy efficiency of many advanced reactors also results in a smaller amount of nuclear waste. Additionally, the waste that is created can be less toxic and may remain toxic for a shorter period of time.

Factory Manufacturing: While traditional reactors are constructed on site, many small advanced nuclear reactors can be constructed in a factory setting and transported to a site for quick installation. For some reactor types, factory construction would allow for large numbers of reactors to be manufactured and deployed much more quickly than traditional reactors, which may be essential to reaching low-carbon generation targets.

Lower Proliferation Risk: The effect of reactor advancements on the risk of proliferation is ambiguous. Some sources state that advanced reactors produce less waste than what could traditionally be used to make nuclear weapons. In addition, advanced reactors are often designed to make fuel and waste less accessible than in traditional reactors. However, advanced reactors also often produce concentrated plutonium waste that may pose a higher proliferation risk than traditional reactors. As proliferation risks are generally perceived to be low for traditional reactors, slight differences due to advancements may not pose significant benefits or drawbacks.

Barriers and Challenges Faced by Advanced Nuclear Reactors

Traditional nuclear reactors and power plants currently face many challenges.

Some challenges faced are economic: currently, the electricity they generate is typically more expensive than electricity generated by renewables and natural gas, so it is difficult for nuclear plants to compete in electricity markets. As a result, nuclear generators often rely on subsidies from the government to remain open and profitable. However, it is worth noting that as more decarbonization leads to greater electricity consumption, the costs of renewables may increase due to storage needs and other costs, which may make nuclear-generated electricity more cost-competitive.

Other barriers faced by nuclear power are political and social: many people are against the development of nuclear power, and some wish for all existing nuclear plants to be shut down. These concerns are generally based on the danger surrounding nuclear waste and the possibility of nuclear accidents, such as the 2011 disaster in Fukushima, Japan, or the 1986 Chernobyl disaster.

Many of the advanced reactor technologies discussed in this explainer have the potential to overcome barriers faced by traditional reactors. Nevertheless, there are still barriers to deployment of advanced reactors. Generally, the greatest inhibitors are substantial costs associated with the development and construction of first-of-a-kind reactors. These costs are inflated by risk premiums - uncertainty due to lack of mature deployment makes first-of-a-kind generators financially risky investments. Although some projections suggest that capital costs will be lower for mature advanced reactors than for traditional ones, it is also possible that there will be substantial capital costs associated with long and complex initial construction phases, creating a significant hurdle for adoption. Finally, even once advanced reactors are built again, they may still be relatively expensive to operate.

Nuclear power has traditionally been supported through a range of policy instruments at the state and federal levels in the United States, including subsidies, the inclusion of nuclear power generation in clean energy credit markets, and loan guarantees provided by the federal government. In particular, as initially authorized by the Energy Policy Act of 2005, the US Department of Energy currently carves out $10.9 billion in loan guarantee authority for advanced nuclear energy projects. Loan guarantees help distribute the financial risk associated with pre-commercialization development and deployment.

The federal government has previously supported advanced nuclear energy via other instruments, as well. In 2017, the Nuclear Energy Innovation Capabilities Act was passed, creating financial and technological incentives to accelerate development of advanced nuclear technologies. Supplementing this was the passing of the Energy and Water Development Appropriations Act where $65 million was granted for research and development into building a versatile test reactor. The SMR Licensing Technical Support Program, housed in the Office of Nuclear Energy in the US Department of Energy, worked with research institutions to accelerate the certification and licensing process between the fiscal years of 2012 through 2017—and in 2020, the US Department of Energy launched the Advanced Reactor Demonstration Program (read more below).

Several proposed policies could impact the rate of development for advanced nuclear energy, including the Nuclear Energy Leadership Act, which was largely incorporated into the American Energy Innovation Act (AEIA). A number of provisions from the AEIA and the companion House bill—jointly termed the Energy Act of 2020—were included as Division Z of the Consolidated Appropriations Act of 2021, which was passed into law in December 2020. RFF researchers are currently working to better characterize how these policy provisions would affect advanced nuclear energy deployment and, consequently, the US electricity generation mix and overall levels of CO₂ emissions in the United States.

In addition to policies focused on innovation, RFF researchers are engaged in the design and assessment of a range of policy options that reward the large-scale deployment of nuclear power and other clean energy sources for their low emissions, including carbon pricing, clean energy standards, and other approaches.

• Congressional Research Service | Advanced Nuclear Reactors: Technology Overview and Current Issues
• Third Way | Advanced Nuclear 101
• MIT | The Future of Nuclear Energy in a Carbon-Constrained World