An "advanced nuclear reactor" is defined in legislation enacted in 2018 as "a nuclear fission reactor with significant improvements over the most recent generation of nuclear fission reactors" or a reactor using nuclear fusion (P.L. 115-248). Such reactors include LWR designs that are far smaller than existing reactors, as well as concepts that would use different moderators, coolants, and types of fuel. Many of these advanced designs are considered to be small modular reactors (SMRs), which the Department of Energy (DOE) defines as reactors with electric generating capacity of 300 megawatts and below, in contrast to an average of about 1,000 megawatts for existing commercial reactors. Advanced reactors are often referred to as "Generation IV" nuclear technologies, with existing commercial reactors constituting "Generation III" or, for the most recently constructed reactors, "Generation III+." Major categories of advanced reactors include advanced water-cooled reactors, which would make safety, efficiency, and other improvements over existing commercial reactors; gas-cooled reactors, which could use graphite as a neutron moderator or have no moderator; liquid-metal-cooled reactors, which would be cooled by liquid sodium or other metals and have no moderator; molten salt reactors, which would use liquid fuel; and fusion reactors, which would release energy through the combination of light atomic nuclei rather than the splitting (fission) of heavy nuclei such as uranium. Most of these concepts have been studied since the dawn of the nuclear age, but relatively few, such as sodium-cooled reactors, have advanced to commercial scale demonstration, and such demonstrations in the United States took place decades ago. The 115th Congress enacted two bills to promote the development of advanced nuclear reactors. The first, the Nuclear Energy Innovation Capabilities Act of 2017 (NEICA), was signed into law in September 2018 (P.L. 115-248). It requires DOE to develop a versatile fast neutron test reactor that could help develop fuels and materials for advanced reactors and authorizes DOE national laboratories and other sites to host reactor testing and demonstration projects "to be proposed and funded, in whole or in part, by the private sector." The second, the Nuclear Energy Innovation and Modernization Act (NEIMA, P.L. 115-439), signed in January 2019, would require the Nuclear Regulatory Commission to develop an optional regulatory framework suitable for advanced nuclear technologies. The 115th Congress also appropriated $65 million for R&D to support development of the versatile test reactor in the Energy and Water Development Appropriations Act, FY2019, along with funding for ongoing advanced nuclear research and development programs (Division A of P.L. 115-244). Continued debate over advanced reactor issues is anticipated in the 116th Congress. A fundamental question may be the role of the federal government in advanced nuclear power development. DOE's budget request for FY2020 focuses the federal role on "early stage research" rather than the more expensive stages of demonstration and commercialization. Controversy is also likely to continue over the need for advanced nuclear power. Supporters contend that such technology will be crucial in reducing emissions of greenhouse gases and bringing carbon-free power to the majority of the world that currently has little access to electricity. However, some observers and interest groups have cast doubt on the potential safety, affordability, and sustainability of advanced reactors. Because many of these technologies are in the conceptual or design phases, the potential advantages of these systems have not yet been established on a commercial scale. Concern has also been raised about the weapons-proliferation risks posed by the potential use of plutonium-based fuel by some advanced reactor technologies.

The National Academies Keck Futures Initiative (NAKFI) Conference in 2013 focused on the Future of Advanced Nuclear Technologies to generate new ideas about how to move nuclear technology forward while making the world safer and more secure. Beyond the public's apprehension concerning the safety of nuclear power, which calls out for better communications strategies, several challenges lie ahead for the nuclear enterprise in the United States. The workforce in nuclear technology is aging, there is an overreliance on large, high-risk reactor designs, and the supply of radioisotopes for nuclear medicine remains unstable-all problems crying out for solutions. The Future of Advanced Nuclear Technologies summarizes the 14 Interdisciplinary Research (IDR) teams' collaborations on creative solutions to challenges designed to propel the policy, engineering, and social aspects of the nuclear enterprise forward.

The construction of nuclear power plants in the United States is stopping, as regulators, reactor manufacturers, and operators sort out a host of technical and institutional problems. This volume summarizes the status of nuclear power, analyzes the obstacles to resumption of construction of nuclear plants, and describes and evaluates the technological alternatives for safer, more economical reactors. Topics covered include: Institutional issues-including regulatory practices at the federal and state levels, the growing trends toward greater competition in the generation of electricity, and nuclear and nonnuclear generation options. Critical evaluation of advanced reactors-covering attributes such as cost, construction time, safety, development status, and fuel cycles. Finally, three alternative federal research and development programs are presented.

The world confronts an existential challenge in responding to climate change, resulting in an urgent need to reduce greenhouse gas emissions from all sectors of the economy. What will it take for new and advanced nuclear reactors to play a role in decarbonization? Nuclear power provides a significant portion of the worlds low-carbon electricity, and advanced nuclear technologies have the potential to be smaller, safer, less expensive to build, and better integrated with the modern grid. However, if the United States wants advanced nuclear reactors to play a role in its plans for decarbonization, there are many key challenges that must be overcome at the technical, economic, and regulatory levels. Laying the Foundation for New and Advanced Nuclear Reactors in the United States discusses how the United States could support the successful commercialization of advanced nuclear reactors with a set of near-term policies and practices. The recommendations of this report address the need to close technology research gaps, explore new business use cases, improve project management and construction, update regulations and security requirements, prioritize community engagement, strengthen the skilled workforce, and develop competitive financing options.

Nuclear energy has provided nearly 20 percent of electrical generation in the United States over the past two decades and currently produces 60 percent of America's carbon free-electricity, but the 99 reactors licensed to operate today in the United States will not last forever. The lack of a comprehensive set of solutions has hampered both commercial nuclear development as well as defense waste cleanup efforts. If nuclear power is to have a future, the U.S. must pursue research, development and deployment of next generation nuclear technologies, including small modular reactors, micro-reactors, Generation IV reactors and future fusion reactors. New designs must be safer, cheaper and efficient, and proliferation resistant. The opportunity for innovation in nuclear technologies has not been this great since the 1960's. Despite the many difficult challenges associated with full deployment, technical, financial, bureaucratic and license-related, there is unprecedented interest from both the public and private sectors.