All nuclear power in the United States is generated by light water reactors (LWRs), which were commercialized in the 1950s and early 1960s and are now used throughout most of the world. LWRs are cooled by ordinary ("light") water, which also slows ("moderates") the neutrons that maintain the nuclear fission chain reaction. At the same time conventional reactors are facing an uncertain future, some in Congress contend that more nuclear power plants, not fewer, are needed to help reduce U.S. greenhouse gas emissions and bring low-carbon power to the majority of the world that currently has little access to electricity. Congress enacted legislation in September 2018 that defines "advanced nuclear reactor" 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 ). Titled the Nuclear Energy Innovation Capabilities Act of 2017 (NEICA), the law requires the Department of Energy (DOE) to take several actions to support advanced reactor development, including studying the need for a versatile fast neutron test reactor that could help develop fuels and materials for advanced reactors. Congress included $65 million for R&D to support development of the versatile test reactor in the Energy and Water Development Appropriations Act for FY2019 (Division A of P.L. A similar definition of "advanced nuclear reactor" is included in the Nuclear Energy Innovation and Modernization Act (NEIMA, P.L. 115-439 ), which was signed January 14, 2019. NEIMA would require the Nuclear Regulatory Commission (NRC) to develop a regulatory framework that could be used for advanced nuclear technologies. Some advanced reactor concepts are relatively new, while others have been under consideration for decades. Not all observers are optimistic about the potential safety, affordability, proliferation resistance 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. Advanced reactor technologies are often referred to as "Generation IV" nuclear reactors, with existing commercial reactors constituting "Generation III" or, for the most recently constructed reactors, "Generation III+." Advanced reactor designs may be grouped into three primary categories: 
Advanced water-cooled reactors , which provide evolutionary improvements to proven water-based fission technologies through innovations such as simplified design, smaller size, or enhanced efficiency; Non-water-cooled reactors , which are fission reactors that use materials such as liquid metals (e.g., sodium and lead), gases (e.g., helium and carbon dioxide), or molten salts as coolants instead of water; and Fusion reactors , which seek to generate energy by joining small atomic nuclei, as opposed to fission reactors, which generate energy by splitting large atomic nuclei. As noted above, most existing conventional reactors in the United States have an electrical generating capacity of 1,000 MWe or more. Unconventional reactors may offer some waste management advantages over existing commercial reactors. Of that, Congress directed $319.5 million (42%) to be used specifically for advanced nuclear technology R&D within the following programs and activities:
S upercritical Transformational Electric Power R&D : $5 million appropriated to develop a supercritical carbon dioxide Brayton cycle for thermal-to-electric energy conversion in sodium-cooled fast reactors; Advanced Small Modular Reactor R&D : $100 million appropriated for this new, one-year subprogram that is to support "cost-shared public-private R&D partnerships" to address technical challenges and accelerate development of SMR reactor designs and supply chains; Advanced Reactor Technologies : $111.5 million appropriated to conduct early-stage R&D on advanced reactor technologies, including SMRs; Versatile Fast Test Reactor : $65 million appropriated for R&D to support development of a versatile fast test reactor, also called the versatile test reactor; Material Recovery and Waste Form Development : $38 million appropriated to activities related to "the improvement of the current back end of the nuclear cycle [waste management and reprocessing]," of which $27 million were specifically directed towards activities supporting advanced nuclear technologies. 115-248 ). 115-244 ). The potential use of plutonium-based fuel in the VTR has drawn opposition because of the usability of such fuel in nuclear weapons. This program would "enable the testing and demonstration of reactor concepts to be proposed and funded, in whole or in part, by the private sector." Such testing and demonstration would take place at DOE national laboratories or other Department-owned sites. NEICA requires DOE to submit a report to Congress on costs and other issues that could be raised by the hosting of reactor testing and demonstration projects, including
DOE's capabilities for safety review and oversight of privately funded advanced reactor research; potential DOE sites that could host privately funded experimental advanced reactors; contractual mechanisms that could be used for such projects; and responsibility for management and disposal of waste. Funding of Demonstration Reactors

A crucial stage in the commercialization of nuclear technology is the construction of demonstration reactors, which are expected to cost several billion dollars apiece, depending on their size and level of technical maturity. Federal Payments for Power and Research Use

The federal government can purchase power generated by demonstration reactors and also pay for research use of the reactors. Title XVII loan guarantees cannot be given to projects that would use federal funds other than the federally guaranteed funding ( P.L. Power Purchase Agreements

Federal agency agreements to purchase power from advanced reactors could substantially improve the financial feasibility of such projects, both at the demonstration and commercialization stages. These include higher-enriched versions of existing uranium fuel as well as new types of fuels that are currently under development. Particular concern has been raised about the availability of high-assay low enriched uranium (HALEU), which would be necessary to power many advanced nuclear reactors.
All nuclear power in the United States is generated by light water reactors (LWRs), which were commercialized in the 1950s and early 1960s and are now used throughout most of the world. LWRs are cooled by ordinary ("light") water, which also slows ("moderates") the neutrons that maintain the nuclear fission chain reaction. High construction costs of large conventional LWRs, concerns about safety raised by the 2011 Fukushima nuclear disaster in Japan, and other issues have led to increased interest in unconventional, or "advanced," nuclear technologies that could be less expensive and safer than existing LWRs.
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.
Other current issues related to advanced reactors include criteria for hosting private-sector demonstration reactors at DOE sites, the licensing framework for non-LWR reactors, longer time periods for federal agreements to purchase power from advanced reactors, and the supply of the high-assay low enriched uranium fuel that would be needed for some advanced reactor designs. There also may be congressional interest about potential federal assistance for demonstration reactors, which are expected to cost billions of dollars apiece. Major options for such assistance include federal cost sharing, loan guarantees, power purchase agreements, purchase of reactor capacity for research uses, and tax credits.