All commercial nuclear power plants in the United States, as well as nearly all nuclear plants worldwide, use light water reactor (LWR) technology that was initially developed for naval propulsion. Cooled by ordinary water, LWRs in the early years were widely considered to be an interim technology that would pave the way for advanced nuclear concepts. After the early 1960s, the federal government focused most of its nuclear power research and development efforts on breeder reactors and high temperature reactors that could use uranium resources far more efficiently and potentially operate more safely than LWRs. However, four decades later, LWRs continue to dominate the nuclear power industry, and are the only technology currently being considered for a new generation of U.S. commercial reactors. Federal license applications for as many as 30 new LWRs have been recently announced. The proposed new nuclear power plants would begin coming on line around 2016 and operate for 60 years or longer. Under that scenario, LWRs appear likely to dominate the nuclear power industry for decades to come. If the next generation of nuclear power plants consists of LWRs, what is the potential role of advanced nuclear reactor and fuel cycle technologies? Do current plans for a new generation of LWRs raise potential problems that advanced nuclear technologies could or should address? Can new fuel cycle technologies reduce the risk of nuclear weapons proliferation? What is the appropriate time frame for the commercial deployment of new nuclear technology? This report provides background and analysis to help Congress address those questions. Prominent among the policy issues currently before Congress is the direction of the existing nuclear energy programs in the U.S. Department of Energy (DOE). DOE administers programs to encourage near-term construction of new LWRs, such as the Nuclear Power 2010 program, which is paying half the cost of licensing and first-of-a-kind engineering for new U.S. LWR designs, and loan guarantees for new reactors now under consideration by U.S. utilities. DOE's Global Nuclear Energy Partnership (GNEP) is developing advanced fuel cycle technologies that are intended to allow greater worldwide use of nuclear power without increased weapons proliferation risks. Advanced nuclear reactors that could increase efficiency and safety are being developed by DOE's Generation IV program, which is looking beyond today's "Generation III" light water reactors. The priority given to these options depends not only on the characteristics of existing and advanced nuclear technologies, but on the role that nuclear power is expected to play in addressing national energy and environmental goals. For example, if nuclear energy is seen as a key element in global climate change policy, because of its low carbon dioxide emissions, the deployment of advanced reactor and fuel cycle technologies could be considered to be more urgent than if nuclear power is expected to have a limited long-term role because of economic, non-proliferation, and safety concerns. As their name implies, light water reactors use ordinary water for cooling the reactor core and "moderating," or slowing, the neutrons in a nuclear chain reaction. The slower neutrons, called thermal neutrons, are highly efficient in causing fission (splitting of nuclei) in certain isotopes of heavy elements, such as uranium 235 and plutonium 239 (Pu-239). Therefore, a smaller percentage of those isotopes is needed in nuclear fuel to sustain a nuclear chain reaction (in which neutrons released by fissioned nuclei then induce fission in other nuclei, and so forth). The downside is that thermal neutrons cannot efficiently induce fission in more than a few specific isotopes. Natural uranium has too low a concentration of U-235 (0.7%) to fuel an LWR (the remainder is U-238), so the U-235 concentration must be increased ("enriched") to between 3% and 5%. In the reactor, the U-235 fissions, releasing energy, neutrons, and fission products (highly radioactive fragments of U-235 nuclei). Some neutrons are also absorbed by U-238 nuclei to create Pu-239, which itself may then fission. After several years in an LWR, fuel assemblies will build up too many neutron-absorbing fission products and become too depleted in fissile U-235 to efficiently sustain a nuclear chain reaction. At that point, the assemblies are considered spent nuclear fuel and removed from the reactor. LWR spent fuel typically contains about 1% U-235, 1% plutonium, 4% fission products, and the remainder U-238. Under current policy, the spent fuel is to be disposed of as waste, although only a tiny fraction of the original natural uranium has been used. Long-lived plutonium and other actinides in the spent fuel pose a long-term hazard that greatly increases the complexity of finding a suitable disposal site. Reprocessing, or recycling, of spent nuclear fuel for use in "fast" reactors—in which the neutrons are not slowed—is intended to address some of the shortcomings of the LWR once-through fuel cycle. Fast neutrons are less effective in inducing fission than thermal neutrons but can induce fission in all actinides, including all plutonium isotopes. Therefore, nuclear fuel for a fast reactor must have a higher proportion of fissionable isotopes than a thermal reactor to sustain a chain reaction, but a larger number of different isotopes can constitute that fissionable proportion. A fast reactor's ability to fission all actinides makes it theoretically possible to repeatedly separate those materials from spent fuel and feed them back into the reactor until they are entirely fissioned. Fast reactors are also ideal for "breeding" the maximum amount of Pu-239 from U-238, eventually converting virtually all of natural uranium to useable nuclear fuel. Current reprocessing programs are generally viewed by their proponents as interim steps toward a commercial nuclear fuel cycle based on fast reactors, because the benefits of limited recycling with LWRs are modest. Commercial-scale spent fuel reprocessing is currently conducted in France, Britain, and Russia. The Pu-239 they produce is blended with uranium to make mixed-oxide (MOX) fuel, in which the Pu-239 largely substitutes for U-235. Two French reprocessing plants at La Hague can each reprocess up to 800 metric tons of spent fuel per year, while Britain's THORP facility at Sellafield has a capacity of 900 metric tons per year. Russia has a 400-ton plant at Ozersk, and Japan is building an 800-ton plant at Rokkasho to succeed a 90-ton demonstration facility at Tokai Mura. Britain and France also have older plants to reprocess gas-cooled reactor fuel, and India has a 275-ton plant. About 200 metric tons of MOX fuel is used annually, about 2% of new nuclear fuel, equivalent to about 2,000 metric tons of mined uranium. While long a goal of nuclear power proponents, the reprocessing or recycling of spent nuclear fuel is also seen as a weapons proliferation risk, because plutonium extracted for new reactor fuel can also be used for nuclear weapons. Therefore, a primary goal of U.S. advanced fuel cycle programs, including GNEP, has been to develop recycling technologies that would not produce pure plutonium that could easily be diverted for weapons use. The "proliferation resistance" of these technologies is subject to considerable debate. Removing uranium from spent nuclear fuel through reprocessing would eliminate most of the volume of radioactive material requiring disposal in a deep geologic repository. In addition, the removal of plutonium and conversion to shorter-lived fission products would eliminate most of the long-term (post-1,000 years) radioactivity in nuclear waste. But the waste resulting from reprocessing would have nearly the same short-term radioactivity and heat as the original spent fuel, because the reprocessing waste consists primarily of fission products, which generate most of the radioactivity and heat in spent fuel. Because heat is the main limiting factor on repository capacity, conventional reprocessing would not provide major disposal benefits in the near term. DOE is addressing that problem with a proposal to further separate the primary heat-generating fission products—cesium 137 and strontium 90—from high level waste for separate storage and decay over several hundred years. That proposal would greatly increase repository capacity, although it would require an alternative secure storage system for the cesium and strontium that has yet to be designed. Safety and efficiency are other areas in which improvements have long been envisioned over LWR technology. The primary safety vulnerability of LWRs is a loss-of-coolant accident, in which the water level in the reactor falls below the nuclear fuel. When the water is lost, the chain reaction stops, because the neutrons are no longer moderated. But the heat of radioactive decay continues and will quickly melt the nuclear fuel, as occurred during the 1979 Three Mile Island accident. DOE's Generation IV program is focusing on high temperature, gas-cooled reactors that would use fuel whose melting point would be higher than the maximum reactor temperature. The high operating temperature of such reactors would also result in greater fuel efficiency and the potential for cost-effective production of hydrogen, which could be used as a non-polluting transportation fuel. However, the commercial viability of Generation IV reactors remains uncertain. DOE's advanced nuclear technology programs date back to the early years of the Atomic Energy Commission in the 1940s and 1950s. In particular, it was widely believed that breeder reactors would be necessary for providing sufficient fuel for a commercial nuclear power industry. Early research was also conducted on a wide variety of other power reactor concepts, some of which are still under active consideration. The U.S. research effort on various advanced nuclear concepts has waxed and waned during subsequent decades, sometimes resulting from changes in Administrations. Technical and engineering advances have appeared to move some of the technologies closer to commercial viability, but significantly greater federal support would be necessary to move them beyond the indefinite research and development stage.