This section discusses (1) nuclear fuel assemblies and their use, (2) the composition of spent nuclear fuel, (3) nuclear fuel cycles, (4) goals to minimize the risks of nuclear proliferation and terrorism, and (5) technology readiness assessments to measure and communicate the risks of using technology in first-of-a-kind applications. Hundreds of nuclear fuel assemblies—bundles of long metal tubes filled with enriched uranium fuel pellets—form the core of a typical nuclear power reactor. Reactors produce energy when uranium atoms split (fission) into smaller elements, called fission products. Some of the uranium atoms do not split but rather transmute into elements with atomic weights heavier than uranium, such as neptunium, plutonium, americium, and curium. With the buildup of fission products in the enriched uranium, the fuel loses its ability to sustain a nuclear reaction, and the fuel assemblies are then replaced with new fuel. Removed assemblies contain spent nuclear fuel, the radiation from which, without protective shielding, can kill a person directly exposed to it within minutes or increase the risk of cancer in people exposed to smaller doses. Figure 1 shows the composition of spent nuclear fuel. Spent nuclear fuel includes only the fuel components and not the assemblies used to contain these components. As shown in the figure, the fuel components of the spent fuel are uranium, plutonium, minor actinides, and fission products. Minor actinides are a group of transuranic by-products produced in nuclear reactor operation that are major contributors to the long-lived hazards of radioactive waste. The term transuranic generally applies to radioactive material containing radionuclides (radioactive elements) with atomic numbers higher than 92 (uranium’s atomic number) and half-lives longer than 20 years in concentrations exceeding 100 nanocuries (a measure of radioactivity) per gram. Some fission products, such as cesium, strontium, iodine, technetium, and other fission products are radioactive and can remain dangerous for hundreds to hundreds of thousands or millions of years. Stable fission products do not emit radiation. As the figure shows, uranium is the primary component in spent nuclear fuel. Uranium and plutonium are part of a group of elements known as actinides—the 15 chemical elements on the periodic table with atomic numbers from 89 to 103, actinium through lawrencium—and are also called major actinides. Major actinides are fissile—they easily undergo fission when hit by a neutron of any energy. Minor actinides consist primarily of neptunium, americium, and curium. Unlike major actinides, minor actinides can be made to fission only when hit by a neutron with high enough energy. Both major and minor actinides pose health and environmental risks, some for hundreds of thousands of years. The remaining component of the material in spent nuclear fuel is fission products, primarily cesium, strontium, iodine, and technetium; and stable fission products. Some fission products, such as cesium and strontium, pose environmental risks for hundreds of years after being removed from a reactor, while iodine and technetium can remain hazardous for hundreds of thousands or millions of years. Other fission products are stable and no longer emit radiation. The term “fuel cycle” may either denote the general process of preparing, fissioning, and disposing of spent nuclear fuel or one of potentially hundreds of specific processes and combinations of technologies that may be used to carry out this process. The details of a specific fuel cycle include the type of fuel, the level of uranium enrichment, the type of nuclear reactor, and the methods for reprocessing, recycling, and/or disposing of spent fuel. After a period of operation, usually every 18 months, U.S. nuclear reactors generally replace some of their fuel and store the spent fuel immersed in pools of water or move them into dry storage containers. As we recently reported, the current nuclear fuel cycle used in most U.S. reactors presents several challenges, including the lack of a geologic repository for permanent disposal of spent nuclear fuel. DOE is proposing to select and demonstrate sustainable nuclear fuel cycles that could address this and other challenges. DOE’s R&D plan defined sustainable fuel cycles as those that would better utilize uranium resources, maximize energy generation, minimize waste generation, improve safety, and limit proliferation and terrorism risks. To achieve this objective, DOE proposes R&D on technologies for three categories of fuel cycles:  Once-through fuel cycle—technologies to more efficiently use uranium than the current open fuel cycle while reducing the amount of radioactive waste generated.  Modified open fuel cycle—technologies that more efficiently use uranium, minimize the amount of radioactive waste generated, and reduce proliferation and terrorism risks using limited or no reprocessing or recycling.  Full recycle fuel cycle—technologies to repeatedly reprocess and recycle nuclear fuels, thereby minimizing the amount of radioactive waste generated and reducing proliferation and terrorism risks. According to DOE’s R&D plan, it is important to ensure that the benefits of nuclear power can be obtained in a manner that limits nuclear proliferation and terrorism risks. The plan states that the Office of Nuclear Energy is responsible for providing technical expertise and leadership on nuclear technology to the federal agencies with overall responsibility for U.S. nuclear nonproliferation policy. These agencies include NNSA, as well as the Department of State, and the Nuclear Regulatory Commission—responsible for overseeing the nation’s 104 commercial nuclear reactors—and other organizations. According to an NNSA official, the goals of U.S. nuclear nonproliferation policy are to: limit the spread of technologies to enrich uranium and to reprocess spent nuclear fuel; strengthen the international safeguards system, which includes international agreements to protect against misuse of nuclear technologies and international design standards for nuclear facilities;  halt the build-up, and eventually draw down, of stocks of separated  develop nuclear fuel cycles and associated technologies that have lower proliferation and terrorism risks, while recognizing that other factors contribute to proliferation and terrorism risks, such as the country in which a nuclear facility is located. In pursuing these goals, NNSA distinguishes between nuclear proliferation and terrorism risks. According to a national laboratory subject matter expert, assessment of proliferation risks considers whether nations that have nuclear facilities for peaceful purposes, such as nuclear power, might divert and misuse these facilities to generate material to build nuclear weapons. International safeguards under the auspices of the IAEA are used to guard against these risks. Generally, because of the technology involved, NNSA considers that proliferation risks will occur over an extended period of time. In contrast, terrorism risks come from groups not necessarily associated with a particular nation. Threats associated with terrorism include the theft of nuclear material and the sabotage of nuclear facilities. As we previously reported, technology readiness assessment provides a systematic way to determine the extent to which a technology critical to ensuring a project’s successful operation is sufficiently developed for its intended purpose. Critical technologies are those that are essential to a project’s success and are either new or are being applied in a new manner. DOE has begun to assess the readiness of technologies for recycling spent nuclear fuel using technology readiness levels, a method pioneered by the National Aeronautics and Space Administration for measuring and communicating the risks associated with critical technologies in first-of-a-kind applications. Using a scale from one (basic principles observed) through nine (total system used successfully in project operations), readiness levels show the extent to which technologies have been demonstrated to work as intended. A higher readiness level indicates a new technology has better demonstrated its suitability relative to a specific set of criteria, and a decision to proceed with an acquisition of the technology will accordingly be lower risk. Demonstration of new technologies at successively larger scales— laboratory scale, engineering/pilot scale and full/prototypical scale—is one way to increase their technology readiness, thereby mitigating the risk of schedule or cost overruns in the design and construction of commercial-scale facilities and limiting investment in potentially ineffective technologies. As we have previously reported, GAO considers level seven (subsystem demonstrated in an operational environment) to be an acceptable level of readiness before proceeding with final design and committing to definitive schedule and cost estimates. DOE’s R&D plan details a systematic approach—that is, the use of scientific methods and engineering principles—to select and eventually demonstrate nuclear fuel cycles and associated technologies. However, the plan does not explain the current readiness of the critical technologies and the estimated time and costs of further developing these technologies; it also does not explain how DOE will collaborate with the nuclear industry and other countries with experience in conducting nuclear R&D in achieving its goals. DOE’s plan lays out R&D objectives for various technical areas and schedules for achieving them. Under the plan, DOE has the goal of selecting nuclear fuel cycle options and associated technologies by 2020 and demonstrating them by 2050. Throughout this selection process, the plan states that DOE will rely on a science-based approach in conducting its R&D. This approach will involve small-scale experiments, theory development, and computer modeling and simulation. The plan states that DOE will develop theories and use the knowledge and data obtained through experiments to, among other things, develop and validate modeling and simulation tools to examine nuclear fuel cycles and associated technologies. DOE is also following a dual-path approach for nuclear fuel cycle R&D— simultaneously pursuing evolutionary and revolutionary tracks across all of its technical R&D areas. That is, according to DOE’s R&D plan, the department is pursuing both advancements of existing fuel cycles and high-risk, high-payoff technologies that, if successful, could replace all or part of the evolutionary technologies. For example, DOE is pursuing a way to economically extract uranium from seawater, which it would consider a revolutionary breakthrough nuclear fuel cycle option, if it were to succeed.