The study is one of the first to address the nuclear waste production of small modular reactors.
Nuclear power is a key component of decarbonizing our economy, but large nuclear reactors are often complex and expensive to build. To make nuclear power more affordable and attractive, developers have identified several small modular reactor (SMR) designs that have greater flexibility and lower initial costs. Various types of SMRs with advanced reactor design features are currently being developed in the United States and around the world.
The researchers suggest that SMRs can be deployed at various scales for locally distributed power generation. SMRs have about one-tenth to one-third the power of large light-water reactors, which are the most common type of nuclear reactor in commercial operation in the United States. SMR technology and economics are widely studied; however, there is less information about their consequences for nuclear waste. “We have just begun to study the attributes of SMR nuclear waste,” said senior nuclear engineer Taek Kyum Kim of the US Department of Energy’s (DOE) Argonne National Laboratory.
Kim and his colleagues at Argonne and the Department of Energy’s Idaho National Laboratory recently published a report that attempts to measure the potential nuclear waste characteristics of three different SMR technologies using metrics developed from the extensive process of the Nuclear Fuel Cycle Comprehensive Assessment published in 2014. SMRs are not yet in commercial operation, several companies are working with the Department of Energy to explore various options for SMRs, and all three projects studied in the report are slated to be built and operational by the end of the decade.
One type of SMR, called VOYGR and being developed by NuScale Power, is based on the current design of a conventional pressurized water reactor, but scaled down and modular. Another type, called Natrium and being developed by TerraPower, is cooled by sodium and runs on metallic fuel. A third type, called Xe-100 and developed by X-energy, is cooled by gaseous helium.
From a nuclear waste perspective, each reactor has both advantages and disadvantages compared to larger LWRs, Kim said. “It is incorrect to say that because these reactors are smaller, they will have more problems with nuclear waste, just because they have more surface area compared to the volume of the core,” he said. “Each reactor has pros and cons that depend on discharge burnup, uranium enrichment, thermal efficiency, and other design features of the reactor.”
One notable factor that affects the amount of nuclear waste produced by a reactor is called burnup, and it refers to the amount of heat energy produced from a given amount of fuel. Sodium and Xe-100 reactors have much higher burnup rates than LWRs, Kim said. More burnup correlates with less nuclear waste because the fuel is more efficiently converted to energy. These designs also have higher thermal efficiency, which refers to how efficiently the heat produced by the reactor is converted into electricity. The VOYGR pressurized water reactor design, due in part to its small size, has a slightly lower burnup rate and thermal efficiency compared to a larger pressurized water reactor.
Spent fuel attributes vary somewhat by design: VOYGR is similar to LWR, Natrium produces more concentrated waste with different long-lived isotopes, and Xe-100 produces lower density but larger spent fuel volume.
“In general, when it comes to nuclear waste, SMRs are roughly comparable to conventional pressurized water reactors, with potential advantages and disadvantages depending on what aspects you’re trying to design for,” Kim said. “Overall, there appear to be no additional major problems in the management of SMR nuclear waste compared to large commercial-scale LWR waste.”
Research funding was provided by the Department of Energy’s Office of Nuclear Energy through the Systems Analysis and Integration Company.