Some believe that nuclear energy is a key component of decarbonizing our economy. However, big conventional nuclear power reactors are complex and expensive. In order to make nuclear energy more available and attractive, developers have designed a variety of small modular reactors (SMRs) that have more flexibility and offer lower initial costs. Different types of SMRs with advanced design features are currently under development in the U.S. and around the world.
Researchers believe that SMRs could be deployed at a variety of scales for locally distributed generation of electricity. SMR have an output of three hundred megawatts or less. This is about one fourth of the one and two tenths’ gigawatts of the conventional pressurized light water nuclear power reactors. The economics and technologies of SMRs have been broadly studied. However, there is less information about their implications with respect to nuclear waste. Take Kyum Kim is a senior nuclear engineer at the U.S. Department of Energy’s (DoE) Argonne National Laboratory. He said, “We've really just begun to study the nuclear waste attributes of SMRs.”
Kim and his team from Argonne and DoE’s Idaho National Laboratory recently issued a report that attempts to measure the potential nuclear waste attributes of three different SMR technologies. They used metrics developed through an extensive process during a comprehensive assessment of nuclear fuel cycles published in 2014. Although SMRs are not yet in commercial operation, several companies have collaborated with the DoE to explore different possibilities for SMRs. The three designs studied in the report are all scheduled to be constructed and operational by the end of this decade.
One type of SMR is called VOYGR. It is being developed by NuScale Power. It is based on a current pressurized water reactor design but has been scaled down and modularized. A second SMR is called Natrium and is being developed by TerraPower. It is sodium cooled and runs on a metallic salt fuel. The third SMR is called the Xe-100 and is being developed by X-energy. It is cooled by helium gas.
With respect to nuclear waste, each reactor offers both advantages and disadvantage over large light water reactors (LWRs). Kim said, “It's not correct to say that because these reactors are smaller, they will have more problems proportionally with nuclear waste, just because they have more surface area compared to the core volume. Each reactor has pluses and minuses that depend upon the discharge burnup, the uranium enrichment, the thermal efficiency and other reactor-specific design features.”
One important factor that influences the amount of nuclear waste produced by a reactor is called burnup. It refers to the amount of thermal energy produced from a certain quantity of nuclear fuel. The Natrium and Xe-100 reactors have significantly higher burnup than LWRs. A higher burnup is correlated with lower nuclear waste production. This is because the fuel is converted more efficiently to energy. These designs also have higher thermal efficiency. This refers to how efficiently the heat produced by the reactor is converted into electricity. The VOYGR pressurized water reactor design has a slightly lower burnup and thermal efficiency than a big conventional pressurized water reactor.
The spent fuel attributes vary somewhat between the designs. VOYGR is similar to the LWRs. Natrium produces a more concentration waste with a different mixture of long-lived isotopes. Xe-100 produces a lower density but a higher volume of spent fuel.
Kim said, “All told, when it comes to nuclear waste, SMRs are roughly comparable with conventional pressurized water reactors, with potential benefits and weaknesses depending on which aspects you are trying to design for. Overall, there appear to be no additional major challenges to the management of SMR nuclear wastes compared to the commercial-scale large LWR wastes.”