Nuclear Waste from Small Modular Reactors

Introduction

Fig. 1: Energy-equivalent waste volumes, by waste type, for the NuScale iPWR, the Terrestrial Energy IMSR, and the sodium-cooled Toshiba 4S SMR. (Image source: I. Nafi, after Krall et al. [4])

In a world where climate change and the utilization of fossil fuels have fueled extensive environmental degradation and an energy crisis, states have been looking into alternatives to fulfill internal demands and slow down climate change. One of these options has been nuclear power generation, which has taken off since the Cold War era and currently helps fulfill the electrical needs of more than 33 countries. [1] However, nuclear energy is a nonrenewable energy source that poses major environmental challenges stemming from the radioactive waste it produces.

Currently, there is no official international regulation of the disposal of radioactive waste. Consequently, different states have adopted 2 main approaches for waste disposal. Countries like Finland, Sweden, and France are leveraging deep permanent disposal methods, while Japan, the U.K, and India are reprocessing spent nuclear fuel. Despite these methods, there still exists a lot of nuclear waste. In fact, a study conducted by researchers at Ontario Tech University estimated that about 370,000 tons of high-level radioactive wastes, waste containing radionuclides as fission products and transuranic elements generated in the reactor core due to fission, were discharged from power reactors since the Cold War, of which 120,000 tons are reprocessed. [2] The amount of high-level wastes generated across global reactors are steadily increasing by 12,000 tons annually, with estimates in 2020 showcasing about 250,000 tons of waste stored in nuclear-energy producing countries. [2]

With nuclear technology advancing with considerations to address some of the key challenges current technology poses, engineers have design small modular reactors (SMRs). These newer generation reactors typically produce up to 300 MW and are much easier to build as parts can be built in the shop and then transported as modules to sites for installation depending on demand. [3] SMRs have attracted international attention and investment as they bring the promise of benefits including quicker and cheaper production and enhanced safety. However, many engineers and developers have failed to carefully consider the waste SMRs can produce as they become more integrated within power generation.

Waste Estimates from SMRs

Due to their smaller size and construction and integration of more advanced nuclear technology, SMRs present unique waste challenges. Fuel spent in SMRs will contain relatively high concentrations of fissile nuclides and a greater chance of neutron leakage due to physical processes that are enhanced in smaller reactor cores and the use of neutron reflectors and chemically reactive fuels and coolants. [4] In order to examine SMR waste production, a team of researchers at the University of Pennsylvania in 2023 compared three distinct SMR designs, water, sodium, and molten salt cooled, to an 1,100-MW(elec) pressurized water reactor to metrics including energy-equivalent volume, radio-chemistry, decay heat, and fissile isotope composition of high, intermediate, and low-level waste streams.

Many SMRs employ a design decision where the reactor core and specific auxiliary systems, including steam generators, pressurizers, and heat exchangers, are all housed within a reactor module. For example, a NuScale integral pressurized water reactor (iPWR) can host up to 12 160-MW iPWRs, each submerged in the common reactor pool, and the primary coolant, either molten salt, water, or sodium, will be heated at the core and circulated upward. [4] In terms of waste generation, for SMRs that harness UO₂ fuel enriched to about 5 wt% U-235, researchers estimated that a 12-module iPWR will reduce burnup from 55 MWd/kg (Megawatt-days per kilogram) to 26-34 MWd/kg, however, it would discharge 21 MT of spent nuclear fuel (SNF)/y and 5.1 m³ SNF/GWth-y, which is comparable to a power station that hosts a single 3,400 MWth PWR. [4] If we look at the mass of fuel consumed per Joule of energy produced (1 Watt is 1 Joule per second), we have

Then, since Uranium has an atomic weight of 235 and releases about 230 MeV of energy during fission, the total energy released per kilogram of U-235 is

If you enrich the Uranium to 5% and then burn it, this means that the energy per kg of fuel rod consumed is

Therefore, the mass of the fuel rod required to generate 1 GWth-y of energy is

Thus to generate 3.4 GWth-y, we'd require about 23 tonnes, which checks out with the 21 tonnes from the research.

For Terrestrial Energy's 400 MWth molten salt reactor, these generate about 5.1 m³ SNF/GWth-y compared to 2.0 m³ SNF-GWth/y from a normal PWR reactor. [4] Other estimates show that the sodium-based 30-MWth Toshiba 4s reactor might generate about 115 m³/GWth-y of contaminated pyro-phoric sodium coolant in need of treatment and disposal with Terrestrial generating about 250 m³/GWth-y. [4] As shown in Fig. 1, across the board SMRs generate significantly larger amounts of SNF and long-lived and short-lived LILW compared to a 3,400 MWth PWR by factors of up to 5.5, 30, and 35, respectively. [5]

Conclusion

In conclusion, small modular reactors (SMRs) have emerged as a focal point for providing reliable and environmentally conscious power amid the escalating concerns over climate change. However, the optimism surrounding these reactors requires a nuanced consideration of their waste management implications. Despite the advantages offered by SMRs, including quick and cheap production and enhanced safety features, their waste disposal presents distinctive challenges, including a higher generation of spent nuclear fuel (SNF) and low-level waste (LILW) compared to traditional reactors. The discernible increase in these waste streams raises pertinent questions regarding the long-term ecological consequences of widespread SMR integration. As nations worldwide explore the potential of SMRs to address both energy demands and environmental concerns, its paramount to prioritize and develop comprehensive waste management strategies ​​to ensure the sustainability and integration of this technology.

© Itbaan Nafi. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute, and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.

References

[1] J. Kim et al., "Global Radioactive Waste Disposal Trends and Prospects," J. Korean Soc. Environ. Eng. 45, 210 (2023).

[2] S. A. Darda et al., "A Comprehensive Review on Radioactive Waste Cycle From Generation to Disposal," J. Radioanal. Nucl. Chem. 239, 15 (2021).

[3] C. L. Vinoya et al., "State-of-the-Art Review of Small Modular Reactors," Energies 16, 3224 (2023).

[4] L. M. Krall, A. M. Macfarlane, and R. C. Ewing, "Nuclear Waste From Small Modular Reactors," Proc. Natl. Acad. Sci. (USA) 119, e2111833119 (2022).

[5] L. Ghimire and E. Waller, "Small Modular Reactors: Opportunities and Challenges as Emerging Nuclear Technologies For Power Production," J. Nucl. Rad. Sci. 9, 044501 (2023).