Findings from research led by a team that included a former U.S. Nuclear Regulatory Commission (NRC) chairperson and experts from Stanford University suggests small modular reactors (SMRs) will generate more radioactive waste than conventional gigawatt-scale nuclear units.

The results were released in a research article published in the Proceedings of the National Academy of Sciences (PNAS), a peer-reviewed journal of the NAS. The authors of the article are Lindsay M. Krall, an affiliate with Stanford University, who currently works in the Research and Safety Analysis Division of the Swedish Nuclear Fuel and Waste Management Co., which deals with all the radioactive waste from nuclear power plants in Sweden; Allison M. Macfarlane, chairperson of the NRC from 2012 to 2014, and current professor and director at The University of British Columbia; and Rodney C. Ewing, a professor and co-director at Stanford University.

Evaluating SMR Waste Streams

SMRs are generally classified as nuclear reactors with a maximum electrical output of 300 MW or less. Many companies, including NuScale Power, GE-Hitachi, Terrestrial Energy, TerraPower, Toshiba, and others, are developing SMRs. Among the benefits touted by SMR proponents are improved reactor safety features, quicker plant construction, and reduced costs. However, the researchers said “remarkably few studies have analyzed the management and disposal of their nuclear waste streams.”

Consequently, the team wanted to compare three distinct SMR designs to an 1,100-MW pressurized water reactor (PWR) in terms of the energy-equivalent volume, radio-chemistry, decay heat, and fissile isotope composition of high-level waste (HLW) and low- and intermediate-level waste (LILW) streams. “Results reveal that water-, molten salt–, and sodium-cooled SMR designs will increase the volume of nuclear waste in need of management and disposal by factors of 2 to 30,” the article says.

The SMR designs evaluated in the study were the NuScale iPWR, the Toshiba 4S sodium-cooled fast reactor, and the Terrestrial Energy Integral Molten Salt Reactor (IMSR). The team said these designs were selected from a list of 16 options because reliable reactor and fuel-cycle specifications were available in pre-license and patent application materials.

How Waste Is Generated

The typical power production process at most nuclear power plants involves fission of uranium fuel, which produces heat used to generate steam, which is directed to a turbine generator to produce electricity. The reaction occurs when neutrons are absorbed by uranium atoms, making them unstable and causing them to fission. The neutron “economy” of a reactor depends on the efficiency of the chain reaction process. Neutrons that are not absorbed by the fuel or fissile nuclides in the core can be lost across the fuel boundary and can activate structural materials surrounding the fuel assemblies, which creates a secondary form of radioactive waste.

The amount of nuclear waste generated in a plant is influenced by the formation and distribution of radionuclides throughout the reactor, which in turn, depends on the geometry, composition, and flow paths of reactor, fuel, moderator, and coolant materials. The probability of neutron leakage is a function of the reactor dimensions. As the name implies, SMRs are smaller than gigawatt-scale reactors, which means there is more surface area per unit volume for neutrons to escape, thus, more neutron leakage is to be expected. In fact, the researchers noted that leakage grows quadratically with decreasing core radius and reactor size.

The PNAS article claims a 3,400-MWth PWR will leak less than 3% of its free neutrons, whereas, a 160-MWth iPWR may leak greater than 7%. Leakage from fast reactors was said to also be high—at least 4% and up to 25%, depending on the fuel composition and other aspects of core design. Overall, “both water and nonwater SMRs entail increased neutron leakage as compared with a gigawatt-scale light water reactor (LWR),” the article says. Furthermore, small increases in neutron leakage are said to have a significant effect on core criticality and power output, and will lead to reduced spent nuclear fuel (SNF) burnup.

More Complicated Waste Management

In the end, the researchers suggest the excess waste volume is attributed “to the use of neutron reflectors and/or of chemically reactive fuels and coolants in SMR designs.” However, they also said volume is not the most important evaluation metric; rather, geologic repository performance is driven by the decay heat power and the radio-chemistry of SNF, for which SMRs provide no benefit. “SMRs will not reduce the generation of geochemically mobile [iodine-129, technetium-99, and selenium-79] fission products, which are important dose contributors for most repository designs.”

More importantly, the researchers said, SMR waste streams will bear significant radio-chemical differences from those of existing reactors. “Molten salt– and sodium-cooled SMRs will use highly corrosive and pyrophoric fuels and coolants that, following irradiation, will become highly radioactive. Relatively high concentrations of [plutonium-239 and uranium-235] in low–burnup SMR SNF will render recriticality a significant risk for these chemically unstable waste streams,” the article says.

Ultimately, the analysis of the three SMR designs shows that, relative to a gigawatt-scale PWR, “these reactors will increase the energy-equivalent volumes of SNF, long-lived LILW, and short-lived LILW by factors of up to 5.5, 30, and 35, respectively.” The article says SMR waste streams that are susceptible to exothermic chemical reactions or nuclear criticality when in contact with water or other repository materials are “unsuitable for direct geologic disposal.” Therefore, a large volume of reactive SMR waste may need to be treated, conditioned, and appropriately packaged prior to geological disposal. “These processes will introduce significant costs—and likely, radiation exposure and fissile material proliferation pathways—to the back end of the nuclear fuel cycle and entail no apparent benefit for long-term safety,” the researchers concluded.

NuScale Reports ‘Factual Error’

Jose N. Reyes, Chief Technology Officer and co-founder of NuScale, in a letter addressed to Professor May R. Berenbaum, Editor-in-Chief of PNAS, said there was “a factual error in the paper.” Specifically, he wrote, “The authors mistakenly assert that NuScale Small Modular Reactors (SMRs) will produce significantly more spent nuclear fuel (SNF) than existing Light Water Reactors. The basis for this statement is their analysis of the NuScale 160 MW thermal core as opposed to the NuScale 250 MW thermal core implemented in NuScale VOYGR plants.

“In January 2021, NuScale provided the characteristics of the 250 MWt core to the National Academies of Science, Engineering, and Medicine (NASEM) Ad hoc Committee to evaluate and assess the ‘Merits and Viability of Different Nuclear Fuel Cycles and Technology Options and the Waste Aspects of Advanced Nuclear Reactors.’ NuScale’s response to the NASEM ad hoc committee questionnaire is publicly available and states that the NuScale fuel has an average fuel burnup of approximately 45,000 MWd/t at discharge and that it has a design basis maximum exposure of 62 GWd/MTHM. These values are within the values typically observed in the existing fleet of LWRs. Therefore, the NuScale 250 MWt design does not produce more SNF than the small quantities typically observed in the existing LWR fleet.”

Researchers Respond

POWER contacted the authors of the PNAS article, asking for their response to Reyes’ claim of a factual error in the paper. The group said:

“First, we want to emphasize that our decision to analyze the NuScale 160 MW thermal core was deliberate and driven by the information available at the time of our study. One of the challenges of completing our analysis was the lack of access to relevant design specifications. For this reason, we focused on the NuScale 160 MWth reactor because the design specifications had been submitted to the Nuclear Regulatory Commission for certification and review. This provided a referenceable source for our analysis. In addition, we filed a request under the Freedom of Information Act to the NRC to obtain the burn-up figure for the NuScale 160 MWth reactor. However, the burn-up figure was redacted from the application; hence, the parameter was calculated as described on page 8 of the SI Appendix.

“In general, we focused on the designs being presented to the NRC. Concerning the parameters for the 250 MWth design, the presentation and information provided to the Academy committee did not provide all the information necessary for a quantitative analysis. Our understanding is that NuScale will be submitting a new application for the 250 MWth design to the Nuclear Regulatory Commission this coming December.

“We emphasize that our study focused on the NRC-certified, 160 MWth NuScale iPWR design because it had been reviewed during the certification process and an adequate amount of information was available on the core design. This is not a ‘factual error’ in our study, but rather a choice we made that is clearly stated in the study. Once the necessary design parameters for the 250 MWth reactor are available, then an analysis can be completed of this larger NuScale reactor. Based on our present understanding, we anticipate that a future analysis will show that the 250 MWth reactor generates less waste per unit energy-equivalent than the 160 MWth reactor but more than a larger LWR. In other words, smaller reactors generate more waste – exactly the point of our paper in PNAS.”

Aaron Larson is POWER’s executive editor (@AaronL_Power, @POWERmagazine).

[Ed. note: This post was originally published June 2, 2022. It was updated on June 10, 2022, to add a response from the author’s of the PNAS article to NuScale’s claim of “a factual error in the paper.”]