Undeniably, ambitions for a nuclear renaissance are thriving. The UK is building its first nuclear power station in 30 years, France plans a new fleet of 14 reactors and, in total worldwide, 100 new reactors are proposed. And it is not only energy firms who are leading the way—Big Tech companies, who require reliable low-carbon dioxide baseload electricity to provide for their energy-thirsty AI technologies and data storage, are supporting nuclear energy too. Microsoft’s Constellation Energy has suggested reopening the Three Mile Island power plant (Reuters 2024) in Pennsylvania, and Amazon recently purchased a nuclear-powered data center (Gardner 2024a).

But unlike Microsoft and Amazon, Google has agreed to buy power generated from a fleet of small modular nuclear reactors that are yet to leave the drawing board, by backing the start-up company Kairos Power and their conceptual molten salt reactor (Gardner 2024b). The primary driver for building small modular reactors like these is to avoid the cost over-runs and extended timescales that have plagued large conventional reactor construction, like the Flamanville light water reactor built and operated by Électricité de France, which was 12 years overdue and more than 10 billion euros (about $10.9 billion in US dollars) over budget. Moreover, the advent of Gen IV reactor designs, often termed “advanced modular reactors,” promises not only electricity generation but also hydrogen and heat, as well as medical isotope production.

Over 80 novel designs of small modular reactors and advanced modular reactors exist, with several under development by start-up and established nuclear technology providers (IAEA 2022). These designs, some of them revolutionary, tend to range in energy output from 30 megawatts of electricity (30 million watts of electricity, or 30 MWe) to 300 MWe. The vendors of these technologies have great ambitions, and rightly so.

Closing the fuel cycle?

But obscured beneath these exciting prospects—and not easily discerned from the glossy marketing material of many reactor vendors—is how the radioactive waste generated from these reactors will be managed and ultimately disposed of. All nuclear fission reactors generate radioactive waste, some of which will be highly hazardous for many generations (up to hundreds of thousands of years). This is true even for those reactor technologies that claim to use radioactive waste as fuel. While actinides may be burned in fast breeder reactors, for example, their long-lived radioactive fission products will remain as waste. There is also radioactive waste encountered by the neutron-activated components of the reactor itself. Consequently, long periods of interim storage and processing are likely to be necessary before such burning can be carried out.

An additional problem for novel advanced modular reactors is that the fuels they use are often novel too, meaning that spent fuel waste from these reactors is poorly understood— completely unknown in some instances, because none has ever been available for research—and so the solutions to manage it safely in the long-term are yet to be developed (IAEA 2019).

Some new nuclear technology vendors assume that radioactive waste from their reactors will be first stored on-site for cooling prior to ultimate disposal in a geological disposal facility. They are right insofar as waste from current nuclear reactors is securely stored at locations where it awaits disposal in a geological facility (for those countries with a policy to build one). Once disposed of, several hundred meters below the ground, the waste is safely isolated from future populations by several layers of containment designed to mitigate the ingress of groundwater and corrosion of the waste, thus preventing the release of harmful radionuclides to the biosphere. It is this aspect of nuclear energy that earns it a place in the EU’s Green Taxonomy, the green handbook for investors that says that nuclear energy will “do no significant harm” to the environment in the long term (EU Taxonomy Overview 2022).

In practice, however, things are not so straightforward. A geological disposal facility is not simply a hole in the ground that any radioactive material can be dumped in. The material form of the waste and its radioactive characteristics—the decay heat, burnup, and radionuclide inventory—require consideration when designing the layers of barrier materials that are engineered to protect the waste from the subsurface environment and vice versa. For example, there are strict maximum temperature limits that the waste must not exceed; if the fuel is not cooled for long enough and the radioactive decay heat is too high, it could damage the engineered layers of containment, particularly clay containment structures. But cooling such waste for longer time periods above ground may render the commercial fueling strategies of certain types of reactor designs unprofitable—and which is more important to an investor? A skeptic may say that profitability is likely to win out over disposability.

Importantly, the waste itself must consist of a passively safe form that will not react chemically or physically with groundwater in the disposal environment. This requirement is likely to be met easily by conventional ceramic uranium dioxide fuel used in large gigawatt light water reactors and proposed for many small modular reactors using light water reactor technology, such as the Westinghouse Electric AP300, the Rolls Royce Small Modular Reactor (SMR), GEH’s BWRX-300 boiling water reactor and Holtec’s SMR-300, all current contenders in the UK Government’s small modular reactor competition.

But this is not the case for novel types of nuclear fuel proposed in many of the Gen IV advanced modular reactor designs, such as Google’s newly backed Kairos Power FHR, which uses a novel carbon-clad tri-structural isotropic fuel, or what is known as “TRISO” fuel. In particular, many of the Gen IV advanced modular reactor designs propose the use of uranium fluoride-containing molten salt fuel, metallic uranium, or uranium/plutonium alloys (Krall, Macfarlane and Ewing 2022).These fuels will require chemical conversion to a passively safe waste form, such as a glass (Bailey et al 2022) or a ceramic (Mason et al 2020), prior to disposal.

Despite over four decades of research in a field established by Rod Ewing and others, such immobilization technologies for these kinds of wastes remain immature (Ewing 1978). For example, the solubility of radioactively-contaminated salts in conventional nuclear waste glasses is extremely low; consequently, the total waste loading would also be very low, leading to a large volume of waste products for storage and ultimate disposal. This may necessitate material innovations in ceramic technologies like hot isostatic pressing, a technique which is being pioneered by the Australian Nuclear Science and Technology Organization for the immobilization of wastes from medical isotope production in their Synroc waste treatment facility in Australia—which the organization says generates a synthetic waste that “mimics the natural ability of rocks to lock up radioactive elements for hundreds of millions of years” (Australian Nuclear Science and Technology Organization 2016). Finally, there are limits on the size of the waste package that can be disposed of in a geological disposal facility, for practical transport and underground operational reasons, that are likely to rule out designs that envisage entire reactor cores being disposed of.

Uncertainty hinders long-term decision making

Some of the more than 80 designs of small modular reactors and advanced modular reactors offer more certainty than others in terms of radioactive waste management and disposal. In general, detailed reactor specifications do not yet exist, so it is difficult to undertake quantitative analysis to determine how much waste, and of what type, will be generated from a given reactor, and what the associated fuel cycle costs will be. The US National Academies of Sciences, Engineering and Medicine estimated that the costs of developing advanced reactor technology and fuel cycles could range from at least several billion up to hundreds of billions of dollars, depending on the current maturity of the technology (NAS, 2023).

The problem is exemplified by the contrasting conclusions of two recently published articles that evaluated the volumes and characteristics of wastes arising from a range of small modular reactors and advanced modular reactors in comparison with a large conventional light water reactor.

Using the information available in the open literature, Krall et al estimated the waste volumes arising from a small light water reactor, a molten salt reactor, and a sodium-cooled fast reactor in comparison with a conventional large gigawatt light water reactor. They concluded that small modular reactors and advanced modular reactors will increase the volume and complexity of intermediate-level waste and spent nuclear fuel when compared to a large light water reactor, leading to additional burden and cost related to decommissioning, waste storage, packing, and disposal. In contrast, a review by Argonne National Laboratory of a small light water reactor, a metal-fuel sodium-cooled fast reactor, and a high-temperature gas-cooled reactor using TRISO fuel, found that there were “no major challenges” to the management of small modular reactors and advanced modular reactor wastes compared to large light water reactors (Kim, Boing and Dixon 2024). Both studies emphasized the large uncertainty in their calculations given that precise designs, operating conditions, and decommissioning technologies of the reactors evaluated are not yet known.

In a recent report, the UK’s independent government advisory group, the Committee on Radioactive Waste Management (CoRWM 2024), summarized the key factors that influence the amount and type of waste generated from a nuclear reactor, including: fuel type, fuel enrichment, burn-up, refuel cycle, reactor size, coolant / moderator choice and whether or not an open or closed fuel cycle (i.e., reprocessing) will be operated. They concluded that small modular reactor and advanced modular reactor technologies could be categorized by the amount of uncertainty that exists in relation to the management and disposal of waste arising from that technology.

Technologies with low waste uncertainty included those utilizing light water reactor-type technology, for which there are already established practices, regulations, and experience in management and long-term storage of wastes. There is high confidence in, and knowledge of, the characteristics of the spent fuel and wastes generated from the decommissioning of light water reactors and, soon, with the construction of the Onkalo disposal facility in Finland, there will also be experience of their disposal. It is due to start accepting spent fuel waste this decade.

A medium level of waste uncertainty was reserved for technologies that have some common features with existing waste, but which may present some novel challenges in terms of disposability. This category includes high temperature gas-cooled reactors using TRISO fuel, which has a spherical kernel of uranium-based fuel at the center, surrounded by layers of carbon, silicon carbide, and graphite. It cannot be easily reprocessed due to the chemical properties of the layers, necessitating co-disposal with the carbon outer layers that entrain radionuclides not typically present in light water reactor spent fuel.

There is an inherent volume increase in spent fuel per unit energy for TRISO fuel when compared with that of large light water reactors, due to packing inefficiency of spheres and the lower density of the fuel (due to the carbon layers). TRISO fuel will use a high uranium 235 enrichment, altering the spent fuel’s radiological and heat characteristics from those of light water reactor spent fuel, which must also be considered during post-closure safety assessment in a geological disposal facility. Some experimental attempts to use these fuels have suffered serious operational issues (Moorman, 2008), thus rendering the disposal of the spent TRISO fuel involved impossible to date. High temperature gas-cooled reactors will also generate a significant volume of graphite waste, for which there is no universally agreed-upon route for disposal (IAEA 2016).

Advanced experimental reactors were deemed to have a high level of waste uncertainty. This included all reactors that envisage use of metallic fuels, since they are chemically reactive and require reprocessing prior to disposal (and reprocessing tends to create voluminous secondary radioactive wastes, also requiring disposal). It also included reactors utilizing molten salt coolants or a combined fuel-moderator molten salt, reactors using liquid metal coolants, for which no management or disposal route is available if they were to become activated, and reactors that envisaged high burn-up, single life cores.

Wastes in the latter category may not be disposable in a geological disposal facility at all. Some may be disposable after overcoming significant technical challenges, at a great cost and involving a protracted, likely several decades-long R&D program to address uncertainties. Evidence that reactor vendors have embraced the magnitude of effort, time, and expense required to overcome these technical challenges is not easy to find, nor is it clear the full extent to which this requirement has been communicated to their investors.

Opportunity for so-called “clean” nuclear energy and a cap on unconstrained costs

Regulatory approval to construct any new type of nuclear reactor technology, among many other factors, requires a robust plan for radioactive wastes arising from operation and decommissioning. Simply put, new reactor designs will not be approved if the reactor vendors cannot demonstrate a suitable proposition for the safe long-term management of the wastes they will create.

Unfortunately, there are too many examples of nuclear sites storing radioactive waste that are now suffering from the lack of such foresight when, decades ago, decisions were taken about nuclear energy without thinking through the consequences for waste management. Such decisions have resulted in staggeringly high and unconstrained costs, of the type that might make investors wary. For example, take the 650-acre Sellafield nuclear site, home to the UK’s central storage of high-level waste and spent nuclear fuel since 1947. The cost of cleaning up 70 years of historic nuclear waste (UK National Audit Office, 2024)—some of it poorly managed in the past because of ill-advised operational decisions—has risen by £1.15 billion (about $1.48 billion in US dollars) in the last 5 years alone, bringing the total clean-up cost, over the 100-year decommissioning mission, to a whopping £216 billion ($279 billion).

However, the development of new small modular reactors and advanced modular reactor designs offers a unique opportunity to allow nuclear energy to really live up to its “clean” label and to put a cap on the costs of nuclear clean-up. It allows reactor vendors to work with waste management organizations and regulators at an early stage to consider spent fuel management, waste treatment, conditioning, storage, and disposal, as well as reactor decommissioning, up front, at the conceptual design phase. If, on paper, waste could be considered at every stage from reactor component material selection to refuelling strategy, the amount and type of waste could be greatly minimized. Or at the very least, methods to safely contain and dispose of wastes that meet with existing storage and disposal plans could be implemented, lowering the overall cost and impact on the environment.

Such foresight would give financial certainty to investors about the lifetime costs of the reactor, for which they will ultimately bear responsibility, as well as meeting the expectations of the public, whose support for new nuclear facilities is contingent on the existence of credible solutions to radioactive waste storage and disposal (UK Government Department for Business, Energy & Industrial Strategy 2021). Companies like Google and others could then be confidently pro-nuclear, and head off the threat to the climate from the energy demands of the continuing digital revolution.

Acknowledgements

The authors wish to thank members of the Committee on Radioactive Waste Management, and the CoRWM secretariat, for their helpful review of this article. We are grateful to all stakeholders who input to our efforts to understand the implications on radioactive waste management and disposal of small and advanced modular reactors.