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M.V. Ramana and Zia Mian, Princeton University
Nuclear energy is on the decline, certainly as a share of global electricity generation. Between 1996 and 2016, this share has come down from 17.6 percent to 10.7 percent (BP 2016). Future prospects don’t seem any better either: the International Atomic Energy Agency’s projections for 2030 and 2050 see nuclear energy maintaining market share under the high scenarios, and declining under low scenarios (Ramana 2016a).
The influential 2003 study on the future of nuclear power from the Massachusetts Institute of Technology attributed the "limited prospects for nuclear power" to "four unresolved problems": costs of generating electricity at nuclear reactors, safety of reactors and nuclear fuel cycle facilities; proliferation and the possible misuse of commercial or associated nuclear facilities and operations to acquire technology or materials as a precursor to the acquisition of a nuclear weapons capability"; and "unresolved challenges in long-term management of radioactive wastes" (Deutch et al. 2003, 2).
One particular problem that has become even more of a challenge in the last decade or more has been increasing costs of construction. Reactors classified as Generation III or III+ have typically costed 6 to 10 billion dollars (for 1100 to 1600 MW of generating capacity), and have taken close to a decade or more to construct; practically all have experienced time and cost overruns in comparison with initial estimates (Schneider and Froggatt 2015). In response, nuclear reactor developers and vendors in several countries have been pursuing the development of Small Modular Reactors (SMRs), with power levels between 10 and 300 MWe, much smaller than the 1000–1600 MWe reactor designs that have become the industry standard.
Proponents of SMRs suggest that these reactors can resolve the four key challenges confronting nuclear power today. In turn, many countries, including the United States, Russia, China, France, Japan, South Korea, India, and Argentina, are investing large amounts of money to support such development. What are the prospects of SMRs solving the problems confronting nuclear power?
Any attempt to deal with the problems identified above — safety enhancement, proliferation resistance, decreased generation of waste, and cost reduction — has to be reflected in some fashion in the design of specific nuclear reactors. But it turns out that each of these priorities can drive the requirements on the reactor design in different, sometimes opposing, directions (Ramana and Mian 2014). Leading SMR designs under development involve choices and trade-offs between desired features and focusing on any one goal might make other goals more difficult to achieve.
Because there a number of SMR designs are under development, it is not possible to examine each of them to demonstrate these trade-offs. SMR designs vary by power output, physical size, fuel geometry, fuel type and enrichment level (and resulting spent fuel isotopic composition), refueling frequency, site location, and status of development. However, the many different kinds of SMRs can be classified into a few families, which share common characteristics.
One way to categorize SMRs is to look at their primary purpose, or stated aim (Glaser et al. 2015).
Ready to Build
The first family of SMRs involves reactor designs intended to demonstrate the technical and commercial viability of these designs as early as possible. These are essentially scaled-down standard light water reactors, usually with steam generators located within the same pressure vessel as the reactor itself (integral Pressure Water Reactor or iPWR). Integration of the primary system has been assessed by some analysts to be "the biggest challenge to SMR development".
These reactors are typically fueled with low-enriched uranium, with enrichment levels of 5% or less. Not only is the enrichment of fuel in the same ballpark as conventional light water reactors, but even the fuel assembly designs are intended to be almost identical to existing designs (although scaled down in height). Because of the similarity of the fuel design, the spent fuel can be reprocessed using traditional and widely understood techniques.
Succeeding the Second Time Around
A second family of SMRs involves reactor designs that were studied in the past but that lost out to the light water reactor design that has dominated nuclear power deployment since the 1970s. Two leading types are the molten-salt reactor (MSR) and the high temperature gas-cooled reactor (HTGR) concept.
MSR designs, as the name suggests, involve nuclear fuel dissolved in salts and that is continuously circulated in and out of the reactor itself. When out of the reactor, the fuel has to be processed, to remove the build-up of various fission products. But handling the highly radioactive molten-salt stream and ensuring that various structural components of the reactor core can tolerate high levels of irradiation as well as corrosion from the highly corrosive salts remain formidable challenges before these designs can be commercialized. These designs have so far not been subjected to independent evaluation of safety by nuclear regulators.
The fuel for HTGRs, on the other hand, is usually in the form of TRISO (tristructural-isotropic) particles, which consist of uranium coated with multiple layers of different materials that can withstand high temperatures and are hard — but not impossible — to reprocess. For use as fuel, the uranium has to be enriched to well above 5 percent as fuel, and graphite as a moderator. Helium is often used as the coolant fluid. Earlier attempts at commercializing similar designs failed (Ramana 2016b).
Reducing the Burden of Nuclear Waste
The next reactor family involves designs that seek to extend uranium resources by using uranium much more efficiently and so lessen the problem of legacy waste. This requires these reactors to be based on the use of fast (energetic) neutrons without any moderator, because fast neutrons are more efficient at fissioning all isotopes of uranium and transuranic elements. The coolant used in these reactors is typically a molten metal, often molten sodium, although some designs involve helium as a coolant.
Comes with Fuel for a Lifetime
Lastly, there are designs intended as "nuclear batteries," with long-lived cores that are designed for possibly unattended operation. They are generally targeted at "newcomer" nations with small electric grids interested in developing nuclear power systems or for remote locations in developed countries. These reactors tend to be liquid metal-cooled fast reactors with fresh fuel having high uranium enrichment levels.
Evaluating all the different SMR designs, even when they are organized in families, against the desired criteria of costs, safety, waste, and proliferation is not straightforward. Each of these criteria has several dimensions, and multiple technical characteristics are needed to effectively implement each criterion. At the same time, the different designs do have some shared technical characteristics, and these characteristics affect how these reactors might score on different desirable criteria.
The economics of nuclear power is a challenge because of both the high cost of constructing each facility and the high cost of generating each unit of electrical energy relative to other options for meeting the same demand. The two are related but distinct. The attraction of SMRs comes from the fact that they are expected to have lower initial expenditures. But this feature will likely make the latter challenge even harder to meet because they miss out on what are called economies of scale: the advantages that come with costs scaling more slowly than output power. For example, a 1000 MW reactor does not require four times as much concrete as a 250 MW reactor. Designers hope that this negative effect possibly could be offset somewhat through economies of mass manufacture. But even with optimistic assumptions about learning rates, hundreds, if not thousands, of reactor units would have to be built in order for mass manufacture effects to counteract the loss of economies of scale (Glaser et al. 2015). There are but 450 reactors operating today around the world after roughly six decades of nuclear power plant construction. Expert elicitation studies also project higher costs for SMRs (Abdulla, Azevedo, and Morgan 2013; Anadón et al. 2012). Thus, the smaller power capacity of SMRs has a largely negative effect on costs of electricity generation, and is unlikely to make nuclear power economically competitive.
There are also specific features of each of these SMR types that would tend to increase costs. For example, the lower fuel burnup in iPWRs means that fueling costs would be higher whereas the special materials used to coat the fuel particles in HTGRs and non-conventional manufacturing techniques also lead to higher fueling costs. In the case of nuclear batteries, the increased cost is a result of needing to fuel the reactor for its entire lifetime up front, that too with fuel with higher enrichment levels.
The small physical size and smaller fissile inventories of SMRs benefit safety. However, in the case of fast reactors, there are other characteristics that affect safety negatively. These include the potential in the core for accidents involving disassembly and reactivity increase as well as the risks from using molten metals as coolants (IPFM 2010; Kumar and Ramana 2008). Proponents of these reactors argue, not surprisingly, that they are safe, but many others view the use of fast spectrum neutrons and molten metal coolants as a significant disadvantage from a safety perspective.
One disturbing trend has been attempts by SMR proponents to emphasize the safety aspects of these reactors to use those features as reasons to get existing licensing requirements diluted (Ramana, Hopkins, and Glaser 2013). The primary motivation for these attempts has been to compensate for higher costs of electricity generation and the consequent inability to compete economically in power markets.
SMRS based on fast neutrons produce a lower amount of radioactive waste per unit of electricity generated. The significance of the lower rate of waste generation, however, is debatable. The problem with siting geological repositories for waste disposal has been local and public resistance. The level of resistance is not particularly sensitive to the amount of waste that might be disposed of in the repository. In other words, even if the repository were to be designed to deal with a significantly smaller volume of spent fuel, there may not be a corresponding decrease in opposition to siting the facility.
Linkage with nuclear weapons
The linkages of nuclear power to the potential for weapon proliferation stems mainly from the front end (uranium enrichment) and the back end (plutonium in spent fuel, and possible processing of spent fuel) of the nuclear fuel chain (Feiveson et al. 2014). All else being equal, the use of fuel with higher levels of uranium enrichment would be a greater proliferation risk, and is the reason why so much international attention has been given to converting highly enriched uranium fueled research reactors to low enriched uranium fuel or shutting them down. Likewise, the chemical processing of fuel allows easier access to the plutonium (or uranium-233, in the case of reactors using thorium), which facilitates proliferation. Practically any mixture of plutonium isotopes could be used for making weapons (DoE 1997; Mark 1993). In the case of both iPWRs and fast reactors, the proliferation risk is enhanced relative to current generation light water reactors primarily because greater quantities of plutonium are produced per unit of electricity generated (Glaser, Hopkins, and Ramana 2013).
Proliferation resistance imposes sometimes contradictory requirements. One way to lower the risk of diversion of fuel from nuclear reactors is to minimize the frequency of refueling because these are the periods when the fuel is out of the reactor and most vulnerable to diversion, and so many SMR designers seek longer periods between refueling. This is the case for SMRs belonging to the fourth family. However, in order for the reactor to maintain reactivity for the longer period between refuelings, it would require starting with fresh fuel with higher uranium enrichment or mixing in plutonium. Therefore, any reduction of proliferation risk at the reactor site by reducing refueling frequency, will be accompanied by an increase in the proliferation risk elsewhere.
SMRs belonging to other families have different impacts on proliferation. In the case of HTGRs, proliferation risk is increased because of the use of fuel with higher levels of uranium enrichment, but is diminished because the spent fuel is in a form that is difficult to reprocess. With MSRs, the continuous processing of fuel, which is integral to reactor operation, could facilitate the extraction of weapon-usable materials (plutonium or uranium-233) from the fuel.
Of the different major SMR designs under development, it seems none meets simultaneously the key challenges of costs, safety, waste, and proliferation facing nuclear power today and constraining its future growth. In most, if not all designs, it is likely that addressing one or more of these four problems will involve choices that make one or more of the other problems worse.
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———. 2016b. "The Checkered Operational History of High Temperature Gas Cooled Reactors." Bulletin of the Atomic Scientists 72 (3): 171–79. doi:10.1080/00963402.2016.1170395.
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These contributions have not been peer-refereed. They represent solely the view(s) of the author(s) and not necessarily the view of APS.