Archive for November, 2017

Edwin Lyman on Small Modular Reactors

November 29, 2017

Small Isn’t Always Beautiful Safety, Security, and Cost Concerns about Small Modular Reactors

UCS, Edwin Lyman September 2013

“…….Less expensive does not necessarily mean cost effective, however. The safety of the proposed compact designs is unproven—for instance, most of the designs call for weaker containment structures. And the arguments in favor of lower overall costs for SMRs depend on convincing the Nuclear Regulatory Commission to relax existing safety regulations………

Congress should direct the Department of Energy (DOE) to spend taxpayer money only on support of technologies that have the potential to provide significantly greater levels of safety and security than currently operating reactors. The DOE should not be promoting the idea that SMRs do not require 10-mile emergency planning zones—nor should it be encouraging the NRC to weaken its other requirements just to facilitate SMR licensing and deployment………

“Small” is defined by the DOE as one that generates less than 300 MWe (megawatts of electric power), which is about 30 percent of the capacity of a typical current commercialpower reactor. “Modular” refers to the concept that the units would be small enough to be manufactured in factories and shipped to reactor sites as needed to meet incremental increases in demand (Smith-Kevern)……….

… greater levels of nuclear plant safety and security cannot be achieved by smart design alone. It must also extend to operation. Without an overarching regulatory framework focused on substantially increasing the level of operational safety, there will be no assurance of greater safety for next-generation reactors either large or small…….

“Affordable” doesn’t necessarily mean “cost-effective.”…….

.. far from increasing design and operational safety standards, proponents of SMRs claim small modular reactors will be so much safer than large reactors that they will not need to meet the same safety standards as large reactors, arguing that they need far fewer operators and security officers, and that they can have disproportionately smaller and weaker containment buildings. SMR advocates claim that they are so safe they can be located close to densely populated areas without the need for extensive evacuation planning. This argument is a crucial part of the case being made by the DOE and others that SMRs can be deployed to replace coal plants at existing sites, many of which are near urban areas. We consider each of those issues below.

“affordable” doesn’t necessarily mean “cost-effective.” According to basic economic principles, the cost per kilowatt-hour of the electricity produced by a small reactor will be higher than that of a large reactor, all other factors being equal. That is because SMRs are penalized by the economies of scale of larger reactors—a principle that drove the past industry trend to build larger and larger plants (Shropshire). For example, a 1,100 MWe plant would cost only about three times as much to build as a 180 MWe version, but would generate six times the power, so the capital cost per kilowatt would be twice as great for the smaller plant (see, e.g., the economies of scale formula used by Carelli et al.)

SMR proponents argue that other factors could offset this difference, effectively reversing the economies of scale. For example, efficiencies associated with the economics of mass production could lower costs if SMRs are eventually built and sold in large numbers. Such factors are speculative at this point, however, and the degree to which they might reduce costs has not been well characterized. A 2011 study found that even taking into account all the factors that could offset economies of scale, replacement of one 1,340 MWe reactor with four 335 MWe units would still increase the capital cost by 5 percent (Shropshire).

The potential cost benefits of assembly-line module construction relative to custom-built on-site construction may also be overstated. Moreover, mistakes on a production line can lead to generic defects that could propagate through an entire fleet of reactors and be costly to fix. The experience to date with construction of modular parts for the nuclear industry has been troubling……..

….Unless the negative economies of scale can be overcome, SMRs could well become affordable luxuries: more utilities may be in a financial position to buy an SMR without “betting the farm,” but still lose money by producing high-cost electricity. In any event, it would take many years of industrial experience, and the production of many units, before the potential for manufacturing cost savings could be demonstrated. In the meantime, as the Secretary of Energy Advisory Board’s SMR subcommittee stated in a November 2012 report, “first of a kind costs in U.S. practice will likely make the early [SMR] units considerably more expensive than alternative sources of power. If the U.S. is to create a potential SMR market for US vendors, it will need to do something to help out with such costs” (SEAB). The report pointed out that if the government decided to provide such help, it would have a “panoply of direct and indirect tools available to support the development of an SMR industry” ranging from “funding SMR demonstration plants, perhaps on U.S. government sites (the DOE is a particularly large user of electricity) to a variety of financial incentives” including “continued cost sharing with selected SMR vendors beyond design certification,” “loan guarantees,” and “production tax credits or feed-in tariffs for those utility generators that are early users of SMR power purchase contracts.”…….

DOE officials have referred to this situation as a “Catch-22.” The economics of mass production of SMRs cannot be proven until hundreds of units have been produced. But that can’t happen unless there are hundreds of orders, and there will be few takers unless the price can be brought down. This is why the industry believes significant government assistance would be needed to get an SMR industry off the ground.

In addition, there appears to be a growing realization that the first SMR production factory cannot fill its order book with domestic units but will need to access a sizable international export market………

In addition to imposing a penalty on the capital cost of SMRs, economies of scale would also negatively affect operations and maintenance (O&M) costs (excluding costs for nuclear fuel, which scale proportionately with capacity). Labor costs are a significant fraction of nuclear plant O&M costs, and they do not typically scale linearly with the capacity of the plant: after all, a minimum number of personnel are required to maintain safety and security regardless of the size……..

SMR vendors are pressuring the NRC to weaken regulatory requirements for SMRs……. . Unless utilities can find a way to justify significantly reducing personnel for smaller reactors, SMRs will need a larger number of workers to generate a kilowatt of electricity than large reactors. Yet a 2011 study of 50 small and medium-sized reactors in Europe concluded that O&M costs must be kept consistently “low”—defined as less than 20 percent of total costs—to maintain SMR cost competitiveness (Shropshire). To reduce both capital costs and O&M costs, SMR vendors are pressuring the NRC to weaken certain regulatory requirements for SMRs……..

SMR Designs……….

NuScale. The NuScale concept is considerably different from both the mPower SMR design as well as from the current fleet of large nuclear reactors. The NuScale design envisions an array of up to 12 reactor modules, each generating 45 MWe of power, submerged under water in a swimming-pool-like structure. Each module would be 65 feet tall and nine feet in diameter, and would be nested within a very small containment structure 82 feet tall and 15 feet in diameter. Unlike the mPower design, the NuScale control rod drive mechanisms would be external to the vessel. Only natural convection cooling of the core would be used both for routine operation and for emergencies: there would be no coolant pumps at all in the primary reactor coolant system (the primary cooling system carries heat from the core to the steam generators). The secondary loop, the system that carries steam from the steam generators to the turbines, would still require motor-driven pumps. NuScale differs from other pressurized water reactors in that the primary coolant is not pumped through the steam generators, but flows around outside of them. The secondary coolant is pumped through the steam generator coils. The designers claim that emergency cooling could be maintained indefinitely in a station blackout by relying on a series of valves that do not require electrical power to open or close and achieve their correct positions (Neve).

Holtec SMR-160. The Holtec SMR-160 will generate 160 MWe. Like the NuScale, it is designed for passive cooling of the primary system during both normal and accident conditions. However, the modules would be much taller than the NuScale modules and would not be submerged in a pool of water. Each reactor vessel would be located deep underground, with a large inventory of water above it that could be used to provide a passive heat sink for cooling the core in the event of an accident. Each containment building would be surrounded by an additional enclosure for safety, and the space between the two structures would be filled with water. Unlike the other iPWRs, the SMR-160 steam generators are not internal to the reactor vessel. The reactor system is tall and narrow to maximize the rate of natural convective flow, which is low in other passive designs. Holtec has not made precise dimensions available, but the reactor vessel is approximately 100 feet tall, and the aboveground portion of the containment is about 100 feet tall and 50 feet in diameter (Singh 2013)

For these and other SMRs, it is important to note that only limited information is available about the design, as well as about safety and security. A vast amount of information is considered commercially sensitive or security-related and is being withheld from the public. ……

SMR Safety In general, the engineering challenges of ensuring safety in small modular reactors are not qualitatively different from those of large reactors. No matter the size, there must be systems in place to ensure that the heat generated by the reactor core is removed both under normal and accident conditions at a rate sufficient to keep the fuel from overheating, becoming damaged, and releasing radioactivity. The effectiveness of such systems depends on the details of their design. Even nuclear fuel in spent fuel pools, which usually have much lower heat loads than reactor cores, can overheat and rupture if adequate cooling is not provided……..

……some vendors are marketing these designs as “inherently safe,” which is a misleading term. While there is no question that natural circulation cooling could be effective under many conditions for such small reactors, it is not the case that these reactors would be inherently safe under all accident conditions. There are accident scenarios in which heat-transfer conditions would be less than ideal and thus natural convection cooling could be impeded. For instance, for the NuScale design a large earthquake could send concrete debris into the pool, obstructing circulation of water or air. Indeed, no credible reactor design is completely passive: no design can shut itself down and cool itself in every circumstance without the need for intervention. Even passively safe reactors require some equipment, such as valves, that are designed to operate automatically. But no valve is 100 percent reliable. In addition, as discussed below, accidents affecting more than one small unit may cause complications that could overwhelm the capacity to cope with multiple failures, outweighing the advantages of having lower heat removal requirements per unit.

Ultimately, how well any safety systems work depends on the accidents against which they are designed to protect. Passive systems alone can address only a limited range of scenarios, and may not work as intended in the event of beyond-design-basis accidents. As a result, passive designs should also be equipped with multiple, diverse, and highly reliable active backup cooling systems. Such systems will necessarily be more complex but the engineering challenges should be manageable with good design of instrumentation and control system architecture. Still, more backup systems generally mean higher costs. Thus, a multiple-backup design philosophy is not generally compatible with the small, compact, stripped-down design of the SMRs currently under consideration………

The need to reduce SMR capital costs is driving one important passive safety system—the containment structure—to be smaller and less robust. None of the iPWR designs has a containment structure around the reactor with sufficient strength and volume to withstand the forces generated by overpressurization and hydrogen explosions in severe accidents. SMRs therefore must rely on means to prevent hydrogen from reaching explosive concentrations. However, neither active means (hydrogen igniters) nor passive means (hydrogen recombiners) of hydrogen control are likely to be as reliable as a robust containment………

Some SMR vendors propose to locate their reactors underground, which they argue will be a major safety benefit. While underground siting would enhance protection against certain events, such as aircraft attacks and earthquakes, it could have disadvantages as well. Again at Fukushima Daiichi, emergency diesel generators and electrical switchgear were installed below grade to reduce their vulnerability to seismic events, but that location increased their susceptibility to flooding. Moreover, in the event of a serious accident, emergency crews could have greater difficulty accessing underground reactors.

Underground siting of reactors is not a new idea. Decades ago, both Edward Teller and Andrei Sakharov proposed siting reactors deep underground to enhance safety. However, it was recognized early on that building reactors underground increases cost. Numerous studies conducted in the 1970s found construction cost penalties for underground reactor construction ranging from 11 to 60 percent (Myers and Elkins). As a result, the industry lost interest in underground siting. This issue will require considerable analysis to evaluate trade-offs.

And if it proves to be advantageous to safety, it remains to be seen whether reactor owners will be willing to pay for the additional cost of underground siting.

Complications of Multiple Reactors at a Site

SMR proponents frequently claim that, like the next generation of large reactors, the probability of reactor core damage can be lower for SMRs than for currently operating reactors. Although true, it is important to note that such claims refer to frequencies of internal events such as pipe breaks. When external events such as earthquakes, floods, and fires are added to a probabilistic risk assessment, however, the Nuclear Energy Institute (NEI)—the policy organization of the nuclear industry—has pointed out that, “the calculated risk metrics for new reactors are likely to increase and therefore be closer to current plants than being portrayed today” (NEI)………..

SMR proponents also point out that the risk to the public from small reactors is lower than that from large reactors, by virtue of the fact that there is less radioactive material in the core. While that is certainly true, it is not the most useful comparison. The relevant factor with regard to societal risk is not the risk per unit, but the risk per megawatt of electricity generated. By this measure, small reactors do not necessarily imply smaller risks if there are more of them.

To see why, consider the impact on risk if one large unit is replaced with multiple smaller units providing the same total power. If the probability of core damage is comparable for small reactors and large reactors, then the total site risk—the probability of an accident multiplied by its consequence—will also be comparable in both cases (see Figure 1). Indeed, the overall site risk for the multiple SMRs could actually be higher than for a single reactor.

The scenario in Figure 1 assumes the damage probabilities and the consequences for the multiple reactors are independent. But they will not be independent unless the potential for common-mode failures and interactions between the multiple reactors are fully addressed.

In order for individual reactor units to remain independent, the number of support staff and amount of safety equipment would need to increase with the number of units on a site. Only through significant sharing of systems and personnel by multiple units, however, could the associated cost increase be moderated. Thus, the SMR vendors want to reduce the number of control rooms and licensed operators that the NRC would ordinarily require for a certain number of units. For example, the NuScale design could have a single control room operator in charge of as many as 12 units, the feasibility of which would have to be verified through performance testing……….

Distribution of SMRs Some SMR proponents argue that the size and safety of the designs of small modular reactors make them well suited for deployment to remote areas, military bases, and countries in the developing world that have small electric grids, relatively low electric demand, and no nuclear experience or emergency planning infrastructure. Such deployments, however, would raise additional safety, security, and proliferation concerns.

First, building many small reactors at a large number of geographically dispersed sites would put great strains on resources for licensing and for safety and security inspections……

Second, deployment of individual small reactors at widely distributed sites around the world could strain the resources of the International Atomic Energy Agency (IAEA) because inspectors would need to visit more locations per installed megawatt around the world. That strain could degrade the IAEA’s ability to safeguard reactors against their misuse for covert nuclear weapon programs. Maintaining robust oversight over vast networks of SMRs around the world would not only be difficult, but also would require the international community to increase funding significantly for the IAEA—a task that has already been extremely difficult to achieve in recent decades.

Third, it is unrealistic to assume that SMRs—especially in the near term—will be so safe that they can be shipped around the world without the need to ensure the highest levels of competence and integrity of local regulatory authorities, plant operators, emergency planning organizations, and security forces. Indeed, many nations where the DOE hopes to export SMRs may not have the resources to safely operate nuclear power plants………

Regulatory Rollbacks The SMR vendors are vigorously seeking regulatory relief from the NRC that would allow them to meet weaker safety and security standards. Such relief would not necessarily involve actual changes to the NRC’s regulations, but could be achieved through a variety of other mechanisms within the existing regulatory framework.

Security of SMRs

The pressure cooker bombs that exploded at the Boston Marathon on April 15, 2013, were a stark reminder of the ongoing terrorist threat in the United States. Nuclear reactors, like all elements of critical infrastructure, must be prepared to withstand terrorist attacks. Fukushima Daiichi demonstrated how rapidly a nuclear reactor accident can progress to a core meltdown if multiple safety systems are disabled. A well-planned and -executed terrorist attack could cause damage comparable to or even worse than the earthquake and tsunami that initiated the Fukushima crisis, potentially in even less time. For these reasons, the NRC requires nuclear plant owners to implement robust security programs to protect their plants against sabotage.

Despite these concerns, SMR proponents argue for reducing security requirements—in particular, security staffing—to reduce the cost of electricity produced by small modular reactors.

In 2011, Christofer Mowry, president of Babcock & Wilcox mPower, Inc., said, “Whether SMRs get deployed in large numbers or not is going to come down to O&M [operations and maintenance]. And the biggest variable that we can attack directly, the single biggest one, is the security issue” (NRC 2011a). His position was echoed by the NEI, which submitted a position paper to the NRC in July 2012 on the issue of physical security for SMRs (NEI 2012). It clearly laid out the industry view:

The regulatory issue of primary importance related to physical security of SMRs is security staffing. The issue has the potential to adversely affect the viability of SMR development in the U.S. Security staffing directly impacts annual operations and maintenance (O&M) costs and as such constitutes a significant financial burden over the life of the facility.  For this reason, evaluation of security staffing requirements for SMRs has become a key focal point.

The paper goes on to say:

[NRC security] requirements, many of which are based on years of operating experience with large LWR [light-water reactor] facilities, may not be appropriate or necessary for SMRs due to the[ir] simpler, safer and more automated design characteristics .

The NRC requires that nuclear power reactors protect against the design-basis threat (DBT) of radiological sabotage. That requirement mandates that armed response forces be deployed round-the-clock at reactors, charged with the sole responsibility for preventing a group of attackers with paramilitary training and weapons from destroying enough plant equipment to result in damage to the reactor core or spent fuel……..

The nuclear industry’s preoccupation with reducing security staffing is somewhat surprising. Even though security labor costs are significant, they are far from being a dominant contributor to overall O&M costs. Security staffing costs range from 15 to 25 percent of total O&M costs.

Reducing the security force at nuclear reactors would appear to be pennywise but pound-foolish……..

one thing is clear: a well-planned terrorist attack could indeed cause the kind of large-break loss-of-coolant event that the plant’s designers say could not occur in a mere accident. If terrorists were able to access the reactor vessel—a feat more likely with reduced security staffing—they could blow a hole in it in short order, utilizing the explosives that are assumed to be within the design-basis threat……….

The primary feature that mPower and other SMR vendors appear to credit in seeking relief from security regulations is underground siting. Underground siting would enhance protection against some attack scenarios, but not all. A direct jet impact on the reactor containment is less likely for an underground reactor, but the ensuing explosions and fire could cause a crisis. Certain systems, such as steam turbines, condensers, electrical switchyards, and cooling towers, will need to remain aboveground, where they will be vulnerable. Plants will require adequate access and egress for both routine and emergency personnel. Ventilation shafts and portals for equipment access also provide potential means of entry for intruders. In addition, if SMR sites have smaller footprints, as vendors are claiming, the site boundary will be closer to the reactor, and thus there will be less warning time in the event of an intrusion and potentially insufficient spatial separation of redundant and diverse safety systems.

In short, knowledgeable and determined adversaries will likely be able to develop attack scenarios that could circumvent measures such as underground siting. In situations such as hostage scenarios, terrorists may even be able to utilize the additional defense afforded by an underground site against off-site police and emergency response. Thus, a robust and flexible operational security response will be required no matter what intrinsic safeguards are added to reactor design……

Conclusions Unless a number of optimistic assumptions are realized, SMRs are not likely to be a viable solution to the economic and safety problems faced by nuclear power.

Indeed, SMRs are likely to have challenges keeping electricity costs low enough to be economically competitive with other sources, including larger reactors. As a result, concerns about costs and competitiveness may drive companies to make decisions about the design and operation of SMRs that undermine any new, inherent safety features not present in current large reactors. For example, designers may reduce other safety features, such as reducing containment strength or the diversity and redundancy of safety systems. Or the NRC may allow SMR owners to reduce the sizes of emergency planning zones and the numbers of operators and security officers per reactor…….