Nuclear power has traditionally been built in very large chunks. Some experts believe there are advantages to building small, modular reactors.
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Mar 2, 2021
by Michael Abrams
Some energy systems work pretty much the same way. Small gas turbine plants can be purchased as factory-built units essentially ready for installation, wind farms are comprised of many multiples of identical turbines, and solar power facilities are fractal down to the level of the photovoltaic panel. This modularity allows for mass production and lower costs.
Nuclear power plants have been the diametric opposite of plug-and-play ethos. For one thing, they have been large—each reactor producing many hundreds of megawatts or more of power—and that size has meant that each one has been manufactured to order. On top of the cost of those large reactors and their primary systems, there’s the price of redundant systems—backups to backups to backups. What’s more, each new plant has required maybe a decade of planning and complex, individualized assessments.
In the early decades of this century when companies and the U.S. government looked to build a new generation of large, light-water reactors, the rule-of-thumb cost was around $1,500 per kilowatt, or between $1 billion to $2 billion for a standard plant. But once companies tried to build new nuclear plants, they quickly realized that those costs were unachievable. The Tennessee Valley Authority finished the Watts Bar 2 plant in 2016 after halting construction on the substantially completed plant in the 1980s; costs for that second reactor are estimated at around $5 billion, or $4,000 per kilowatt. Building a nuclear plant from scratch would likely cost somewhere between $8 billion and $10 billion.
Costs that high not only make nuclear projects difficult to finance, but also present the possibility that plants could be cancelled after sinking billions of dollars. That’s what happened in South Carolina, when the project to add two 1,117‑MW reactors to the Virgil C. Summer Nuclear Generating Station were abandoned after several years of construction. The total price tag for that project, which will never produce a single watt of electricity: $9 billion.
“Their consumers are now going to pay for some very large holes in the ground,” said Robert Rosner, a theoretical physicist at the University of Chicago and co-founder of the Energy Policy Institute at the university. “The gigawatt-scale plants have this huge problem: There is no guarantee they will ever make their money back.”
The solution, say some nuclear advocates, is to give up on gigawatt-scale, bespoke plants and—following in the footsteps of other energy system—go small and modular. Small modular reactors (SMRs), which would generate tens or a few hundreds of megawatts, would have a lower price tag to be sure. But they could also prove to be safer and take advantage of more advanced technology.
“There’s a whole list of circumstances where a one-gigawatt reactor makes absolutely no sense, no matter what it costs, where a small reactor of the order of a 100 megawatts—plus or minus 50 percent—makes a lot of sense,” said Rosner, who is coauthor of the 2011 report, Small Modular Reactors—Key to Future Nuclear Power Generation in the U.S.
Ironically, however, even concepts that are predicated on being small, modular, and fast to build seem locked into decades-long development cycles.
Products, Not Projects
Small reactors are not new—the Shippingport Atomic Power Station, the first commercial reactor in the world, had an electric output of 60 MW. A Russian-built 70‑MW power plant made up of two 35‑MW reactors installed on a barge is floating in the Arctic Ocean, serving the Siberian town of Pevek. And two HTR-PMs, each of which can output around 100 MW of electricity, are being tested in eastern China.
The key to reviving the nuclear power industry, however, is building these small reactors not as projects, but as factory-made products. That’s easier said than done. “Usually, a bunch of nuclear engineers go in a room and then they come out after a year or two, and they have a design that doesn’t have a lot of foundation in realty, and nobody can make it, and the projects dies,” said Kurt Terrani, a senior staff scientist at Oak Ridge National Laboratory.
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The SMR company closest to exiting the R&D “valley of death” is Oregon-based NuScale. The company has been refining its Power Module, a 60-MW reactor plus steam generator, pressurizer, and control rods, all designed to fit in a 76 ft. by 15 ft. containment vessel, for the past decade. Unlike the site-built gigawatt-scale behemoths, such as unit could be built in a factory and shipped to wherever it might be needed.
Of course, a traditional nuclear power plant is scaled to serve a metropolitan area or even a small state, while 60 MW would barely cover the electricity demand of a small city. That’s where the concept of modularity comes in: a utility could buy as many small reactors as needed and add more down the line to accommodate growth, rather than gambling on huge chunks of power.
In terms of reactor physics, the NuScale concept is fairly bog standard: low-enriched uranium, light-water cooling. In essence, their reactor is just a smaller version of the nuclear plants already in operation. That NuScale didn’t go with a more revolutionary design to mitigate waste or utilize an alternative fuel cycle is no accident. To do so would require the Nuclear Regulatory Commission to come up with an entirely new licensing framework, said José Reyes, cofounder and chief technology officer at NuScale.
“Pressurized water-cooled reactors have benefited from billions of dollars of research and development and millions of hours of operating experience over the past 50 year,” Reyes said. “NuScale went with a more traditional approach to assure a design that is cost-competitive and capable of near-term deployment.”
One area where the Power Module differs from today’s reactors is in safety. NuScale has incorporated some of the lessons of the Fukushima accident—where safety systems failed when backup generators were destroyed—in its design, which does not require pumps and electricity to cool itself in an emergency. Instead, it relies on passive convection to continue drawing heat away from the core.
“Our SMR design safely shuts down and self-cools, indefinitely, with no operator action, no ac or dc power, and no additional water,” said Diane Hughes, NuScale’s vice president of marketing and communications.
The containment vessel will also sit underground in a giant pool capable of absorbing radiation from a leak. Multiple reactors would share the same pool. Being underground, they are also earthquake- and airplane-resistant. The company believes that its design is robust enough that utilities could site the reactors much closer to population centers, rather than in remote locations surrounded by an emergency planning zone.
So far, the concept and design have been convincing enough to win funding from the DoE and to move NuScale farther along in the regulatory process than any of its would-be competitors.
“NuScale’s small modular reactor technology is the world’s first and only to undergo design certification review by the U.S. Nuclear Regulatory Commission,” Hughes said. And the company has its first customer lined up: Utah Associated Municipal Power Systems is on deck to build a 12-module plant at the Idaho National Laboratory. It’s scheduled to go into operation in the mid-2020s.
Opportunity to Innovate
NuScale set out to design a reactor that was small enough to transport to site, essentially complete. Not everyone agrees, however, that building out a power plant in 60-MW modules is optimal.
“The whole idea of SMRs is that smaller is better,” said Jacopo Buongiorno, a professor of nuclear science and engineering at MIT and the director of the Center for Advanced Nuclear Energy Systems. “But within the class of small reactors, larger is still better. If you can design a reactor that is still simple, that is still passively safe, that can still be built in a factory, but that generates 300 megawatts, that for sure is going to be more economically attractive than the same thing that generates 60 megawatts.”
Buongiorno points to GE’s BWRX-300 concept as a potentially better option. It, too, is a light-water reactor with fuel rods and passive cooling. But its larger size makes it a more of a plug-and-play replacement for coal plants.
“In a way, it’s elegant and really beautiful,” said Buongiorno, who is also a consultant for the company. “Out comes the coal plant and in goes an SMR. I can still use the transmission lines, the switch yard, the cooling towers.” And the turbine doesn’t have to be custom made—the reactor uses a standard, off-the-shelf model that GE has sold for decades.
Other companies see the small modular form factor as an opportunity to innovate in other aspects of nuclear reactor design. Holtec International, a parts supply for the nuclear industry, has designed a 160-MW reactor that eschews conventional water cooling for a passive air-cooling system that makes it effective for the most arid environments. The cooling system is designed to remove heat from the core and the spent fuel pool “during normal and all postulated events,” said Tom Marcille, the company’s vice president of reactor technologies.
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Holtec’s SMR-160 is intended to be installed deep underground; the steel containment vessel is strong enough to keep the core covered during any conceivable disaster. “The safety level achieved by the above systems is incomparably better than that for large reactors,” Marcille said. “As an example, the SMR-160’s core is not uncovered under any accident scenario.”
Other SMR designs are dispensing with solid fuel altogether. These reactors would instead dissolve uranium in a molten salt. Some of these designs are miniaturized versions of the Molten Salt Reactor Experiment built by the Oak Ridge National Laboratory in the late 1960s, which pumped the salt mixture through the core and back out to heat exchangers; the salt was both fuel and coolant.
Moltex Energy, headquartered in the United Kingdom, has settled on a two-salt system: a coolant salt circulates to carry heat out of the reactor while the salt containing the fuel is held within long, hollow rods or pins.
The separation enables a simpler design, and the coolant can move rather leisurely through the reactor. A traditional reactor needs a nuclear grade pump with nuclear grade electrical systems to remove heat in an emergency.
“Well, in case that pump fails, you need another back up pump, and in case that fails you need another one, and in case that fails you need another one,” said Rory O’Sullivan, the CEO for North America for Moltex Energy. “They have four different safety trains for each safety system. We don’t need the pump or electricity in the first place. We’re completely eliminating all of those costs.”
The two-salt system offers some other advantages. For one, the hot salt could be stored in insulated tanks rather than sent to a heat exchanger, which would enable a reactor to better fill in when renewable power sources such as wind or solar cut out. For another, the pins could utilize spent fuel, which still retains most of its energy potential. “25 percent of the world’s spent fuel is in the U.S.—that’s like 25 to 40 gigawatts of nuclear power,” O’Sullivan said. “We want that stuff.”
The one downside to molten salt reactors is that the salts usually contain fluoride, which is extremely corrosive. Simplifying the mechanical design of the cooling system cuts down on the parts in danger of corroding, but the pins that will contain the fuel are still at risk. To mitigate corrosion, a sacrificial anode—a zirconium wire—will hang inside the pins; The fuel will corrode the zirconium first before it attacks the cladding. That should enable the pins to remain intact during their expected five-year lifespan.
Make or Break for Nuclear
Moltex is aiming for build costs at around $2,000 per kW—more than wind or solar, but less than newly built coal or gas plants, let alone competing nuclear concepts. “We’ve believe we’ve come up with a concept that can radically reduce the cost of nuclear power,” O’Sullivan said, “and that’s by eliminating the fundamental hazard so you don’t have to put in expensive engineered systems to contain it.”
Other SMR companies are less aggressive with their cost estimates—NuScale has its scopes on a cost of around $3,600 per kW, while GE is aiming for less than $2,500—but still come in under conventional nuclear power. “At 2,500 per kilowatt, boy you can compete—you can compete against natural gas in the US with no carbon tax,” Buongiorno said. “If they can deliver at that cost, they will be very hard to ignore.”
Proof of whether those costs can be achieved will be actual construction and commissioning. “This decade will be very telling,” said Chicago’s Rosner. “It’s the make or break decade for nuclear.”
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Furthest along is NuScale, which in September 2020 announced its SMR design had been issued a standard design approval from the U.S. Nuclear Regulatory Commission. That means the design can be referenced in an application for a construction permit—a big step, and one that had not been before achieved by a small modular reactor design. In August 2020, the NRC had completed its Phase 6 review and issued a Final Safety Evaluation Report (FSER).
The company also announced in November that it had uprated its Power Module to 77 MW, which should improve its economics by around 25 percent.
What’s driving these companies is the perception that nuclear power has a future in a world where zero-carbon energy is prized. And if the designs can jump off the drawing boards and become mass-produced power sources, they have the potential to be transformative: Clean, safe energy available whenever we want it.
The key is getting the cost and scale right.
“I personally think that if the industry comes up with at least one competitive product, the flood gates might open,” Buongiorno said. “It’s the 21st century. We should not be stuck in marriage with these gigawatt scale beasts that we have deployed around the world.”
Michael Abrams is a science and technology writer in Westfield, N.J.