From Science Magazine: “Smaller, safer, cheaper: One company aims to reinvent the nuclear reactor and save a warming planet”

From Science Magazine

Feb. 21, 2019
Adrian Cho

NuScale researchers want to operate 12 small nuclear reactors from a single control room. They built a mock one in Corvallis, Oregon, to show they can do it.

To a world facing the existential threat of global warming, nuclear power would appear to be a lifeline. Advocates say nuclear reactors, compact and able to deliver steady, carbon-free power, are ideal replacements for fossil fuels and a way to slash greenhouse gas emissions. However, in most of the world, the nuclear industry is in retreat. The public continues to distrust it, especially after three reactors melted down in a 2011 accident at the Fukushima Daiichi Nuclear Power Plant in Japan. Nations also continue to dither over what to do with radioactive reactor waste. Most important, with new reactors costing $7 billion or more, the nuclear industry struggles to compete with cheaper forms of energy, such as natural gas. So even as global temperatures break one record after another, just one nuclear reactor has turned on in the United States in the past 20 years. Globally, nuclear power supplies just 11% of electrical power, down from a high of 17.6% in 1996.

Jose Reyes, a nuclear engineer and cofounder of NuScale Power, headquartered in Portland, Oregon, says he and his colleagues can revive nuclear by thinking small. Reyes and NuScale’s 350 employees have designed a small modular reactor (SMR) that would take up 1% of the space of a conventional reactor. Whereas a typical commercial reactor cranks out a gigawatt of power, each NuScale SMR would generate just 60 megawatts. For about $3 billion, NuScale would stack up to 12 SMRs side by side, like beer cans in a six-pack, to form a power plant.

But size alone isn’t a panacea. “If I just scale down a large reactor, I’ll lose, no doubt,” says Reyes, 63, a soft-spoken native of New York City and son of Honduran and Dominican immigrants. To make their reactors safer, NuScale engineers have simplified them, eliminating pumps, valves, and other moving parts while adding safeguards in a design they say would be virtually impervious to meltdown. To make their reactors cheaper, the engineers plan to fabricate them whole in a factory instead of assembling them at a construction site, cutting costs enough to compete with other forms of energy.

Spun out of nearby Oregon State University (OSU) here in 2007, NuScale has spent more than $800 million on its design—$288 million from the Department of Energy (DOE) and the rest mainly from NuScale’s backer, the global engineering and construction firm Fluor.

The design is now working its way through licensing with the Nuclear Regulatory Commission (NRC), and the company has lined up a first customer, a utility association that wants to start construction on a plant in Idaho in 2023.

NuScale is far from alone. With similar projects rising in China and Russia, the company is riding a global wave of interest in SMRs. “SMRs as a class have a potential to change the economics,” says Robert Rosner, a physicist at the University of Chicago in Illinois who co-wrote a 2011 report on them. In the United States, NuScale is the only company seeking to license and build an SMR. Rosner is optimistic about its prospects. “NuScale has really made the case that they’ll be able to pull it off,” Rosner says.

For now, NuScale’s reactors exist mostly as computer models. But in an industrial area north of town here, the company has built a full-size mock-up of the upper portion of a reactor. Festooned with pipes, the 8-meter-tall gray cylinder isn’t exactly small. It resembles the conning tower of a submarine, one that has somehow surfaced through the dusty ground. NuScale built it to see if workers could squeeze inside for inspections, says Ben Heald, a NuScale reactor designer. “It’s a great marketing tool.”

Not everyone thinks NuScale will make the transition from mock-up to reality, however. Dozens of advanced reactor designs have come and gone. And even if NuScale and other startups succeed, the nuclear industry won’t build enough plants quickly enough to matter in the fight against climate change, says Allison Macfarlane, a professor of public policy and geologist at George Washington University in Washington, D.C., who chaired NRC from 2012 through 2014. “Nuclear does not do anything quickly,” she says.

Nuclear power scheme-Twelve-pack of power. C. BICKEL/SCIENCE

A nuclear reactor is a glorified boiler. Within its core hang ranks of fuel rods, usually filled with pellets of uranium oxide. The radioactive uranium atoms spontaneously split, releasing energy and neutrons that go on to split more uranium atoms in a chain reaction called fission. Heat from the chain reaction ultimately boils water to drive steam turbines and generate electricity.

Designs vary, but 85% of the world’s 452 power reactors circulate water through the core to cool it and ferry heat to a steam generator that drives a turbine.

The water plays a second safety role. Power reactors typically use a fuel with a small amount of the fissile isotope uranium-235. The dilute fuel sustains a chain reaction only if the neutrons are slowed to increase the probability that they’ll split other atoms. The cooling water itself serves to slow, or moderate, the neutrons. If that water is lost in an accident, fission fizzles, preventing a runaway chain reaction like the one that blew up a graphite-moderated reactor in 1986 at the Chernobyl Nuclear Power Plant in Ukraine.

Even after the chain reaction dies, however, heat from the radioactive decay of nuclei created by fission can melt the core. That happened at Fukushima when a tsunami swamped the emergency generators needed to pump water through the plant’s reactors.

NuScale’s design would reduce such risks in multiple ways. First, in an accident the small cores would produce far less decay heat. NuScale engineers have also cut out the pumps that drive the cooling water through the core, relying instead on natural convection. That design eliminates moving parts that could fail and cause an accident in the first place, says Eric Young, a NuScale engineer. “If it’s not there, it can’t break,” he says.

NuScale’s new reactor housings offer further protection. A conventional reactor sits within a reinforced concrete containment vessel up to 40 meters in diameter. Each 3-meter-wide NuScale reactor nestles into its own 4.6-meter-wide steel containment vessel, which by virtue of its much smaller diameter can withstand pressures 15 times greater. The vessels sit submerged in a vast pool of water: NuScale’s ultimate line of defense.

For example, in an emergency, operators can cool the core by diverting steam from the turbines to heat exchangers in the pool. During normal operations, the space between the reactor and the containment vessel is kept under vacuum, like a thermos, to insulate the core and allow it to heat up. But if the reactor overheats, relief valves would pop open to release steam and water into the vacuum space, where they would transfer heat to the pool. Such passive features ensure that in just about any conceivable accident, the core would remain intact, Reyes says.

To prove that the reactor will behave as predicted, NuScale engineers have constructed a one-third scale model. A 7-meter tall tangle of pipes, valves, and wires lurks in the corner of a lab at OSU’s department of nuclear engineering. The model aims not to run exactly like the real reactor, Young says, but rather to validate the computer models that NRC will use to evaluate the design’s safety. The model’s core heats water not with nuclear fuel but with 56 electric heaters like those in curling irons, Young says. “It’s like a big percolator,” he says. “We set up a test and watch coffee being made for 3 days.”

Making a reactor smaller has a downside, says M. V. Ramana, a physicist at the University of British Columbia in Vancouver, Canada. A smaller reactor will extract less energy from every ton of fuel, he argues, driving up operating costs. “There’s a reason reactors became larger,” Ramana says. “Inherently, NuScale is giving up the advantages of economies of scale.”

But small size pays off in versatility, Reyes says. One little reactor might power a plant to desalinate seawater or supply heat for an industrial process. A customized NuScale plant might support a developing country’s smaller electrical grid. And in the developed world, where intermittent renewable sources are growing rapidly, a full 12-pack of reactors could provide steady power to make up for the fitful output of windmills and solar panels. By varying the number of reactors producing power, a NuScale plant could “load follow” and fill in the gaps, Reyes says.

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