小型堆：小身材大能量

The next big thing in nuclear energy is small. Designs for dozens of small modular reactors have been developed or are in progress . And a few small reactors are already operating in India, Pakistan, Russia, and elsewhere. A small modular reactor can produce up to 300 megawatts (MW) of electricity (many designs produce much less). But size isn’t the only thing that makes these new, advanced reactor designs stand out.

Small nuclear reactors produce energy in much the same way as larger reactors. A chain reaction splits apart atoms. Usually, they are atoms of a certain isotope of uranium, called U-235 . (Natural uranium is roughly 99 percent U-238, which won’t sustain a nuclear reaction.) Splitting U-235 produces smaller atoms, spare neutrons, and energy . The spare neutrons collide with other uranium atoms to keep the reaction going .

Small nuclear reactors could operate in a fixed location, Or, they could be deployed where needed for emergencies, research and military bases, or other needs. Designs of small reactors fall into four main groups. One category uses water as its coolants. High-temperature gas- cooled reactors make up a secon category. Other categories include fast -neutron reactors and molten salt reactors.

For example, the NuScale Power Module is a water-cooled reactor. The whole reactor vessel sits inside a containment vessel. Fission of uranium in the reactor’s nuclear core heats water flowing through the core in tubes . The hot water rises due to convection. On the way up through the riser, conduction transfers heat to water in separate water tubes in a steam generator. The steam leaves through a steam line and can be used for direct heat or to power an electric generator. Meanwhile, the transfer of heat has cooled the water from the riser. Gravity makes that water then sinks back down other tubes so it can again cool the nuclear core.

The HTR-PM , developed in China, is an example of a high-temperature gas-cooled reactor . The reactor core uses graphite to moderate the fission of uranium. A helium circulator sends in inert helium gas to cool the reactor core. Hot helium gas exits the core and transfers heat to a steam generator. The cooled helium is then sent back to the core.

Whatever the design, small reactors need more concentrated fuel than large reactors. That helps them go much longer between refueling . That means less time spent and fewer chances for something to go wrong in fuel handling.

Small nuclear reactors “can be built to match applications,” says Jess Ge hin . He’s chief scientist for nuclear science and technology at Idaho National Laboratory in the United States. Remote or developing communities, mining operations, and other places really don’t need the gigawatts of power that a large plant might produce.

Small nuclear reactors can provide distributed energy . That means spreading power generation out geographically by using multiple small plants in a variety of locations, rather than having power generation centralized in one spot. “Now that advanced nuclear reactors are smaller, they make sense (for Puerto Rico)”, says Jesus Nunez, chief executive officer for the Nuclear Alternative Project, based in the United States. Currently, Puerto Rico’s electricity supply depends on diesel oil deliveries. And its large power plants tend to be on one part of the island. The island is prone to hurricanes and other storms.

Small nuclear reactors don’t emit greenhouse gases. Many people also hope to move away from fossil fuels, which drive human-caused climate change. But industries on the island are energy-intensive, notes Ramon Martinez, chief nuclear officer for the Nuclear Alternative Project. “To be able to replace those fossil fuel plants, you will need a sustainable energy source,” he says.

Small reactors also offer the possibility of cost savings through standardized production . “More of these components can be fabricated in a factory,” Gehin explains, referring to the parts of a small plant. The reactor vessel and the containment vessel can be made in one place and then sent to a site. There, they can be put into a larger housing built on site.

In the United States, for example, NuScale Power plans to deliver the first of its 60-MW modules in 2027. Each reactor will be almost twenty meters tall and nearly three meters in diameter. The containment vessel will measure approximately twenty-three meters tall and less than five meters in diameter. At that size, the equipment could fit on flatbed rail cars or on ships.

Small nuclear reactors would miss out on economies of scale in operating costs. However, capital costs, plus interest and other financing charges, often make up more than half of the price charged for large power plants’ electricity. Plus, if something goes wrong, parts to fix large reactors often must be specially made.

Most small reactors are also modular . “Rather than building large plants, you stack multiples of these smaller plants together and build up to meet your power demand,” Gehin explains. Electricity demand is growing significantly in developing nations, he notes . “So you want to be able to match that over time. ” Groups of small reactors can also replace large coal plants. In the United States, for example, plans call for twelve NuScale units, producing 720 MW in all, to be brought online at a site in Utah.

“ Our short 36-month construction schedule brings the technology to market sooner, reducing interest during construction and recognizing revenue earlier,” says Diane Hughes. She heads up marketing and communications at NuScale.

Small nuclear reactors are easier to move than their large predecessors. They could operate in a fixed location, or they could be deployed where needed, for emergencies, research, military support, or other needs.

Supporters say small reactors offer more flexibility. Electricity production can be adjusted by changing rod positions for different designs . Some designs can also route turbine steam to a condenser for short periods. Or, one or more modules could go off-line for a while.

“You could deploy them with renewables,” Gehin adds. The reactor would provide a steady supply while renewables produce variable power. Battery storage could help balance supply and demand.

Supporters say small reactors also offer safety advantages . Large reactors generally need active systems to circulate coolants. If backup systems fail, a meltdown can occur . Passive gravity and convection features can do all or much of this work in small nuclear reactors. So, a failure of backup power is less worrisome. Also, there’s more surface area compared to the amount of heat produced. That makes it easier to disburse or divert extra heat.

Protecting a small nuclear reactor from malicious harm or natural disasters should also be easier. “In the NuScale plant, the modules and fuel pool are located below grade in a Seismic Category 1 building that is capable of withstanding a Fukushimatype seismic event and can withstand hurricanes, tornadoes, and floods,” Hughes says. The system’s controls also don’t use types of software that are vulnerable to internet cyber- attacks, she adds.

Small nuclear reactors are still expensive. Small reactors might someday compete well with larger nuclear plants. In various places with competitive markets, however, the levelized cost of natural gas or renewable energy is cheaper. Even adding battery storage to solar power or wind power would likely cost less. And battery power could provide the need for constant energy or smooth out the variability that happens with renewable energy.

Small reactors are late on the scene to deal with climate change. Critics doubt whether small reactors could be deployed quickly enough to make a difference before different environmental tipping points are reached. Renewable energy with battery storage could do the job sooner, they argue.

Small reactors still use radioactive fuel. Handling that fuel requires safety precautions. And any spent fuel must be disposed of. Some facilities might reprocess the waste to get more concentrated uranium. But whatever is left will stay radioactive for thousands of years . Plant sites would also eventually need to be decontaminated.

Safety systems are larg e ly untested. Cooling systems and other design features for small reactors may well seem safe. However, critics say that safety in practice is what really matters. And that’s not yet proven.

Critics also worry because some supporters of small reactors say they don’t need all the regulatory safeguards put in place for large reactors . “If they make enough compromises in safety in other areas, then that product winds up being less safe,” says Ed Lyman, director of nuclear power safety for the Union of Concerned Scientists in the United States.

People and politicians may not want more nuclear fission power . The memory of nuclear disasters at Fukushima, Chernobyl, and other places still makes some political leaders wary. Germany, for example, has been shifting away from nuclear power. And even if a country’s leaders might accept nuclear power, people in local communities may still worry.

Decisions will ultimately be made on a case-by – case basis . In the meantime, scientists and engineers continue work to fine-tune different designs.

“There are going to be great opportunities to work in this field,” Gehin said. “The future looks brighter for nuclear than it has at any time. ”