SMRs will be small enough to be pre-assembled in a factory and shipped to location. These easy-to-install reactors could potentially shave years and millions of dollars off the construction of nuclear power plants, and could make it economical to bring nuclear power to rural areas or developing countries that lack infrastructure. That?s why SMRs are being hailed as the next generation in nuclear technology.
How It Works
First, don?t let the name fool you. "These are not going to fit in your backyard," says Paul Genoa, a policy director with the Nuclear Energy Institute. "They?ll still be industrial facilities, but the footprint will probably be like that of a small shopping mall, but with more land around it." SMR plants could fit inside the footprint of the old coal-fired plants they?re expected to replace, Genoa says.
An SMR would generate one-tenth to one-third the energy of a conventional reactor. Rather than producing 1000 megawatts of electricity, for example, an SMR might produce 300Mw or less. For example, the company NuScale Power is developing a 45Mw SMR that would be able to supply electricity to 45,000 American homes for a year, making it well suited for smaller towns and cities where a conventional reactor would be overkill. And because SMRs are modular, they?re scalable. The power plant can install additional SMRs as electricity demand grows.
There are three main varieties of SMR in development.
Light-Water SMRs
These are basically a scaled-down version of the light-water reactors already working in the United States. Inside a light-water reactor, heat from the uranium core turns water into steam, which spins turbines that generate electricity. The same thing happens in a light-water SMR, with a few modifications.
Unlike traditional reactors, which position the generators outside the reactor, some SMRs, such as the Babcock & Wilcox 125Mw "mPower" reactor, locate the generators inside the reactor. John Kelly, the energy department?s deputy assistant secretary for nuclear reactor technologies, says this makes manufacturing easier and eliminates the piping between reactors and generator, which is a safety liability. (If a pipe breaks, it becomes difficult to deliver coolant back to the hot core.)
Some light-water SMRs also incorporate what engineers call passive safety features?in an emergency, they could cool a reactor core even if the power goes out. At Fukushima Daiichi in Japan, the site of last year?s post-tsunami nuclear disaster, the plant relied on electrically driven pumps to deliver water to the hot core and cool it down. When the power went out and diesel backups failed, operators had to resort to desperate measures to prevent total catastrophe.
By contrast, small reactors such as the Westinghouse SMR would rely on gravity and thermodynamics to circulate coolants. As the radioactive core heats the water surrounding it, that hot water becomes less dense and flows upward toward the heat exchangers that turn the heat into electricity. As the water loses heat to the exchangers, it cools, becomes more dense, and falls back toward the core?no electricity required.
"The new plans are elegant in their simplicity," Genoa says. "Passive features allow reactors to go without operator interaction, and without pumps to move water around." To further improve on safety, several SMRs are meant to be installed and operated underground.
The light-water SMRs in development have been slightly less efficient than normal reactors, meaning less of the uranium?s potential energy is turned into electricity. But small light-water reactors may eventually deliver electricity that is less expensive than what larger reactors can produce simply because construction and installation costs would be lower. The Nuclear Regulatory Commission expects to approve the first light-water SMR power plants in the early 2020s.
Gas-Cooled SMRs
The idea behind gas-cooled reactors, Genoa says, is to rule out even the possibility of a meltdown. "It is physically impossible for the reactor to get hot enough to damage the fuel," he says. That?s because rather than using water as a coolant, gas-cooled SMRs would use helium.
As water boils it can build up pressure inside a reactor. Under extreme heat it can also react with zirconium alloys in the core. At Fukushima Daiichi, water-zirconium reactions caused a hydrogen explosion that blew the roofs off several reactors.
But unlike water, helium doesn?t boil or react. This allows the gas-cooled reactor to operate safely at temperatures up to 1000 degrees C, which increases the reactor?s efficiency. While a light-water reactor typically extracts roughly 34 percent of its core?s potential energy, a gas-cooled reactor would operate at more than 40 percent efficiency. A gas-cooled reactor developed by the Japanese Atomic Energy Research Institute has achieved 45 percent efficiency, and General Atomics? Modular Helium Reactor achieves up to 47 percent.
To accommodate the high heat needed to achieve such high efficiencies, engineers must modify other elements of the gas-cooled reactor. The fuel requires a heat-tolerant carbon coating, for example, and metal parts of the reactor are replaced with ceramics, Genoa says. Because gas-cooled reactors require these new technologies, the Nuclear Regulatory Council estimates they won?t come on line until the mid-2020s.
Fast Reactors
Normal nuclear reactors use what are called moderators to slow down neutrons and control the chain reactions that happen during fission. That?s because the "fast neutrons" created when uranium splits are less likely to cause fission in the neighborhood?and keep the chain reaction going?than slightly slower neutrons are.
Fast reactors, though, are optimized for fast neutrons, which allows them to extract 60 times more energy from uranium than a typical light-water reactor can. That also means that fast reactors can digest the nuclear waste of other reactors, reducing the waste?s radiotoxicity while extracting energy in the process.
Fast reactors already in development include Argonne National Lab?s 175Mw reactor, Advanced Reactor Concept?s sodium-cooled ARC-100, and the 25Mw Hyperion Power Module. But because uranium is still in abundant supply, and because fast reactors can be used to breed weapons-grade plutonium, these SMRs are not economical (or legal) at this point.
Potential Snags
Before any SMR can be used in a power plant, the Nuclear Regulatory Commission must create regulations for it. Any new reactor design raises a slew of new questions. Since SMRs are smaller and have lots of passive safety features, are fewer operators needed per reactor? Should the 10-mile evacuation radius mandated for traditional reactors be smaller for a smaller reactor? What are the proper safety protocols for an SMR? Once the NRC figures out how to adapt current regulations, it could go certify SMR designs and issue licenses to operate new power plants.
SMRs may be the reactors of the future, but Genoa says traditional reactors aren?t going away anytime soon. "Small reactors are not a substitute for big reactors, but we can?t build a big reactor everywhere," he says. "Just like when you go to the auto store and you can choose a sedan, a minivan or a truck, the nuclear market needs more options."
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