More than half a century ago, the first commercial nuclear power reactors went critical in the United Kingdom and the United States. In the decades since, technology has brought us 3-billion-transistor chips, manned spaceflight, and violin-playing robots. Nevertheless, the basic design of commercial nuclear power reactors has changed not a whit. They seem to be trapped in a land that technology forgot.
Yes, conservatism can be a good thing, perhaps nowhere more so than in the design of nuclear reactors. Electric utilities aren't known for daring, and you can't reasonably expect them to risk several billion dollars on a reactor without a track record. On the other hand, you can't pin hopes for a nuclear renaissance on designs that were fresh back when color TV and transatlantic jet travel were novelties. You need the promise of something much better, and no fewer than a dozen advanced reactor designs are in the running to offer it.
The backers of these designs are eyeing potentially enormous businesses, as "waking giant" countries China and India pursue major electrification schemes. In the United States and Europe, a significant shift to nuclear is far from assured, but several factors seem to be pushing that option, including climate change concerns and awareness of the hidden costs of fossil fuels.
The new reactor designs fall into three categories. First, there are the new light-water reactors, which aren't radically different from what's out there right now but add better safety features. Then there are the small modular reactors that produce less than 300 megawatts but can be scaled up. Need more power? Just add more modules to your plant. Finally, there are the really-out-there designs, known in the industry as Generation IV.
There are too many worthy, intriguing designs for us to describe here. So, after talking to a dozen nuclear experts, we simply chose seven reactor designs that struck us as the most innovative and interesting. We picked reactors of different kinds and at different development stages, including those that are only a hair's breadth from regulatory approval and others that are literally still on the drawing board.
Did we leave out a new reactor design that you think beats all these here? Will new reactors reenergize the nuclear industry? Leave your comments below.
NEW + IMPROVED
Next-Gen Light-Water Reactors
To understand the new generation of nuclear reactors, you need to start with the basics. Think of a reactor as a tightly grouped array of thin, 4-meter-long, heat-emitting rods stacked in the center like a bunch of rigid metal asparagus. Surrounding those rods is a pressure vessel full of ordinary, or "light," water. Each rod is filled with uranium fuel pellets, which when close to one another emit neutrons and lots of heat. The water in your basic thermal nuclear reactor needs to do three things: get hot; cool the nuclear fuel, which would otherwise overheat and cause a meltdown; and reduce the speed of the neutrons emitted by the nuclear reactions in that fuel. Here's why. When a neutron hits a uranium nucleus, that nucleus then fissions into two smaller nuclei while emitting more neutrons. Then those neutrons hit other uranium nuclei, which fission, emitting neutrons that hit other nuclei, and so on. That's a nuclear chain reaction. It seems counterintuitive, but the neutrons must be slowed—the technical term is "moderated"—to increase the rate at which they split the uranium nuclei they encounter in the fuel rods. Without the water to moderate them (graphite is another commonly used moderator), the neutrons would move too quickly for the uranium nuclei to absorb them, and the nuclear reaction would simply fizzle.
In a light-water reactor, ordinary water accomplishes both cooling and moderation. There are two main types. Pressurized-water reactors, or PWRs, are associated historically with Westinghouse Electric Corp., in which the vessel housing the fuel rods is kept at 160 atmospheres, so the water flowing past the core never turns to steam. The other kind are boiling-water reactors, pioneered for commercial uses by General Electric, which work, as the name suggests, by boiling the water that cools the reactor. Right now PWRs vastly dominate the nuclear landscape. The heat is used to produce steam that drives a turbine, which spins a dynamo to generate electricity. In both types, the steam is always produced by water flashing on extremely hot pipes.
Given their long history, light-water reactors won't be going away anytime soon. One of the leading contenders for the next generation belongs (unsurprisingly) to Westinghouse, in the form of a souped-up PWR known as the AP1000. So far, it's the only new PWR design that's been approved by the U.S. Nuclear Regulatory Commission. (Although other countries have nuclear certification processes of their own, some borrow heavily from the NRC, which is influential internationally.) This new breed of PWR, which also includes a French model called an EPR, is known in the industry as Generation III or III+.
Passive safety features will shut down this reactor without any power, pumps, or people
How it works:
The AP1000 core is similar to standard PWR cores: The fuel produces heat that turns water into steam, which drives a turbine.
The reactor's designers have significantly improved the safety features on an otherwise standard PWR. Whereas conventional reactors rely on motor-powered valves and water pumps to deal with accidents, the amended AP1000 design has safety systems that rely on airflow, pressure changes, and gravity. For example, if a coolant pipe breaks, both the pressure and temperature rise inside the containment vessel . Those changes trigger a water-flooding emergency system [2, 3, 4]. The water inside the sealed containment vessel heats up and turns into steam. The steam rises to the top, where the steel shell has been cooled by air circulating around the vessel. Thus cooled, the steam condenses back into water. This cycle reduces the pressure and temperature, and the nuclear chain reaction ends. Unlike other pressurized-water reactors, the AP1000 needs no safety features beyond the passive ones.
Water—particularly superheated water—corrodes metal, and so the pipes, joints, and other conduits must be periodically checked, maintained, and replaced. According to one estimate, the AP1000 will use as much water per megawatt as a regular PWR.
Westinghouse is building four AP1000s in China. Construction on Sanmen 1, which will be the world's first operating AP1000, began in March of last year and should be completed in 2013. Three U.S. utilities have announced plans to build six AP1000 units, with one scheduled for commercial operation in 2016. However, construction can't start until the NRC grants the reactor its final approval, which the agency says will not happen before mid-2011.
Manufacturer: Westinghouse Electric Co.
HQ: Cranberry Township, Pa.
Type: Pressurized-water reactor
Power: Thermal, 3415 MW; electric, 1117 MW
Fuel: Enriched uranium clad in fuel assemblies similar to those in ordinary PWRs
Refueling: Every 18 to 24 months
Waste: Spent fuel, consisting of leftover uranium 235 and other highly radioactive waste, similar to standard PWR waste.
Europe's Evolutionary Power Reactor will be the world's largest pressurized-water reactor
How it works:
An EPR core is similar to a standard PWR core, but larger.
The reactor is a descendant of the time-tested N4 and Konvoi reactors, the most modern reactors in France and Germany. An EPR's turbines can be maintained while it is in service; its manufacturers claim this will make for very little downtime and a lifetime of 60 years. The Union of Concerned Scientists has referred to the EPR as the only new reactor design under consideration in the United States that "appears to have the potential to be significantly safer and more secure against attack than today's reactors" . The EPR also has the highest-ever efficiency (36 percent) in converting thermal energy into electric compared to other light-water reactors, whose efficiency typically runs at about 33 to 34 percent.
Some analysts have expressed doubts that the EPR is the world's safest reactor. Their main concern is the spent fuel: The reactor's higher burn-up rate makes the waste more radioactive, raising concerns about proliferation.
Four EPRs are now under construction: one each in Finland and France and two 1650-MW units in Taishan, China, which is already planning to build two more. The Finnish plant will be the world's first EPR and the first Generation III+ reactor. A handful of U.S. utilities plan to build at least four EPR plants after the NRC finishes its review.
Type: Pressurized-water reactor
Power: Thermal, 4500 MW; electric, 1650 MW
Fuel: The reactor can use 5 percent enriched uranium oxide clad in fuel rods similar to those of conventional PWRs. It can also use fuel with up to 50 percent mixed uranium plutonium oxide.
Refueling: Every 24 months, at most
Waste: Spent fuel, consisting of leftover uranium 235 and other highly radioactive waste.