The image of nuclear power is not going to be helped by the recent disaster in Japan. But that hasn't stopped people from thinking about new ways to build nuclear power plants either to make them safer, more efficient, or less prone to spreading weapons technology.
What follows is a look at a few designs for nuclear plants ranging from the drawing board to some that are actually being built now.
The image of nuclear power is not going to be helped by the recent disater in Japan. But that hasn't stopped people from thinking about new ways to build nuclear power plants either to make them safer, more efficient, or less prone to spreading weapons technology.What follows is a look at a few designs for nuclear plants ranging from the drawing board to some that are actually being built now.REUTERSThe traveling wave reactor is a design first proposed in the 1950s. The idea is to have a reactor that generates less dangerous waste and breed its own fuel. This would reduce the amount of uranium that needs to be mined, as well as allow for reusing spent fuel because the “burnup” – the amount of fuel used up – is higher than in conventional designs.The fuel is spent nuclear fuel, which is mostly (about 96 percent) uranium-238. Uranium-238 won’t sustain a nuclear chain reaction, so it needs to be helped along with a block of uranium-235, plutonium-239, or even thorium. A small piece of one of those materials can start the process, which in a cylinder of nuclear fuel 60 centimeters long can last decades.Conventional nuclear reactors have to have the spent fuel periodically taken out, as they can't function by simply burning through the entire fuel supply at once. A traveling wave reactor only needs to have its fuel replaced once, when the fuel in the core is used up.Such reactors could be cooled in a variety of ways. One proposal is to use liquid sodium, which melts at the same temperature water boils at, but while it would allow the reactor to run at higher temperatures, sodium burns and can explode when exposed to air or water.TerraPower, a company backed by former Microsoft exec Nathan Myhrvold, is currently exploring the design, but there are still regulatory and technical hurdles.
TerraPowerThe supercritical water reactor (SWR) envisions using water as a coolant at even higher pressures than are common in conventional reactors today. Current reactors use water at 150 atmospheres in pressurized water designs; an SWR uses pressures of 250 or more. Pressurizing water raises the temperature at which water boils, and at a certain point the steam and liquid water are the same density. That eliminates the need for any kind of steam generator.One issue in conventional reactors is that if the water boils, it creates voids in the reactor core, which are basically areas where the water makes bubbles of steam. This can affect how well the water moderates the reactions in the core, and makes reactor behavior harder to predict. SWRs avoid this problem because the water isn’t boiling in the first place.Supercritical water is a much better carrier of heat than ordinary water is, so the reactors would be more efficient, allowing for smaller cores and less nuclear fuel.There are still some technical challenges to deal with. The fuel rods in an SWR would have to be clad in something other than the zirconium alloys that are currently used in the industry. The combination of very hot water (nearly 400 degrees Celsius, or 750 degrees Fahrenheit) means that some other materials would have to be used. DOEWestinghouse is producing a reactor called the AP1000, which in many respects is a typical pressurized water design. But the AP 1000 makes an attempt to simplify the design and provide safety systems that don’t require as much human intervention.Part of the design is cooling the reactor using air flow around the containment vessel. The idea is to allow the warm air to rise, just as it does in a chimney, which cools the reactor vessel. On top of the reactor containment vessel is a pool of water, which in the event of an accident would drain onto the containment vessel, cooling it off. There is enough water, Westinghouse says, for three days.In addition, the containment vessel can be examined from the outside, so any corrosion is apparent. In this way, cooling the reactor requires less input from the operators.Thus far the AP1000 is being built in the U.S., as it is the type of plant Westinghouse plans to build in Georgia. The company is already building a similar plant in China.
Westinghouse
