The Politically Correct Nuke: MIT Students Help Design a Nuclear Power Plant That They Hope Will Revive the Industry
Wardell, Charles, Whole Earth
To truly understand the renewed buzz for nuclear, you have to travel to the Massachusetts Institute of Technology in Cambridge. Here, Andrew Kadak, professor of nuclear engineering, holds two billiard-size balls that many believe represent the future of nuclear energy. The balls are the "pebbles" in something called a pebble bed reactor, a new type of plant that proponents say is safer and more efficient than current plants. It could even crank out electricity for less than a gas-fired plant, savings that would presumably be passed on to you. More important, considering our anxiety toward nuclear energy, it's immune to meltdowns. The technology could be implemented, possibly at Three Mile Island, within five years.
When Kadak, formerly vice president of the American Nuclear Society, came to MIT in 1997, nuclear power seemed doomed. So in January 1998, he challenged eleven students to design "a politically correct reactor" that would win acceptance from regulators and the public while giving gas a run for its energy-generating money.
All existing US commercial reactors are "light water" reactors. They're powered by half-inch cylindrical pellets of uranium--like cutoffs from a 1/2-inch dowel--stacked up in 14-foot-long metal rods. Hundreds of rods are lowered into a water-filled reactor core. The uranium atoms give off neutrons, some of which crash into other uranium atoms, splitting them, generating heat, and knocking free more atom-splitting neutrons--the process known as fission. The water in the core carries the heat away to drive an electric turbine.
Kadak's students rejected light-water technology for this reason: If the coolant leaks away, the core heats up enough to melt. Instead, they found something they considered safer: a pebble bed research reactor that had run for twenty-two years in Germany ("until Chernobyl came along and Germany got out of nuclear," Kadak says). It relied on fission too, but was fueled by eight-ball-sized pebbles, and rather than water coolant, it used helium gas.
The main safety feature is the fuel itself. Each pebble consists of roughly 10,000 "microspheres" of uranium dioxide the size of a pencil point. Each is in turn coated with several layers of graphite, and a silicon carbide outer shell. While fission heats the pebbles to as much as 1,100 [degrees] C, the coatings trap all radioactivity inside. Once the fuel is spent, the coatings isolate radioactive decay particles for a million years--four times longer than it takes them to completely decay. Of course, they still need a permanent burial place.
With the pebble bed, a Three Mile Island-type event couldn't happen, Kadak says. Even if the helium coolant completely leaked out of the core, the fuel wouldn't get hotter than 1,600 [degrees] C, well below the 3,000 [degrees] C or so needed to melt uranium dioxide. Plus, the graphite coating is a great heat absorber.
A commercial pebble bed would produce 110 megawatts of electricity--one-tenth that of a large, light-water plant. Its core would consist of a giant, upside-down bottle, 3 1/2 meters in diameter and 8 meters high, filled with more than 400,000 pebbles. Pneumatic tubes would pull pebbles out of the spout at its base. They would be continuously scanned, put back in the top if still usable, and sent to a sealed container if spent. All of this would happen automatically.
Kadak's students weren't alone in their fascination with the pebble bed. A few months into the project, Kadak learned that a South African electric utility called Eskom was doing similar research. Before he knew it, Eskom and Exelon had partnered to create a next-generation nuclear reactor. The MIT team, meanwhile, has since received more than $1 million funding to investigate fuels, reactor core physics, safety, and waste issues. It hopes to build its own research reactor at the Idaho National Engineering & Environmental Laboratory in Idaho Falls. …