A Path for Nuclear Power
A novel but tested technology, the pebble-bed reactor, can make fission energy safe.
The Pebble-Bed Alternative
Both Three Mile Island and Fukushima suffered melted-down reactor cores because of a loss of coolant. Multiple backup systems guarded against such a loss, but failure occurred anyway. In response, the newest nuclear power plants (called Generation III and III+ plants) incorporate passive safety features, which require no operator intervention or active controls in the event of an operational emergency. But as an engineer I would be more trusting of a nuclear reactor that would use its own nuclear reactions to shut itself down if reactor coolant is lost.
A high-temperature reactor that uses helium instead of water can do just that. Uranium fuel—a mix of uranium-235 and uranium-238—is contained not in rods but in tennis-ball–size spheres called pebbles. The helium heated by the pebbles can be used to generate steam (as in water-cooled reactors) to drive a turbine, or used directly to power a gas turbine to generate electricity.
One such high-temperature, gas cooled reactor was under development in South Africa by Pebble Bed Modular Reactor (Pty), a company with international participation by Westinghouse (from the United States), Mitsubishi (from Japan), and Nukem (from Germany). A series of modular reactor nuclear power plants using this design were to be built in South Africa, but due to the 2008 global financial crises, the program was canceled in 2010. In July 2007 I visited Pebble Bed Modular Reactor’s headquarters in Centurion, near Johannesburg, where they had an engineering and management team of about 700. I also toured their test facility at Pelindaba, the location of South Africa’s state-owned Nuclear Corporation.
Pebble bed reactor nuclear fuel is formed in such a way as to moderate fission, contain pressure buildup, and accommodate fuel deformation. Uranium dioxide nuclear fuel, coated with mass diffusion and radioactive fission product containment layers of pyrolytic carbon and silicon carbide, is formed into poppy seed–sized fuel particles. Some 15,000 of these are embedded in a graphite sphere, which is encased in a thin carbon shell, sintered, annealed, and machined to a uniform diameter of 6 centimeters.
In engineering designs, the cylindrical pebble bed reactor vessel, 27 meters high and 6 meters wide, is packed with about 450,000 pebbles. Due to gravity, the pebbles gradually filter down from the top to the bottom of the packed bed. At the bottom, the pebbles fall into a chute and are moved pneumatically, from helium pressure, into inspection equipment that checks their physical integrity. Pebbles that are intact are returned to the reactor vessel, whereas those that are spent are routed to a holding tank. A typical pebble will make six loops through the reactor during its three years of fuel life. The process allows for refueling with new pebbles while the reactor is operating, limiting shutdown time.
Inside the reactor vessel, helium gas coolant flows around and between the stacked pebbles, emerging at about 1,650 degrees Fahrenheit. Helium is inert, so it does not become radioactive from the exposure. The heated helium drives a gas turbine, connected to a 165 electrical megawatt generator. The helium flow continues through a number of other power-generating processes, then re-enters the pebble bed reactor to be heated again. This closed-cycle process has a predicted 41 percent efficiency— higher than the 30 to 35 percent efficiency of current pressurized-water and boiling-water power plants.
In the event of a complete shutdown of helium flow in a pebble bed reactor, the temperature would rise at most to 2,900 degrees, well below the thermal limit of graphite. At such high temperatures, the uranium-238 nuclei, which are more plentiful than uranium-235, absorb more neutrons (due to an effect called Doppler broadening) and the reactor output decreases, lowering the temperature until equilibrium is reached. The greatly reduced heat is then transferred passively by radiation, conduction, and convection to the steel reactor vessel, which is designed to eject the heat without human intervention.