National Geographic : 1991 Aug
As the temperature of the core increased, the reactor's unique metal fuel-an alloy of uranium, plutonium, and zirconium expanded, letting enough neutrons escape to slow down the chain reaction. Within ten minutes, reactor power had dropped from 100 percent to almost nothing and temperature had returned to nor mal, without operator intervention. The device that performed so well was EBR II, a research breed er reactor in operation since 1964. It had been modified by Argonne scientists to test its inherent shutdown capability. Such safety features as well as simplified designs are to be incorporated into the new generation of reactors. Among the new candidates: AP600: Westinghouse's Advanced Passive 600-megawatt reac tor would be a simplified version of the pressurized water reactor, the type most widely used around the world. Compared with a con ventional plant the same size, designers say, the AP600 would require half as much concrete, 80 percent less electric cable, 60 per cent fewer valves and pipes, 50 percent fewer large pumps and heat exchangers, and 35 percent less ducting. Although its core would be larger than that of a conventional reactor, the AP600 would operate at a lower temperature, making it more forgiving of human errors- "like driving a car at 40 miles per hour instead of 60," says program director Howard J. Bruschi. MHTGR: Designed by General Atomics of San Diego, the Modular High Temperature Gas-Cooled Reactor would circulate helium coolant rather than water through a core composed of bil lions of kernels of uranium, each coated with protective layers of carbon and silicon carbide and bonded in fuel rods. Because these pellets can withstand temperatures higher than any the MHTGR can generate, they could not release radioactive material, even if all the helium leaked from the system. That makes it a true "walk-away" reactor, says Linden Blue, General Atomics' vice chairman. "You can have the worst kind of accident, go home and have lunch, take a nap, and come back and deal with it. Not to worry. No need to panic." PRISM: Further into the future, General Electric's Power Reac tor Innovative Small Module would use liquid sodium, a more efficient carrier of heat than water or helium, to cool its alloy fuel. As a breeder, PRISM would also make more fuel than it consumes. And it might be capable of recycling nuclear wastes as fuel, reduc ing the time that the most long-lived wastes would have to be stored -from hundreds of thousands of years to only hundreds. Other innovative designs are the PIUS (Process Inherent Ulti mate Safety) reactor by ABB Atom, CANDU 3 (Canadian Deu terium Uranium) by AECL, and the SBWR (Simplified Boiling Water Reactor) by General Electric -all of which aim for greater simplicity. "We looked at every component with a view of taking it out," says GE's Daniel R. Wilkins. The safety features of these machines have earned cautious consideration from environmentalists. "If we're going to save the planet, we're going to have to phase out fossil fuels," says Jan Beyea of the Audubon Society. "And though we are optimistic about the technology for solar power, the world is too important to put all our eggs in that basket. So we think it makes sense to put resources into testing these so-called idiot proof reactors." TINY BUT POWERFUL Each one precisely crafted,fuel pellets for the MHTGR dwarfed by a penny-contain a thousand times more energy by weight than coal. Manufactured by General Atomics in San Diego, the pellets start as drop lets of uranium (facing page), which are then solidified and coated with layers of carbon and silicon carbide to give them a protective shell. This permits them to withstand temperatures as high as 36000 F, giving the MHTGR a greater margin of safety than conventional reactors. A Comeback for NuclearPower?