June 11, 2005
Few Differences for New Nuclear Plants
WASHINGTON -- The new-generation nuclear reactors being talked about after a pause of three decades are not much different from those of the past, though the designs should make them safer, more efficient and easier to build.
Two designs likely to be pursued adopt a passive safety system requiring less involvement by operators to shut the system down and ensure that the reactor core doesn't overheat. A third design would have more redundant and isolated safety systems than current reactors plus a double-walled concrete containment dome better able to withstand an airplane crash.Still awaiting Nuclear Regulatory Commission approval, all three designs are "evolutionary" advancements from the "light-water" reactors in use in the United States and Europe today. These reactors use ordinary water to slow, or moderate, the fission process as well as for emergency cooling if needed. A Generation IV gas-cooled reactor would be the next step in design advancements, probably after 2030, in the United States.
The three reactor designs attracting the most interest are being developed by Westinghouse, a subsidiary of the British company BNFL; General Electric; and the French conglomerate AREVA, whose Framatome subsidiary designed France's reactors. All three manufacturers say their new designs have been simplified to increase safety and have fewer moving parts, valves and pumps.
Here are some characteristics of each of the top three light-water reactor designs and a next-generation gas-cooled reactor:
The Westinghouse AP1000:
This would have one-third fewer pumps, half as many valves, and more than 80 percent fewer pipes than current reactors. It can be built using modular units manufactured in a factory and transported to the reactor site, cutting construction time to three years.
It relies on a largely passive safety system. The cooling water for use in event of a buildup of excess heat is above the reactor core and uses gravity and natural circulation for emergency cooling if needed. In current reactors, cooling water must be pumped into the core.
General Electric's ESBWR:
This has a 1,500 megawatt boiling water design, meaning the cooling water is not under pressure and is allowed to boil with steam passing over the top of the reactor into the turbines.
ESBWR stands for "Economic Simplified Boiling Water Reactor," reflecting that its design removes many complexities of current reactors. It has 25 percent fewer pumps, valves, motors, piping and cabling and is designed to respond more quickly to a loss of coolant situation. Modular construction and a smaller plant size allow for faster construction.
A 1,500 megawatt pressurized water reactor that's an evolutionary design based on the French and German reactors designed by Framatome and Siemans. It is a simplified design using existing technologies, with fewer parts.
While it maintains an active rather than passive safety system, the EPR has a number of design improvements, including a double-wall concrete containment dome for greater protection against an aircraft crash. The design also extends the dome over the spent fuel pool and two of the four safety buildings.
If there is a severe accident and meltdown, the reactor vessel is designed to capture the core melt in a cavity below the containment building.
Generation IV reactors:
These reactor technologies reflect a "revolutionary" step from the "Generation III" and earlier design light-water reactors. Development for commercial use won't occur until 2030.
They produce more heat and less waste with different cooling mechanisms than the light water reactors, and would be able to produce hydrogen as a replacement for fossil fuels to power everything from cars to electric lamps. An international effort has been under way since 2000 to examine various technologies, using a gas such as carbon dioxide, water, liquid metal or even molten salt for cooling.
A gas-cooled reactor known as the pebble bed is being developed in South Africa and was touted for the U.S. market until Exelon, the Chicago-based utility, pulled out of the project. Instead of fuel rods, the pebble bed uses coated graphite pebbles filled with uranium fuel. The decay heat is transferred to helium, an inert gas, that eventually moves to a gas turbine to produce electricity.
The Energy Department is planning a $1.25 billion program to build a gas-cooled Generation IV experimental reactor in Idaho. It would produce both hydrogen and electricity and could become a prototype for future commercial reactors.