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ORNL is doing its part to make nuclear energy for power production safer, less expensive, and more efficient.

Nuclear Energy:
Assuring Future Energy Supplies

Nuclear power produces one-fifth of our nation’s electricity. However, no nuclear power plants have been ordered in the United States since 1978 because of concerns about waste storage, radiation, accidents, and costs. Yet, nuclear power is poised to make a comeback, partly because of concerns about a possible shortage of power plants (suggested by the inadequate power supply in California in early 2001) and the long-term effect of coal-fired power plants on world climate. (Replacing coal plants with nuclear plants will reduce greenhouse gas emissions.) Also, the nuclear industry surpasses the coal power industry in lowering electricity production costs and boosting actual output compared with potential output.

One of 103 nuclear power plants in the U.S.
One of 103 nuclear power plants in the U.S.

Whether the nuclear power industry will experience a resurgence will probably depend on three factors. First, our nation must find an acceptable way to isolate nuclear wastes from the environment, including the growing amount of radioactive spent fuel stored under water at commercial nuclear power plants. The U.S. government now sees this problem as an energy security issue. ORNL researchers have analyzed the environmental impacts of nuclear waste disposal at two proposed sites. Second, the public must be convinced that reactors are essentially safe. Third, nuclear power must become economically competitive with natural gas, the fuel of choice for most new power plants. If new reactors are designed to cost only $1000 for each kilowatt of generating capacity, then nuclear power may enjoy a revival in our nation.

Three American companies—Exelon, Entergy, and Dominion Resources—are seriously considering building new nuclear power plants, ranging from pebble-bed modular reactors (PBMRs) to gas-turbine modular-helium-cooled reactors (GT-MHRs)—both advanced reactor concepts on which ORNL will be working. Also, the licenses of some of the nation’s 103 nuclear power plants are likely to be renewed, allowing them to operate for another 30 years.

ORNL, which pioneered the design of the core, fuel elements, and helium coolant systems for high-temperature gas-cooled reactors (HTGRs), is already playing a role in engineering nuclear energy’s comeback. In February 2002 the Department of Energy designated ORNL and the Idaho National Energy and Environmental Laboratory (INEEL) as the lead laboratories for gas-cooled-reactor technology.

SECURE NUCLEAR PLANT DESIGN

An IRIS power plant could be expanded through the addition of modules.
The diameter of the IRIS containment is 25 meters, compared with 40 meters for today’s pressurized-water-reactor containment. An IRIS power plant could be expanded through the addition of modules.

Recognizing the importance of nuclear power as part of a diverse energy portfolio that will allow us to reduce dependence on imported fuels, DOE’s Office of Nuclear Energy is leading a road-mapping activity involving 150 persons from industry, universities, and national labs. The goal is to draw up and select the best designs for next-generation nuclear power plants that will be built over the next 30 years. DOE has declared that these designs must provide improvements in economic competitiveness, safety, and proliferation resistance, while showing a reduction in radioactive waste production. In February 2002, DOE announced its “Nuclear Power 2010” initiative—a new public-private partnership aimed at building and operating new nuclear power plants in the United States before the end of this decade.

In the quest to design a near-term advanced reactor that is cheaper to build and quicker to license, an international consortium of researchers from universities, industrial firms, utilities, and laboratories in nine nations is developing the International Reactor Innovative and Secure (IRIS) nuclear power plant. ORNL is the only DOE national laboratory participating on this team led by Westinghouse Electric Company.

“IRIS is a pressurized-water reactor with safety features that have eliminated five out of six most extreme possible nuclear accidents,” says Gordon Michaels, manager of ORNL’s Nuclear Technology Program. “Our researchers are developing computational methods and assessing options for the safe and efficient control of the reactor core and for plant surveillance and diagnostics. They are also preparing a probabilistic risk assessment tool to determine the safety of the conceptual design.”

According to Dan Ingersoll, leader of the Radiation Transport and Physics Group in ORNL’s Nuclear Science and Technology Division (NSTD), the IRIS concept meets DOE’s strategic goals in several ways. “It will be more economical because it is more compact,” he says. “Whereas today’s large nuclear power plants generate 1500 megawatts of electricity, a single IRIS module would generate 100 to 300 megawatts. IRIS is small and simple enough that it could get the reactor operation cost down to $1000 per kilowatt and may be built in developing countries.

This schematic of the innovative and secure reactor, or IRIS, design shows the components of the reactor and steam generator in a single vessel.
This schematic of the innovative and secure reactor, or IRIS, design shows the components of the reactor and steam generator in a single vessel. ORNL plans to seek funding to build a first-of-a-kind, power-producing IRIS reactor in East Tennessee in 10 to 12 years.

“IRIS is an integral reactor, which means that the steam generators will be inside the reactor vessel rather than outside, as is the case with conventional light-water reactors. This design avoids the risk of a large pipe break because the large pipes that typically connect the vessel and the steam generators are eliminated entirely. The small pipelines that carry steam from the reactor vessel to the turbine are inside a pressurized containment.

“If a small pipe were to break and release steam and water, the water pressure in the reactor vessel would drop and the air pressure in the containment would rise until the pressures were equal inside and outside the vessel. As a result, the reactor core stays covered with enough water to keep it cool, despite the earlier water loss. Also, the vessel is designed so that natural circulation of water occurs even if the power to the water pumps is lost, making this reactor even safer.”

IRIS will be more resistant to proliferation because potential terrorists would have much less access to the fuel. Today’s reactors are opened up every 18 months to remove and replace fuel. This shuffling of fuel makes it more susceptible to theft. In an IRIS reactor the fuel residence time is 5 to 8 years. Then the fuel will be pulled out and sent to a repository or reprocessing plant. Because IRIS is designed for high burnup of fuel, the spent fuel will contain less usable fuel and thus be less attractive for diversion.

CHEAPER NUCLEAR PLANT CONCEPT

One way to reduce nuclear power plant costs dramatically is to build a reactor that more efficiently converts its heat into electricity. The route to efficient energy conversion is to operate a new reactor at higher temperature than current reactors. Two industrial teams, backing two different reactor concepts, propose to do just that. An international consortium involving British Nuclear Fuels and Eskom plans to build the world’s first pebble-bed modular reactor in Koeberg, South Africa, using technology developed in the 1950s and 1960s by ORNL and German teams. And a U.S. industrial team led by Entergy, a Southeastern utility that operates 10 commercial power reactors, is evaluating the deployment of a gas-turbine modular-helium-cooled reactor.

The fuel elements for the helium-cooled PBMR would be mobile tennis-ball-sized, graphite-coated spheres—or “pebbles”—that slowly move though the reactor. The GT-MHR is powered by stationary fuel elements in the shape of prismatic blocks. Both fuel forms are built up from tens of thousands of coated TRISO particles—particles that employ a dense layer of silicon carbide to trap radioactive fission products. Both reactors would operate at gas temperatures of 900°C with an efficiency in excess of 43% (compared with 31% for today’s pressurized-water reactors); in other words, more than 43% of the energy in the fuel core would be converted to electricity.

Schematic of a
gas-turbine modular helium-cooled reactor.
Schematic of a gas-turbine modular helium-cooled reactor.

“ORNL is the lead laboratory for providing technical support to the Nuclear Regulatory Commission in licensing reviews of the pebble-bed reactor,” Michaels says. ORNL and INEEL jointly manage a new DOE-NRC program for irradiating German pebble fuel in the Advanced Test Reactor at INEEL. The post-irradiation test program, headed by Gary Bell of ORNL’s Fusion Energy Division, will probably involve examination and accident-simulation testing of the fuel.

“The Entergy-led consortium has also expressed interest in ORNL’s research and laboratory capabilities,” Michaels reports. “We are being told that our ability to re-establish the technology for fabrication of gas-reactor fuel may be essential to deployment of a new U.S. GT-MHR.”

NUCLEAR FUEL FABRICATION

In a new fuel fabrication research and development (R&D) program, ORNL is collaborating with General Atomics Company to produce TRISO fuel, a type of fuel similar to what ORNL researchers developed for HTGRs and what is now proposed for GT-MHRs and PBMRs. The work, led by David Williams of NSTD, involves find-ing the best ways to produce uranium-oxide and plutonium-oxide kernels, using an internal gelation process similar to the sol-gel process developed years ago at ORNL, and to coat them to prevent the escape of the particle fuel’s fission products. The coatings deposited on the kernels consist of pyrolytic carbon, silicon carbide, and a porous carbon buffer. The resulting product resembles black beads the size of salt grains.

The goal is to create fuel beads that allow reactors to operate at higher temperatures so that they can more efficiently convert their heat to electricity. The coated fuel should also be meltdown proof—that is, it should act as a miniature containment system that would prevent the release of fission products to the environment during a highly unlikely loss-of-coolant accident.

These micrographs show the structure of the coatings of TRISO-coated particles incorporated into pebbles for the proposed pebble-bed modular reactor.
These micrographs show the structure of the coatings of TRISO-coated particles incorporated into pebbles for the proposed pebble-bed modular reactor. ORNL researchers have developed coatings for nuclear-fuel particles that better contain the radioactive fission products within the particles.

TRISO fuel would also be used in the Advanced High Temperature Reactor, a long-term concept being developed at ORNL under the leadership of NSTD’s Charles Forsberg. Using molten salts as a coolant, this highly efficient, power-producing reactor would deliver heat at a high enough temperature to produce hydrogen from water more cheaply than electrolysis. Hydrogen will be used for automotive and building fuel cells.

ORNL researchers recently helped design and manage a nonproliferation program to test “mixed-oxide” (MOX) reactor fuel fabricated from depleted uranium and plutonium extracted from both U.S. and Russian nuclear weapons. “Now, we are helping the Russians design and build a MOX fuel fabrication plant to provide fuel for Russian fast reactors,” Michaels says. “We are also assisting DOE and industry in preparing MOX fuel for use in four U.S. commercial reactors.”

ADVANCED MATERIALS FOR REACTORS

“In a reactor, neutrons and heat must be controlled,” Michaels explains. “Materials used to construct reactors must survive neutron irradiation and heat and still have the right properties to allow reactors to be run at higher temperatures. You must get heat out of the reactor and take it to where you want it to produce power. Toward that end, we are developing advanced materials for future reactors.”

Modern ferritic and other steels have been developed for use in advanced reactors operating at temperatures of up to about 600°C. If the operating temperature were raised 200°C to increase reactor thermal efficiency, these materials would become deformed and eventually rupture. ORNL and Japanese researchers recently discovered that a high density of oxygen-rich clusters uniformly dispersed in steel can greatly reduce its deformation up to 800°C, making this modified alloy suitable for higher-temperature applications. ORNL and French researchers are conducting further research in this area in projects funded by ORNL’s Laboratory Directed R&D (LDRD) program and DOE’s International Nuclear Energy Research Initiative program. According to ORNL’s David Hoelzer, who leads the LDRD project, modified steels could revolutionize the future development of gas-cooled reactors.

Michaels says the United States needs to both attract more American students into nuclear engineering and improve the country’s nuclear infrastructure by building advanced reactors, reprocessing plants, and uranium enrichment facilities. “Otherwise,” he says, “we risk becoming dependent on foreign nations for nuclear technology. It would not help our nation’s energy security if we become a nuclear know-nothing.” Fortunately, DOE has a program that aims to revive nuclear engineering as a hot field.

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