Software System for Nuclear Safety
Annealing of Reactor Pressure Vessels
Fusion Helium Exhaust
Gender of Trees
New Superconducting Wire
DC Power Transmission
Wall Ratings, Thermal Shorts
ission, fusion, or fossil energy? Biomass energy? Life is full of choices, but when it comes to energy, ORNL recommends development and use of all these energy alternatives. For example, we're supporting national efforts to improve nuclear safety and extend the life of nuclear power plants. We're participating in worldwide efforts to improve the design of the International Thermonuclear Fusion Reactor.
We're also investigating and promoting ways to save energy in the home, office, and factory. Recently, we've created a new high-temperature superconducting wire, a converter that could cut costs of high-voltage direct-current power transmission, a procedure to help users measure R-value in walls, and a computer model that predicts energy losses from wall-floor connections, or thermal shorts.
Encompassing both production and end-use technologies, ORNL's energy research and development (R&D) program is one of the premier enterprises of its kind in the world. Its strong applied focus is underpinned by fundamental investigations in the basic energy sciences and by the integration of many diverse technical skills.
Energy-production R&D is one of ORNL's oldest programs, dating back to the mid-1940s. Today, fission reactor R&D emphasizes nuclear safety work for the Nuclear Regulatory Commission and development of advanced gas-cooled reactors in cooperation with industry. Fusion energy R&D is a major component of DOE's Magnetic Fusion Program and involves collaboration with other research institutions, both nationally and internationally. Biomass energy R&D includes both conversion to end-use fuels and energy crops, with ORNL serving as technical manager for the national program on energy crop development. Fossil energy R&D includes materials research, coal combustion, and bioprocessing.
End-use technologies cover a wide range of applications for buildings, industries, and transportation. An important component of the buildings R&D program, which includes both thermal envelopes and equipment, is the Buildings Technology Center, a user facility for testing elements of buildings and equipment. Our contributions include advanced air conditioning and refrigeration systems and testing of insulation and roof systems. Industrial energy efficiency R&D focuses on advanced materials for heat exchangers and other industrial uses, advanced bioprocessing concepts, industrial gas turbines, and alternative chemical feedstocks. Transportation R&D involves materials, propulsion technologies, alternative fuels, transportation data, and policy analysis.
The High Temperature Materials Laboratory, another user facility, houses several laboratories to support DOE's Office of Transportation Technologies and other DOE materials research programs. A significant part of ORNL work on energy R&D is moving technologies from the laboratory to the commercial sector. As a result, industry is involved in almost every energy technology program. In addition to such major federal clients as DOE, the Department of Transportation, the Environmental Protection Agency, and the Nuclear Regulatory Commission, customers include members of the nuclear power, automotive, biochemical, electric utility, refrigeration, and building industries.
Software System Helps Ensure Nuclear Safety
scientists involved in the fabrication, storage, and transport of nuclear fuel used in reactors must address nuclear safety concerns. Is there sufficient shielding surrounding the spent fuel to protect people from radiation? Will spent fuel storage or transport casks stay within a temperature limit and avoid rupture under internal heating from nuclear fuel or external heating from fires in hypothetical accidents? Is the facility or cask design sufficient to prevent a criticality incident involving an uncontrolled release of energy and radiation?
To analyze various accident scenarios accurately and comply with safety regulations of agencies such as the U.S. Nuclear Regulatory Commission (NRC) and the International Atomic Energy Agency, people throughout the world use ORNL's popular Standardized Computer Analysis for Licensing Evaluation (SCALE) code system. Over the past decade SCALE has been used to analyze the disassembly and shipment of the damaged Three Mile Island reactor and the transport of highly enriched uranium from Kazakhstan to the Oak Ridge Y-12 Plant storage facility for Project Sapphire.
This geometric model of a transport cask loaded with assemblies of spent nuclear reactor fuel is a visualization of the SCALE code system.
In 1995, to improve users' abilities to evaluate safety concerns, SCALE Version 4.3 was completed, tested, and released for public distribution. This modular, versatile version of SCALE offers automated data processing and computer modeling to predict the composition of spent fuel from a reactor and determine the distribution of radiation and heat in a cask. Among its improvements are visualization capabilities (color graphics of model geometries) and simple procedures for installation on a variety of workstations and personal computers. We have trained people throughout the United States, Europe, and Asia in how to use SCALE, and it has been important in ORNL collaborations with Japan, Germany, Italy, Thailand, the Netherlands, the United Kingdom, and the Ukraine.
When nuclear fuel is burned in a reactor, its composition changes, reducing its ability to achieve a critical configuration. Traditionally, criticality safety assessments have not taken credit for these reductions. If "burnup credit" can be accurately considered in safety analyses, more spent fuel could be safely loaded into storage and transport casks. ORNL and Sandia National Laboratories provided technical underpinnings for such a recommendation, which was submitted by DOE to the NRC in June 1995. SCALE has been used to weigh in on physics and analysis issues linked to burnup credit. If burnup credit is allowed, far fewer shipments would be needed to move spent fuel from reactors to a final disposal site, reducing the risk of a transportation accident and costs of spent fuel disposition.
The development and maintenance of SCALE is sponsored by the NRC and DOE's Office of Environment, Safety and Health. The work to investigate technical issues related to burnup credit is sponsored by DOE's Office of Civilian Radioactive Waste Management.
ORNL Assessing Annealing of Reactor Pressure Vessels
Heat could be a key to the future of the nuclear power industry. Nuclear energy, of course, provides heat that is converted to electrical power. But uncontrolled heat generation could lead to costly power plant shutdowns. Controlled use of heat could extend the life of reactor pressure vessels.
Here's the problem. Aging nuclear-reactor pressure vessels, especially those containing nickel alloys and copper impurities, may become embrittled by neutrons from the fissioning fuel core. If the hot vessel walls ever become too brittle and are chilled by cold water injected to replace the accidentally lost pressurized coolant (pressurized thermal shock), cracks could develop because of the vessel's reduced fracture toughness. The water that cools the fuel could then leak out, causing the fuel to overheat and melt portions of the vessel wall, releasing radioactivity to the containment building.
Here's a possible solutionthermal annealing. In Russia, embrittled vessels have been emptied of water and heated electrically to about 454°C (850°F) for about a week. This thermal annealing process has restored ductility to the vessels. The theory is that vessel walls will remain ductile if atoms dislocated by neutron irradiation are free to move around. But if clusters of displaced atoms and resulting voids (point defects) and copper-rich precipitates block the motion of dislocations, the material becomes brittle. In thermal annealing, the heat tends to dissipate the defect clusters and coarsen the copper-rich precipitates, greatly decreasing their number density. As obstacles to dislocation motion fall, fracture toughness and ductility are recovered in the vessel wall.
American reactor pressure vessels differ from Russian ones in that their welds run in both longitudinal and circumferential directions (a typical Russian vessel has only a circumferential weld). As a result, thermal annealing could introduce potentially damaging temperature gradients in the vessel's nozzle region where water enters and leaves. If the vessel expands in this region near a weld, the stresses might damage pipes and adjacent components.
To determine whether thermal annealing is safe and effective for U.S. reactor vessels, DOE's Light Water Reactor Technology Center at Sandia National Laboratories is demonstrating annealing on unirradiated pressure vessels at the abandoned Marble Hill and Midland-2 nuclear power plants near Louisville and Detroit respectively. The NRC has asked ORNL to help it evaluate these demonstrations and the engineering feasibility of annealing U.S. reactor pressure vessels.
ORNL researchers are conducting thermal and stress analyses of the annealing process to assess the adequacy of the annealing vendor's instrumentation plan. The vendor's instruments should (1) verify that the required annealing temperature has been achieved and maintained for the required time and (2) provide data that can be used to confirm that components and structures on or near the vessel were not damaged during annealing.
To maintain the ability to cool the fuel core of a nuclear reactor, integrity of the reactor pressure vessel must be preserved. To prevent fracture of the vessel, irradiation damage levels must be limited in the belt-line region below the inlet and outlet nozzles. As the vessel ages, it may be necessary to alleviate radiation damage in this region by thermal annealing.
The results from this project are expected to assist the NRC in evaluating the adequacy of the operating plan called for in its thermal annealing rule and regulatory guide. The plan requires an evaluation and demonstration that the reactor's operability will not be detrimentally affected by thermal annealing. It is hoped that when the project is completed in June 1997, the evaluation will shed more light than heat on the issue of whether annealing is appropriate for American reactor pressure vessels.
The research is sponsored by the Nuclear Regulatory Commission.
What To Do With Weapons Plutonium
As nuclear warheads are dismantled in accordance with international agreements, large plutonium stockpiles are created. Also created is a problem: How can these bomb-grade nuclear materials be kept away from terrorists?
To consider ways to reduce the proliferation hazard of plutonium inventories in the United States and Russia, on December 4-5, 1995, ORNL served as the host site for a Joint U.S.Russia Technical Summit Meeting on Plutonium Disposition. During the meeting, U.S. and Russian representatives exchanged important technical information concerning proposed options, such as encapsulating plutonium in glass, burying it in 4-kilometer-deep bore holes, or burning it in commercial power-producing nuclear reactors.
ORNL was picked to host the first meeting of this type in the United States because of its U.S. leadership role for evaluation of reactor alternatives in the plutonium disposition program. ORNL researchers have examined the feasibility of extracting plutonium from dismantled weapons, converting it to a mixed oxide fuel (plutonium oxide mixed with depleted uranium fuel), and burning it in Canadian heavy-water reactors and U.S. light-water reactors. The Russians are studying the possibility of burning plutonium fuel in their light-water reactors.
The meeting was completed successfully with the development of a detailed outline of the report to be produced by June 1996 to support the summit meeting between Presidents Clinton and Yeltsin. ORNL researcher Kent Williams was appointed U.S. co-chairman of a newly created Cross-cutting Cost Analysis Team, which will study the economics of the various plutonium disposition options. ORNL has been charged with working with the Russians to develop cost analysis methodology for Russian projects. This collaboration should create workable solutions that will safeguard the world's dangerous weapons materials and may extend its energy supplies.
This work was sponsored by DOE, Office of Fissile Materials Disposition.
Guiding Studies of Fusion Reactor Helium Exhaust
Special people from around the world are needed to harness the energy source of the sunnuclear fusion. International cooperation is the key to developing nuclear fusion reactors as a safe, abundant supply of energy. To design and build a fusion reactor that can produce electric power from fusion energy, nations are pooling their resources. Collaborative research in the international fusion community has led to the launching of the International Thermonuclear Experimental Reactor (ITER) Project, perhaps the largest international scientific venture ever undertaken.
|ORNL studies at the DIII-D Tokamak in San Diego show that more of the energy-wasting helium "ash" formed as a by-product of fusion reactions is exhausted if divertor pumps are "on" (red) rather than "off" (black).|
For the past decade, ORNL's fusion researchers have been intensely involved in multinational research efforts. In 1995, one of our contributions was recognized by ITER's Confinement and Transport Expert Group: Some of our researchers were invited to the group's meeting to report on the status of worldwide research on helium transport and exhaust in fusion reactors.
In nuclear fusion, two nuclei of heavy hydrogen (deuterium, which abounds in the ocean, and tritium) will fuse if held closely together and heated to a high enough temperature to overcome their natural repulsion. The products of this fusion reaction are neutrons, considerable energy, and helium "ash." If helium is not removed from the burning plasma, this impurity will gradually build up, quenching self-sustaining fusion reactions.
Our helium transport and exhaust group has been designing and performing experiments, collecting and analyzing data, and developing and validating research models. A goal of this work will be to develop a helium transport database that will guide design work on the ITER. It will provide information on results of experiments at small fusion devices, which can be extrapolated to the future ITER, a much larger reactor designed to show the technological feasibility of producing electricity from fusion energy.
It has been shown that magnetic fields can guide helium ions away from the plasma boundary into a diverter chamber from which the ions are exhausted. Based on data obtained from present-day experiments, researchers seek to optimize divertor configurations that more effectively remove helium. By solving the problem of helium exhaust, we will move closer to harnessing a special energy source that is virtually inexhaustible.
The research is sponsored by DOE, Office of Energy Research, Office of Fusion Energy Science.
Identifying Gender of Fast-growing Trees
Trees that grow up fast are in high demand. They can be used for fuel in transportation vehicles and for wood products. While they are growing, they can capture carbon dioxide from the atmosphere, helping to put the brakes on climate change.
Fast-growing trees are being cloned and nurtured for these purposes. One promising species is the hybrid willow tree. However, if the climate change predicted by some scientists ushers in higher temperatures and drought, hybrid willow trees of only one gender will be favored: male willow clones are generally more tolerant of dry conditions than are female willows.
It's not easy to tell a male willow from a female willow because trees of this species do not express gender until age 6 to 20 years. However, ORNL researchers in collaboration with Swedish scientists through the International Energy Agency have identified a potentially useful method for early identification of hybrid willow gendera DNA marker.
Nicholas McLetchie employs genetic engineering techniques like those used to determine the gender of a hybrid willow tree in an early stage of development.
The marker is present in all female hybrid willows and absent in all males. The marker will ultimately be used to isolate and characterize the DNA sequence responsible for gender selection. Use of this DNA sequence should greatly increase researchers' ability to identify highly productive, drought-resistant trees for biofuels. What was once a willow problem is no longer worth weeping over.
The research was supported by DOE, Office of Transportation Technologies, Biofuels Systems Division.
Superconducting Wire Developed
By learning how to lay a proper foundation, ORNL researchers have formed a promising new high-temperature superconducting wire. Combining a specially textured substrate, buffer layers that maintain the texture, and deposited films, the new wire can carry 710 times more current per unit area than conventional wirewithout energy-wasting resistance. This development may pave the way for manufacturing practical, energy-saving wires for underground transmission cables, transformers, current limiters, large electric-power generators, and large motors used in paper and steel mills.
Twelve researchers from three ORNL divisions have produced a roll-textured, buffered metal, superconducting tape with a critical current density of 710,000 amperes per square centimeter in liquid nitrogen. The higher the current density, the greater the amount of electric current that can be transmitted through the wire. Standard household wires typically carry less than 1000 amperes per square centimeter.
High-temperature superconducting material chilled by liquid nitrogen (which costs 2% the price of liquid helium, the coolant for low-temperature superconductors) offers virtually no resistance to the flow of electric current. New electric devices wired with superconductors could take up less space, use less energy, and cost less. Although demand for electricity is expected to double by the year 2030, these devices could help reduce U.S. requirements for new power plants.
To make superconducting wire, the underlying substrate for deposited superconducting film must be flexible. It must be a perfect template with correctly oriented crystalline grains that align film grains to make a path for electric current. It must be reliably reproduced to form long lengths of wire.
At ORNL nickel-metal tapes are prepared using special rolling and heat treatment procedures. Thin buffer layers of palladium, cerium oxide, and yttria-stabilized zirconia are then placed on the nickel tapes by vapor deposition processes, such as electron-beam evaporation. The high-temperature superconductor yttrium-barium-copper oxide (YBCO) is then deposited on the conditioned surface by pulsed-laser deposition, a technique in which target materials in a vacuum chamber are vaporized by laser light so that they deposit on a template. Because nickel and YBCO are incompatible (nickel and copper atoms tend to trade places), the buffer provides a chemical barrier between the nickel and the superconductor while maintaining the texture.
Fred List (left) and Patrick Martin observe sputtering of cerium oxide on textured nickel, the substrate for high-temperature wire capable of carrying large amounts of current. Photograph by Tom Cerniglio
One problem with high-temperature superconducting material has been that it tends to lose its superconductivity in an applied magnetic field. We have shown that the performance of our YBCO wire in a background magnetic field at liquid nitrogen temperature (77 K) is excellent, exceeding that of bismuth-based, powder-in-tube wires. This excellent performance is essential for applications in transformers, motors, and generators, where magnetic fields are present. In a 1-tesla field at 77 K, the critical current density of the YBCO wire is 150,000 amperes per square centimeter, and high critical currents are maintained in fields of up to 5 tesla. In addition, the critical current density at 64 K in a magnetic field of 1 tesla is as good as that of the metallic superconductors at 4.2 K, the boiling point of liquid helium.
High-current short wires 3 millimeters wide and 15 millimeters long have been produced using the ORNL process called rolling-assisted biaxial textured substrates, or RABiTSTM. ORNL has applied for patents on RABiTSTM in the United States and certain foreign countries. A nonexclusive licensing agreement has been signed with Midwest Superconductivity of Lawrence, Kansas, for use of the technology in research and development. The agreement includes an option for rights to commercialize superconducting wire and tape. In a planned cooperative research and development agreement, Midwest has entered into a partnership with Westinghouse Science and Technology Center and Southwire Company for the development phase of the project with Oak Ridge.
Now that we have a super substrate, we may soon see practical use of superconductivity.
Funding for the project was provided jointly by DOE, Office of Energy Efficiency and Renewable Energy, Office of Utility Technologies and by DOE, Office of Energy Research, Division of Materials Sciences.
Reducing Costs of DC Power Transmission
One way to be sure to get power to the people when electricity is in high demand is to connect utility networks. An electrical utility in need can then draw power from another utility's network or even from small private generators of electricity. Transmitting high-voltage, direct-current (HVDC) power between networks requires back-to-back converter systems to overcome tiny inherent differences in generating frequency and phase between networks.
Back-to-back converters, which change alternating current (ac) to direct current (dc) and back again, are expensive components in HVDC transmission stations and switchyards. To reduce the cost of HVDC converter stations, researchers at ORNL have completed developing and testing the world's first multilevel back-to-back converter system for HVDC transmission.
View of the back-to-back converter developed at ORNL to change alternating current to direct current and back again.
The multilevel converter synthesizes sinusoidal voltagesine waves that resemble staircase steps on a computer screenusing multiple levels of voltage from different capacitors. The invention eliminates three out of four major HVDC converter station components found in traditional transformer-coupled stations. No longer needed are bulky and expensive transformers, large ac and dc filters, and switch-operated capacitors that correct power factors (the ratio of power to the product of voltage and current). As a result, engineering and installation costs are significantly reduced.
The converter developed at ORNL offers nearly distortion-free output voltage. It may also be used in other high-voltage applications such as unified power flow controllers (which enable power systems to be operated closer to peak capacity), industrial motor drives, electric traction drives for high-speed rail transportation (including that propelled by power-hungry magnetic levitation), and reactive power compensators that stabilize power system voltage.
Deployment of multilevel converter-based systems at HVDC stations throughout the nation will enable the use of two instead of three transmission cables for ac systems, reducing power transmission and distribution costs and real estate costs by at least 33%. In some designs, the earth is used as the return current path, so the savings can be as high as 66%. In addition, dc transmission does not produce electromagnetic fields, which have been a major health concern of the public. It's good to get power to the people, and it's even better to minimize the impact on their pocketbooks.
Funding for this development was provided by DOE Office of Energy Management to the Power Systems Technology Program in the Energy Division.
New Wall Ratings and Thermal Shorts
What's the R-value in your walls? The answer used to be the R-value of your wall insulation in wood-frame walls. But now R-value, or resistance to heat flow, can be measured in the whole wall, not just the insulation, using ORNL's new wall testing and rating procedure. We are performing whole-wall testing and rating for users at the Buildings Technology Center, a DOE user facility at ORNL. We have already worked with 7 firms and are currently working with another 4; about 36 firms expressed some interest. Users pay $3000 to $15,000 each for testing in our new Rotatable Guarded Hot Box and for computer modeling. Total wall R-value ratings allow comparisons of thermal resistances of dissimilar walls. This capability is important because of the growing use of alternative wall systems and construction materials such as metal. We have put the performance information in a database (http:www.cad.ornl.gov/kch/demo.html) so home designers, builders, realtors, and buyers can predict ratings for new wall systems and materials. Several state energy agencies are considering adopting our procedure.
The whole-wall R-value is obtained by comparing the hot box measurement of the clear wall with results from the HEATING computer model, which simulates heat leakages at corners, doors, windows, and studs. This figure shows details for a standard 2 ¥ 4 inch wood frame wall and a metal frame wall, which is gaining popularity in residential construction.
At the junction of a conventional concrete floor and masonry wall in a typical building, heat flows at a high rate, causing energy losses and occasional moisture problems. How bad is this "thermal short"? We have made the first known measurement of a thermal short in the Rotatable Guarded Hot Box. The energy losses and potential for excessive moisture condensation are worse than were expected. Based on the heat-loss measurements, a recommended simplified computer model that predicts energy losses from wall-floor connections has been developed and was presented to the American Society of Heating, Refrigeration, and Air-Conditioning Engineers in June 1995. The work is part of a cooperative research and development agreement with Enermodal. The information could lead to improvements in construction of floors and walls to make better connections and help seal the building envelope.
Funding for the research is provided by DOE, Office of Building Technologies.