RADIATION EFFECTS IN MATERIALS: CULTIVATED AT OAK RIDGE
The high state of development of the science and engineering of irradiated materials is due in large part to the contributions of the Laboratory. Three overlapping areas are covered by this research: radiation effects, the development and radiation characterization of materials for nuclear reactors, and the production of new materials with properties that are applicable in a variety of technologies.
In 1946 while at the University of Chicago, Eugene Wigner, first suggested that neutron irradiation could displace atoms, causing changes in materials properties. In short order, colleagues at the Clinton Engineering Laboratories' Metallurgy Division carried out experiments in the Graphite Reactor to confirm this hypothesis, but it was not until the 1950s and 1960s that studies by the Solid State Division staff at the Graphite Reactor, the Bulk Shielding Reactor, and the Oak Ridge Research Reactor revealed the magnitude and pervasiveness of radiation-induced changes.
In 1953, the Metallurgy Division began work on the aircraft propulsion nuclear reactor. After irradiation of the high-temperature structural alloy Inconel 600, ORNL scientists measured extreme reductions in its ability to resist rupturing under stress. It was later shown that helium produced by neutron interactions with the alloy accumulated between the metal crystals, leading to the poor performance. The problem was solved by alloying Inconel 600 with titanium. This element effectively neutralized boron, which was responsible for the helium-producing reactions.
Irradiation experiments on light-water-reactor pressure vessel steels began at ORNL in the mid-1950s. At that time, it was unknown if such steels could exhibit levels of fracture resistance high enough to ensure the integrity of reactor pressure vessels after they had been exposed to neutrons from the fuel core. Later experiments involved the largest specimens ever irradiated (about 50 kg each), and demonstrated that high levels of post-irradiation fracture resistance could be maintained. Extensive experiments have also been carried out to investigate the effects of radiation on embrittled materials.
In the 1970s the Metals and Ceramics Division (formally the Metallurgy Division) received large infusions of funding for research on the physical mechanisms underlying radiation effects and the liquid-metal-cooled fast breeder reactor and the fusion reactor. Burgeoning work in this field was a worldwide phenomenon, and U.S. efforts were paralleled, by similar work in the Great Britain, France, Germany, the Soviet Union, and Japan.
During this period in the Solid State Division, the emphasis was on the fundamentals of calculating damage to materials caused by the displacement of atoms by particle irradiation. Two theoretical contributions to the field of damage production were developed at ORNL. These are the damage production computer code MARLOWE and the Norgett-Torrens-Robinson (NRT) standard method of calculating damage production. MARLOWE is widely used today to obtain detailed information about displacement production for a variety of projectiles and materials. The NRT method is now the standard through the world for obtaining the number of displacements per atom for a given material and type of particle irradiation.
Today, the main efforts in radiation effects on materials are centered in the Metals and Ceramics Division in groups led by Tim Burchell, Lou Mansur, Randy Nanstad, and Arthur Rowcliffe. The emphases in these groups are: radiation effects in carbon materials, physical mechanisms of radiation effects, materials for present-day light-water reactors, gas-cooled reactor pressure vessels, and materials development for fusion reactors, respectively. The Advanced Neutron Source will also benefit from work in these groups.
The effects of neutron irradiation on carbon materials are being investigated on two fronts. Shape changes, creep behavior, and changes in mechanical, fracture, and thermal-physical properties induced by neutron irradiation are being determined for several nuclear graphites. Also carbon-carbon composites, a newer and distinctly different class of carbon material, are of interest as a potential structural material for high-temperature control rods, and other reactor components.
Research into the physical mechanisms of radiation effects has yielded principles for design of radiation-resistant materials as well as prediction of their behavior in fission and fusion reactors. Ion irradiation has also been used to develop new materials and properties that are applicable to a variety of advanced technologies unrelated to fission and fusion. In fact, research into improving the surface, mechanical, and physical properties of polymers received an R&D 100 award from R&D Magazine recognizing it as one of the top 100 technological developments in the nation in 1992.
The division's work on the development of materials for fusion reactor systems has made it a key participant in the design of the proposed International Thermonuclear Experimental Reactor.
The largest single concern of the group dealing with radiation effects centers on the relationship between the effects of irradiation on embrittlement of commercial gas-cooled reactors and the fracture toughness of reactor vessel materials. Researchers are also evaluating the effects of radiation on components being designed for the Advanced Neutron Source.
The main facilities at ORNL for experiments in radiation effects are the High Flux Isotope Reactor (HFIR) for neutron irradiations and the Multiple Ion Facility for charged particle irradiations. The HFIR began operation in 1965 and continues today as a workhorse for the basic, fusion, and pressure vessel programs. The Multiple Ion Facility was built up for materials science research over the past 20 years. It is unique in that it can be used to irradiate materials with up to three ion beams simultaneously. Today it is used heavily for experiments on the basic mechanisms of radiation effects and for related work on ion beam treatment of materials.
Activity in radiation effects has always been broader than the direct needs of nuclear technology. While nuclear materials is a subfield of materials science and engineering, the capability to irradiate materials can be viewed as a new dimension, like temperature. Virtually all processes and properties can be affected by irradiation. Even materials and properties can be created. With this powerful tool, new insights into the fundamental behavior of materials have also been gathered.
Irradiation of materials is at once a source of engineering problems and a basis for unique capabilities for producing and understanding new materials and properties. ORNL's extensive capabilities for radiation effects research will serve well in the design, construction, and operation of the Advanced Neutron Source. This Reactor will make ambitious demands on our understanding of the irradiation behavior of materials. It will also serve as an irradiation test bed for materials that will benefit basic research and the technologies of fission and fusion reactors of the next century.
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