Program Organizers: L. K. Mansur, Metals and Ceramics Division, Oak Ridge National Laboratory, PO Box 2008, Oak Ridge, TN 37831-6376; C. L. Snead, Jr., Applied Technologies Division, Brookhaven National Laboratory, PO Box 5000, Upton, NY 11973-5000
Wednesday, PM Room: Grand H
February 7, 1996 Location: Anaheim Marriott Hotel
Session Chairperson: K. G. Lynn, Physics Department and Department of Applied Science, Brookhaven National Laboratory, PO Box 5000, Upton, NY 11973-5000
2:00 pm Invited
MECHANISMS OF RADIATION-INDUCED DEGRADATION OF VESSEL MATERIALS: L. K. Mansur, K. Farrell, Metals and Ceramics Division, Oak Ridge National Laboratory, PO Box 2008, Oak Ridge, TN 37831-6376
In reactor components, fast neutrons are typically considered the source of various radiation effects caused by atom displacements such as embrittlement, swelling and irradiation creep. However, in principle, other reactions can contribute to atom displacements, such as those caused by thermal neutron capture recoils,-induced electron interactions, and transmutation reaction recoils. In reactor pressure vessels, special circumstances may be encountered where these reactions may become significant with respect to fast neutron induced displacements. General considerations governing relative contributions are described, and recent knowledge gained by evaluating a particular case, the HFIR reactor vessel is reviewed.Research sponsored by the Division of Materials Sciences, U.S. Department of Energy under Contract DE-AC05-840R21400 with Lockheed Martin Energy Systems.
2:30 pm
FLUENCE DEPENDENCE OF TENSILE PROPERTIES OF FERRITIC STEELS IRRADIATED AT 5060[[ring]]C IN A HIGH NEUTRON FLUX: S. T. Mahmood, K. Farrell, Metals and Ceramics Division, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, TN 37831-6376 (Present address: General Electric Vallecitos Nuclear Center, Pleasanton, CA 94566)
Five ferritic steels representing materials used for nuclear reactor pressure vessels, nozzle forgings and vessel support structures were irradiated at 50-60deg.C in a very high flux to fluences in the range 1.5xl020 to 3.5x1023 n/m2 (0.05 dpa ) at nine fluence intervals. Tensile strengths were increased and ductilities reduced in a consistent pattern, with little or no dependence on chemical composition. An exponential expression of the form [[Delta]][[sigma]][[gamma]] = Á(1-e-[[beta]][[theta]]t)n' with n' equal to about 0.7, gives a singular description of the fluence dependence of the change in yield strength. This is shown to be consistent with a new model of radiation strengthening based on clusters of self interstitial atoms. Comparison of these high flux data for A212B steel with literature data show no significant effects of damage rate over a range of flux spanning five orders of magnitude. Research sponsored by the Division of Materials Sciences, U.S. Department of Energy under contract DE-AC05-840R21400 with Lockheed Martin Energy Systems. One of the authors (S. T. Mahmood) was supported by an appointment to the Oak Ridge National Laboratory Postdoctoral Research Program administered by the Oak Ridge Institute for Science and Education.
2:50 pm
METASTABLE REVERSAL OF THE MARTENSITIC PHASE TRANSITION IN Nb3Sn INDUCED ENERGETIC-ELECTRON IRRADIATION: C. L. Snead, Jr., Applied Technologies Division, Brookhaven National Laboratory, P.O. Box 5000, Upton, NY 11973; R. C. Birtcher, M. A. Kirk, Materials Science Division, Argonne National Laboratory, Argonne, IL 60439
Nb3Sn upon cooling undergoes a structural martensitic phase transition from the cubic A-15 structure to a tetragonal one commencing at about Tm - 50 K, with the c/a of the tetragonal structure increasing as the temperature is decreased until the ratio becomes constant at Tc. In conducting electron irradiations between 0.4 and 3.0 MeV and measuring resistivity changes at 20 K in order to determine the damage threshold energy, it was observed that in all irradiations the resistivity initially decreased, contrary to all experience with irradiated metals. For electron energies above the energy determined to be threshold for the production of Frenkel defects(about 0.4 MeV) the resistivity eventually increased with increasing dose as expected. For energies below threshold, however, the decrease in resistivity was seen to approach a limiting value of about 0.4% of the initial resistivity. The resistivity vs. temperature plot shows a deviation of the curve to higher-resistivity values commencing at Tm and increasing the deviation with decreasing temperature as the tetragonality increases. We ascribe the observed decrease in resistivity with electron irradiation as a metastable reversal of the phase toward a state of reduced tetragonality(reversal of the phase transition). This resistivity decrease is completely recovered in annealing to slightly above Tm. High-voltage electron microscopy at 20 K confirms the decrease in tetragonality by a disappearance of the twin boundaries associated with the tetragonal phase. The microstructure responsible for the resistivity changes will be discussed. Work performed under the auspices of the Department of Energy.
3:10 pm
RADIATION DAMAGE PROBED BY POSITRONS: Bent Nielsen, K. G. Lynn, Departments of Applied Science and Physics Department, Brookhaven National Laboratory, PO Box 5000, Upton, NY 11973-5000; O. W. Holland, Solid State Division, Oak Ridge National Laboratory, PO Box 2008, Oak Ridge, TN 37831-6033; A. van Veen, Delft University of Technology, 2629 JB Delft, The Netherlands
The sensitivity of the positron annihilation technique to vacancies and small vacancy clusters, not yet resolvable by the electron microscope, is one of the major potentials of the technique in studies of radiation damage. Defects formed by energetic ions, electrons and neutrons in metals and semiconductors have been studied using positrons. Defect evolution during annealing has been followed (vacancy migration, void nucleation, etc.) The role played by impurities (N and H) has been examined - including the observation of effects such as enhanced defect production and strongly reduced vacancy mobility. Further examples of secondary defect formation are presented and the underlying mechanisms are discussed. Supported by the U.S. Department of Energy, Division of Materials Sciences, Office of Basic Energy Sciences under Contract No. DE-AC02-76CH00016 and Contract No. DE-AC05-84-OR21400.
3:30 pm BREAK
3:50 pm
TECHNOLOGICAL DEVELOMENTS CONNECTED WITH RADIATION EFFECTS RESEARCH ON LIGHT-WATER REACTOR VESSEL STEELS: R. K. Nanstad, Metals and Ceramics Division, Oak Ridge National Laboratory, PO Box 2008, Oak Ridge, TN 37831-6151
The effects of radiation on ferritic pressure vessel steels have been studied
for over four decades and are driven primarily by commercial light-water
reactor applications. Work has focused on two themes: the ability to predict
the effects of radiation on existing reactor vessels and the fabrication of
radiation-resistant steels. The need for integrity analyses of vessels with
radiation-sensitive steels has resulted in advances in the development of
elastic-plastic fracture mechanics, which have found widespread application in
other technologies. The need to understand compositional factors has led to
improvements in microstructural characterization techniques at the atomic
level. Advances in fabrication of heavy-section steel components for miclear
reactors have also found application in other technologies. All these
developments are discussed and descriptions of current research are
presented.
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