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The method of molecular dynamics (MD) has been used to simulate atomic displacement cascades in iron with energies up to 50 keV. Since this corresponds to the damage energy of the average iron recoil from an elastic collision with a 2.3 MeV neutron, these simulations have reached well into the regime relevant to fast neutron irradiation. The final cascade damage state is that present when the PKA energy has been dissipated and atomic motion returns to a level characteristic of the irradiation temperature; this typically requires a simulation time of 10 to 20 ps. This state has been characterized in terms of several parameters: the total number of surviving point defects, the fraction of the surviving point defects contained in clusters, and the size distribution of the point defect clusters formed during the cooling of the cascade. The energy dependence of the MD primary damage parameters has been compared with values predicted by the industry-standard NRT secondary displacement model and spectrum-averaged values have been obtained for several relevant neutron energy spectra. The effect of neutron energy spectrum on total point defect survival is relatively weak, supporting the use of NRT dpa as a damage correlation parameter. However, a potentially significant spectrum dependence is observed in the point defect cluster size distributions.
displacement cascades, ferritic steels, modeling, molecular dynamics, point defects, pressure vessels, radiation damage
Senior Research Staff, Oak Ridge National Laboratory, Oak Ridge, TN
Lead Scientist, Pacific Northwest National Laboratory, Richland, WA