Evaluation of neutron energy spectrum effects and RPV thru-wall attenuation based on molecular dynamics cascade simulations

Abstract

Displacement cascade formation in iron has been investigated by the method of molecular dynamics (MD) for cascade energies up to 40 keV. The results of these simulations have been used in the specomp code to obtain effective, energy-dependent cross sections for two measures of primary damage production: (1) the number of surviving point defects expressed as a fraction of the those predicted by the standard secondary displacement model by Norgett et al., 1975 [M.J. Norgett, M.T. Robinson, and I.M. Torrens, Nucl. Eng. Des. 33, (1975) 50–54] and (2) the fraction of the surviving interstitials contained in clusters that formed during the cascade event. The primary knock-on atom (PKA) spectra for iron obtained from the specter code have been used to weight these MD-based damage production cross sections in order to obtain spectrally-averaged values for several locations in commercial fission reactors and test reactors. An evaluation of these results indicates that neutron energy spectrum differences between the various environments do not lead to significant differences between the average primary damage formation parameters. In particular, spectrum-averaged defect production cross sections obtained for PWR and BWR neutron spectra were not significantly different. A representative application of the new defect production cross sections is provided by examining how they vary as a function of depth into the reactor pressure vessel wall. A slight difference was noted between the damage attenuation in a PWR vessel and a BWR vessel. This observation could be explained by a subtle difference in the energy dependence of the neutron spectra. Overall, the simulations indicate that spectrum-averaged defect production cross sections do not vary much among the various environments in light-water moderated fission reactors. As such, the results support the use of atomic displacements per atom (dpa) as a damage correlation parameter and provide guidance for choosing the primary damage source term in kinetic embrittlement models.