Primary Damage State in Neutron-Irradiated Iron

Abstract

An attempt was made to characterize the primary damage state produced in finite bcc iron samples by neutron irradiation at 0°K, in a series of computer studies. Distributed neutron spectra appropriate to graphite‐moderated reactors, water‐moderated reactors, and fission neutrons, together with monoenergetic neutron sources with energies up to 10 MeV, were used. The number of primary knock‐on atoms displaced by neutron collisions and their energy spectrum were computed using the Monte Carlo method. Anisotropic neutron scattering as a function of energy was described using tabulated experimental data. Damage states produced by these primary knock‐on atoms, as a function of their energy, were computed by tracing out elastic atomic collision cascades in the bcc iron lattice. A damage state was described in terms of the displacement spike volume distribution produced, the total number of displaced atoms, and the size distribution of vacancy and interstitial atom clusters in a displacement spike. A description of how the damage state depends upon sample geometry and shape, the incident neutron energy spectrum, and the incident neutron angular distribution is given. It is shown that the damage state produced in samples of different size and shape can be significantly different and the irradiation conditions required to produce equivalent damage states in different sample sizes are defined. The principal content of the article pertains to the primary basis for the change in mechanical properties caused by neutron irradiation, as indicated by current theories.