Computer Studies of Neutron Irradiation and Annealing in Finite BCC Iron Specimens

Abstract

The primary damage state in neutron-irradiated α-iron was computed as a function of specimen size and neutron energy. A Monte Carlo technique was used to compute the concentration and energy spectrum of primary knock-on atoms (PKA) produced by neutrons. In a second set of calculations, the number of displaced atoms produced by a PKA was computed as a function of its energy. These calculations consisted of simulating the entire elastic collision cascade of an energetic atom in the exact crystal structure of α-iron. In addition to providing the displacement function for a PKA as a function of energy, they also gave an atomic level description of the defect distribution in a collision cascade (displacement spike). By combining the results of these two sets of computations it was possible to describe the damage state in an iron specimen as a function of its size and of neutron energy. The results strongly suggest that the damage state produced in small specimens is essentially different than that produced in large specimens, as are their annealing characteristics. The conditions of tension test specimen size and shape which insure that annealing kinetics data obtained from irradiated wires are applicable to these specimens were determined. Calculations on the mechanisms for damage saturation, on annealing behavior, and on saturation and embrittlement in pressure shells as a function of depth, are also described.