On the Use of Electron Beams to Characterize the Microstructure of Radioactive Materials

Abstract

When we consider solid materials that have been neutron irradiated, we are confronted with a myriad of microstructural effects. Irradiated materials have very complex microstructures. The characterization of such microstructures is an important part of the nuclear industry, which depends on the utilization of materials as fuels and structural materials in neutron environments. Let us consider some of the microstructural effects of neutron irradiation. First, there is the physical displacement of atoms from their normal positions. This effect leads to an immediate disturbance and relaxation event that results in the formation of small vacancy-rich zones. With some thermal annealing these zones become dislocation loops and tangles. Under conditions of high neutron flux at elevated temperatures most metals can form voids by the precipitation of vacancies. Second, there are chemical effects. Transmutations change the chemical composition resulting in the formation either of solid solutions or of precipitated phases. In a neutron flux the kinetics of phase changes are affected by altered diffusivities and by the creation of metastable phases. Third, we have severe temperature effects. There are such steep temperature gradients that high thermal stresses are imposed. Mass transport occurs by diffusion, effusion and convection along steep temperature gradients. Sudden temperature changes produce thermal shock, and of course, phase changes can occur as the temperature is changed.