Radiation damage production and accumulation in solids can be divided into two stages. In the production stage, the impinging particle gradually gives off its kinetic energy to lattice atoms in the form of energetic recoils. These deposit their energy by generating secondary and higher order recoils that result in a displacement collision cascade. The outcome of this stage, of the time scale of a few to 100 picoseconds, is a population of point or clustered defects known as the primary state of damage. In the second stage, which can extend over seconds, defects that survive recombination within their nascent cascade migrate over long distances, interacting with the microstructure. These freely migrating defects (FMD) are responsible for the changes in the macroscopic properties of metals under irradiation, such as void swelling, embrittlement, radiation enhanced diffusion, etc. Such changes in mechanical properties are most often detrimental and severely limit the flexibility in materials choice and operating temperature when designing a fusion power plant. Under most conditions, such as those that would be present in a magnetic fusion energy plant, or when bombarding with fission or spallation neutrons, irradiation takes place at a certain dose rate and temperature, but in a continuous manner. However, in an Inertial Fusion Energy (IFE) reactor, or when using a pulsed neutron source such as that recently proposed by Perkins [1], the irradiation flux is pulsed and the interplay between temperature, flux and pulse frequency controls the kinetics of damage accumulation. For sufficiently low pulse frequency, and at elevated temperature where the defects migrate fast, it may be expected that annealing between pulses may result in a significantly decreased rate of damage accumulation compared to that seen under steady state conditions. On the other hand, very high neutron fluxes in the pulse itself may severely limit recombination therefore leading to extremely fast rates of damage accumulation even at elevated temperatures.
