Swelling associated with the formation and growth of cavities is among the most damaging of radiation-induced degradation modes for structural materials in advanced nuclear reactor concepts. Ion irradiation has emerged as the only practical option to rapidly assess swelling in candidate materials. For decades, researchers have tried to simulate the harsh environment in a nuclear reactor in the laboratory at an accelerated rate. Here we present the first case in which swelling in a candidate alloy irradiated ~ 2 years in a nuclear reactor was replicated using dual ion irradiation in ~ 1 day with precise control over damage rate, helium injection rate, and temperature and utilize physical models to predict the effects of radiation in reactors. The capability to predict and replicate the complex processes surrounding cavity nucleation and growth across many decades of radiation dose rate highlights the potential of accelerated radiation damage experiments. More importantly, it demonstrates the capability to predict the swelling evolution and the possibility to predict other features of the irradiated microstructure evolution that control material property degradation required to accelerate the development of new, radiation-tolerant materials.

Understanding the void swelling and phase evolution of reactor structural materials at very high damage levels is essential to maintaining safety and longevity of components in Gen IV fast reactors. A combination of ion irradiation and modeling was utilized to understand the microstructure evolution of ferritic-martensitic alloy HT9 at high dpa. Self-ion irradiation experiments were performed on alloy HT9 to determine the co-evolution of voids, dislocations and precipitates up to 650 dpa at 460 degrees C. Modeling of microstructure evolution was conducted using the modified Radiation Induced Microstructure Evolution (RIME) model, which utilizes a mean field rate theory approach with grouped cluster dynamics. Irradiations were performed with 5 MeV raster-scanned Fe2+ ions on samples pre-implanted with 10 atom parts per million He. The swelling, dislocation and precipitate evolution at very high dpa was determined using Analytical Electron Microscopy in Scanning Transmission Electron Microscopy (STEM) mode. Experimental results were then interpreted using the RIME model. A microstructure consisting only of dislocations and voids is insufficient to account for the swelling evolution observed experimentally at high damage levels in a complicated microstructure such as irradiated alloy HT9. G phase was found to have a minimal effect on either void or dislocation evolution. M2X played two roles; a variable biased sink for defects, and as a vehicle for removal of carbon froth solution, thus promoting void growth. When accounting for all microstructure interactions, swelling at high damage levels is a dynamic process that continues to respond to other changes in the microstructure as long as they occur. Published by Elsevier B.V.

Owing to its advantages such as higher dose rates and shorter time required, ion irradiation, including in-situ ion irradiation of nano-foils on electron microscopes, is being proposed as a surrogate for neutron/reactor irradiation in materials testing for the research of irradiation induced/enhanced degradation of reactor structural materials. This raises a key question: can the microstructural evolution be matched across different irradiation conditions? Here we report our recent study using a rate-theory based cluster dynamics approach on the time dependent defect accumulation in elemental molybdenum under varying ion and neutron irradiation conditions, within a low temperature and low dose regime. The approach solves a large system of coupled differential equations each equation describing the rate of change in the concentration of a particular defect/cluster (defined by its nature and number of constituent point defects) caused by direct production (for primary defects), diffusion flux across spatial grids (for mobile defects, under ion irradiation), and capturing and/or emission interactions with other defects/clusters. Our results suggest that defect evolution under different dose rates can be very well matched in either thin foil ion irradiation or bulk neutron irradiation, by using different irradiation temperatures to counteract the effect of the dose rate variation. Across the two types of irradiation, less perfect matching is obtained mainly due to the strong surface sink effect in the thin foil, but the matching can be improved by combining the temperature shift strategy with selection of a single depth in the foil (as opposed to all depths averaged) for matching, or a greater foil thickness.