The Role of Damage Rate on Cavity Nucleation with Co-Injected Helium in Dual Ion Irradiated T91 Steel

Abstract

The desire for greener solutions to fossil fuels for baseload power generation drive an interest in new nuclear power designs. The development of radiation-tolerant materials for new nuclear power plants requires extensive research, development and deployment programs. Typically, radiation effects studies are conducted using materials test reactors followed by expensive and time-consuming post-irradiation examination because of the resulting neutron induced radioactivity. Additionally, the flux of neutrons and gamma rays in a reactor dictate the radiation damage rate, temperature, and helium generation rate interdependently making the analysis for the underlying mechanisms of cavity nucleation difficult. Ion irradiation experiments allow for the separation of single variable dependencies to uncover the processes and understand the mechanisms underlying cavity nucleation with orders of magnitude higher damage rates compared to reactor irradiations and no induced radioactivity. The objective of this thesis is to understand the roles of temperature, co-injected helium, and high damage rates on the nucleation of cavities in T91 heat 30176 by comparing dual ion irradiation and reactor irradiation. A systematic study using dual ion irradiation was conducted at 17 dpa with variations in temperature from 406°C to 570°C, irradiation damage rates from 5 × 10-5 to 3 × 10-3 dpa/s and helium injection rates from 0 to 4 appm He/dpa to compare with reactor irradiations in the BOR-60 reactor from 376°C to 524°C at 15-19 dpa and 6-9 × 10-7 dpa/s. Transmission electron microscopy was used to characterize the dislocation loop population and cavity size distributions in each irradiation experiment. Bimodal cavity size distributions were observed with helium co-injection rate. Dual ion irradiation exhibited the expected bell-shaped dependence on void density with temperature. With increasing helium co-injection rate, cavity densities increased. With increasing irradiation damage rate, cavity density decreased. The cavity size distributions and dislocation loop size distributions were identical between irradiation in the BOR-60 reactor and dual ion irradiation with a shift in the irradiation temperature and helium injection rate to compensate for the increased damage rate. The cavity growth rate equation with a helium trapping-release process was used to analyze the results. With these results and model, several conclusions were drawn. The role of helium on bubble nucleation is determined by where helium is trapped in the microstructure. At low helium injection rates helium is in vacancy clusters that evolve into bubbles. At high helium injection rates, the implanted helium atoms became bound to dislocation loops in the microstructure as the nucleating bubbles saturated with helium. The dislocation loops arrive at a saturated trap behavior earlier in the damage process in reactor compared to dual ion irradiation. The mechanism of bubble to void transition shifts from being driven by helium accumulation to the critical bubble at low damage rates to being driven by spontaneous nucleation from stochastic vacancy fluctuation at high damage rates. At high temperatures, helium accumulation to a critical radius is the only void nucleation mechanism. An increase in the helium injection rate was needed to emulate reactor irradiation with ion irradiations to offset the decreases in the cavity nucleation rates imposed by the increased temperature and damage rate in ion irradiations. This work provided substantial insight into the complex nucleation of bubbles and voids across a range of temperatures, helium injection rates, and orders of magnitude in damage rate.