Understanding Neutron Atypical Features in Ion Irradiation Testing of Nuclear Materials

Abstract

Testing nuclear materials performance under extreme radiation conditions is required in materials screening and development for advanced reactors. However, the current testing reactors around the world cannot satisfy such needs due to their low efficiency in damage introduction. Ion accelerators have been widely used for the past decades to emulate reactor neutron damages, with efficiency in damage creation at least three orders of magnitudes higher than a testing reactor. However, such accelerated testing brings complexity, including the surface effect and the injected interstitials effect. We propose the methods to quantitively determine the region affected by the injected interstitial effects and the surface effects in ion-irradiated metals.

The free surface effects and the injected interstitials effect on void swelling were systematically studied based on a series of self-ion irradiation of single crystal pure Fe. The void denuded zone width ∆x obtained from the summation of the width measured from TEM images and sputtering thickness loss calculated by using SRIM. ∆x is linearly proportional to D_vK1/4, where D_v is the vacancy diffusivity and K is the averaged dpa rate in the near-surface region. The activation energy of ∆x is 1.65±0.03 eV for Fe self-ion irradiation. The sudden drop of void densities near the surface determines the region affected by the surface effect. Rate theory simulations by using MOOSE code were used for comparison. The deviation of voids sizes from the expected dpa dependence is used to define the region influenced by the interjected interstitial effects.

A safe zone map for Fe irradiated by Fe ions was established. For the 1 MeV irradiation, although voids appeared after irradiation, the whole region is affected by both the surface effect and the injected interstitials effect, and there is no safe analysis zone. Therefore, the capability to accurately identify the safe zone is critical for any attempt to use local dpa dependence to predict swelling as a function of damage level. Especially for the low energy irradiations, which were used very frequently in previous studies, it may have a narrow region that can be safely used for the analysis.