2018 | AFC ACCOMPLISHMENTS 148 There is an ongoing need to evaluate the effects of neutron irradiation on the structure and mechanical properties of existing and candidate nuclear reactor core structural materials. However, performing neutron irradiations is expensive, time consuming, creates radioactive material, and the available neutron irradiation facilities across the world have become much more limited. Using ion irradiations to estimate neutron irradiation effects has been a known technique for more than 40 years. Ion irradiations are quicker to perform, much less expensive, and do not produce radioactive material. But until recently it was not a favored technique because there was a concern that ion irradiations do not sufficiently estimate neutron irradiation effects. In recent years, there has been a strong push to understand and improve ion irradiation techniques with the intent of providing a better simulation of neutron irradiation effects.The results of one such study to better understand ion irradiations are presented here. Project Description: When a material is bombarded with neutrons, the majority of the Fate of Injected Ions During Ion Irradiation of Core Structural Materials Principal Investigator: Mychailo B.Toloczko, PNNL Collaborators: Jing Wang, PNNL neutrons readily leave the material after collisions with atoms within the material. However during ion irradiations, nearly all of the ions remain in the material, and for high dose ion irradiations that are needed to assess the performance of core structural materials over the lifetime of the reactor or clad and duct during high burn up conditions, a large number of ions are deposited.The depth distribution of where these atoms initially come to rest is readily determined, and this distribution has a strong peak. Microstructural examinations of ion irradiated materials are typically conducted at regions away from this peak without regard for the possibility that the injected atoms may diffuse away from their original stopping point, potentially affecting microstructural evolution in the investigated region. For example these atoms could redistribute to grain boundaries, and it would be unclear if the enhancement of an element on a grain boundary was due to the inherent behavior of the material or due to the injected atoms. Thus it is important to understand the behavior of self-ions in order to fully assess the ability for ion irradiations to simulate neutron irradiations. Novel material analysis techniques are being used to understand the details of ion irradiation conditions allowing ion irradiations to be more effectively applied to estimating neutron irradiation effects in reactor core structural materials. Understanding the fate of injected atoms is especially difficult when the ion being used is also a primary constituent element of the material being studied.This use of "self- ions" is actually strongly preferred because there is less chance that the injected atom will strongly affect the microstructure.While it possible to track the combined concentration of the injected self-ions and the base element using traditional composition measurement tools such as energy dispersive x-ray spectroscopy (EDS) in a transmission electron microscope, the actual fate of the injected atoms cannot be tracked by this method. Researchers at Pacific Northwest National Laboratory (PNNL) have devised a way to track these injected atoms by ion irradiating a material using self-ions of low natural abundance isotopes of an element. Because of their low natural abundance, these injected atoms are readily distinguished from the constituent element in the material using mass sensitive composition measurement tools such as secondary ion mass spectroscopy (SIMS) and atom probe tomography (APT). Initial results if self-ion fate studies are presented here.