Nuclear Science User Facilities 90 Neutron Irradiation Performance of Fe–Cr Base Alloys James Stubbins – University of Illinois at Urbana-Champaign – Figure 1. A comparison of (a) dislocation decoration in Fe irradiated to 1 dpa at 300℃, and (b) uniform dislocation loops in Fe–10Cr irradiated to 1 dpa at 300℃. Advanced nuclear reactors come with inherent safety and improved energy effi- ciency, but they also require harsh operating conditions, such as elevated temperatures, high-dose neutron exposure, and corrosive environment. Fe–Cr-base ferritic/martensitic (F/M) alloys are considered lead candidate materials for in-core (cladding, ducts, and wrappers) and out-of-core (pres- sure vessel, piping) structural applica- tions in advanced reactors. Compared to austenitic alloys, F/M alloys have higher thermal conductivity, a lower thermal expansion coefficient, and superior resistance to swelling and helium embrittlement. However, at relatively low temperatures, F/M alloys are susceptible to irradiation hardening and embrittlement. The irradiation hardening and embrittlement in F/M alloys are known to be related to two types of defects created by neutron irradia- tion: (1) dislocation loops and (2) precipitates such as Cr-rich α' phase, Ni, Mn, Si-rich G-phase, and Cu-rich phase. It is critical to understand the microstructural evolution and governing mechanisms, in order to design advanced radiation-resistant alloys that can meet the requirements of next-generation reactors. Project Description In this research program, a coordi- nated set of ATR neutron-irradiation experiments with post-irradiation examination (PIE) on Fe–Cr base alloys were carried out, and several rapid-turnaround experiment (RTE) projects focusing on the PIE of irradiated specimens are ongoing. The suite of alloys ranges from pure This project included several firsts including the first irradiations of low dose (0.01 and 0.1 dpa) specimens at ATR, first use of MR- CAT for in situ tensile tests of ATR specimens, and first use of NSUF facilities for Positron Annihilation Research.