Nuclear Science User Facilities 82 Investigation of Dislocation Loop Hardening and Stability in Irradiated RPV Steels Peter Wells – University of California, Santa Barbara – Figure 1. Atom probe maps from the Ni-free steel (LA) showing evolution of both CRP and dislocation loop enrichment under annealing. One potential barrier to extending light water reactor (LWR) lifetimes to 80 years is embrittlement of their massive reactor pressure vessels (RPV). Embrittlement is primarily due to the formation of nm-scale precipitates as well as solute defect clusters, which cause hardening and a corresponding increase in the ductile-to-brittle transition tempera- ture. Safely extending the lifetime of the US fleet of LWRs will require robust prediction models that not only include the effect of irradiation variables such as neutron flux, fluence, and operating temperature, but also alloy chemistry.This research aims to better understand the role of alloy Ni content in the formation and thermal stability of hardening features formed under irradiation.