The objective of this project is to understand inter-phase localized fracture, a non-hardening irradiation embrittlement mechanism not previously considered in reactor pressure vessel (RPV) steels. Irradiation embrittlement of RPV steels is one of the most crucial challenges facing continued operation of light water reactors, and is categorized as hardening-type or non-hardening embrittlement. Most studies have focused on the former, ascribing hardening embrittlement to dislocation pinning on loops and nanoclusters. On the other hand, non-hardening embrittlement is typically attributed to P radiation-induced segregation causing grain boundary decohesion. However, a new form of non-hardening embrittlement recently identified in dual-phase low-alloy steels (e.g., RPV steels) is localized fracture caused by stress concentrations at ferrite/bainite inter-phase boundaries – this mechanism has never before been considered in RPV steels. Because the ferrite phase fraction in RPV steel increases with irradiation, we hypothesize that inter-phase localized fracture will be exacerbated, playing a significant role in the overall irradiation embrittlement. We will conduct scanning electron microscopy (SEM) in situ uniaxial and triaxial straining with subsequent characterization of regions of interest, enabling us to directly visualize inter-phase localized fracture. Work will focus on as-received control specimens and proton irradiated specimens of SA508 RPV steel, intentionally selected to encompass a range of ferrite phase fractions. We have already characterized the dislocation loops, nanoclusters, and estimated irradiation embrittlement in these specimens through our ongoing NSUF program. By leveraging these previous results, the proposed project can have heightened impact by enabling us to develop a unified mechanism that captures both hardening-type and non-hardening type embrittlement in RPV steels.

The DOE Office of Nuclear Energy (DOE-NE) mission is to advance nuclear energy science and technology to meet U.S. energy, environmental, and economic needs. Toward this end, DOE-NE has identified five mission-oriented goals: (1) enable continued operation of existing reactors, (2) enable deployment of advanced reactors, (3) develop advanced fuel cycles, (4) position the US to maintain leadership in nuclear energy technology, and (5) enable a high-performing organization. This project will discover and understand a new mechanism for nuclear reactor pressure vessel (RPV) irradiation embrittlement. RPV irradiation embrittlement is the most crucial challenge facing the continued operation of existing light water reactors (LWRs), directly addressing Goal 1. Further, understanding this new embrittlement mechanism will enable us to develop accurate component reliability and lifetime models for advanced reactors such as small modular reactors (SMRs), which are planning to use low-alloy steel vessels, thus addressing Goals 2 and 3. Identification of new RPV degradation mechanisms that will have implications to the global fleet of reactors will directly address Goal 4. Finally, the proposed project leverages previous NSUF results to heighten the scientific impact of the proposed work, as well as utilizes some of NSUF’s newest capabilities for in situ measurement of deformation and stress concentrations in irradiated materials. Thus, this project directly fulfills Goal 5 by enhancing DOE-NE’s return on investment. Additionally, the impact of this project on LWR life extensions will add value to the DOE-NE LWR Sustainability Program. Moreover, the new embrittlement mechanism identified will need to be incorporated into degradation models within the DOE-NE Nuclear Energy Advanced Modeling & Simulations (NEAMS) Program.