2018 | ANNUAL REPORT 55 acterization techniques available include small-angle neutron and X-ray scattering (SANS and SAXS), transmis- sion electron microscopy (TEM), atom probe tomography (APT), and X-ray diffraction (XRD). UCSB-1 also included two lab-on-a-chip experi- ments, including diffusion multiples, to characterize multi-constituent alloy thermo-kinetics under irradiation, and in situ He injection assemblies to produce samples to evaluate the effects of a wide range of He/dpa ratios. The library concept irradiation was extended to what might best be described as a “reading-club” experimental campaign. UCSB organized various collaborations around different sub-experiments, with researchers expert in different post-irradiation examination (PIE) techniques.As an example, the Fe-Cr series sub-experiment for one irradia- tion condition involved APT studies led by Professor Emmanuelle Marquis at the University of Michigan (UM). TEM characterization was led by Dhriti Bhattacharyya at the Australian Nuclear Science andTechnology Organization (ANSTO), and irradia- tion hardening measurements were performed at UCSB.These data were then combined and analyzed by UCSB to develop a new microstructure- based hardening model that has been successfully extended to predict yield stress changes (∆σy ) in tempered martensitic structural steels.The combination of expertise and insight provided by a reading-club approach resulted in a whole that is greater than the sum of its parts. Accomplishments High-fluence Embrittlement of RPV Steels Light-water reactor pressure vessels (RPVs) are exposed to a low flux of neutrons that cause irradiation hardening and embrittlement, which manifests as a growing degradation of their fracture resistance with increasing fluence. Plant life extension of up to 80 years requires rigorous proof that the RPV maintains a very large safety margin to protect against brittle frac- ture under all conceivable conditions, including severe accident transients. Embrittlement manifests as an upward shift in the temperature marking the transition from brittle cleavage to ductile fracture. RPV embrittlement is reasonably well understood and predicted up to the normal licensed plant life of 40 years; however, limited surveillance data is available for extended life, and current regulatory models underpredict accelerated test- reactor data at high fluence, as shown in Figure 2a. Current embrittlement regulations reflect the strong effect of Cu and Ni on embrittlement and are associated with the rapid forma- tion of Cu-rich precipitates (CRPs) that harden and embrittle the steel. Theoretical models long ago predicted a new embrittlement mechanism, associated with the formation of so-called “late blooming” Mn-Ni-Si precipitates (MNSPs), which cause severe and unexpected hardening and embrittlement at high fluence, even in nominally radiation-tolerant, Cu-free RPV steels.As studies from UCSB long ago confirmed, the MNSPs are real and highly embrittling.The multifaceted question is this: at what combina- tion of fluences, alloy compositions, irradiation temperatures and fluxes do MNSPs form? Further, it is important to know what MNSPs are, how much of them develop, and how they relate to hardening and embrittlement? The UCSB-1 experiment was enor- mously successful in addressing this challenge.Work by then Ph.D. student Peter Wells considered split melt alloys with controlled variations in Cu and Ni contents.APT and SANS showed large volume fractions (f) of MNSPs, which are approximately independent of the alloy Cu content at the high UCSB-1 experiment fluence [1]. The MNSP f increases approximately linearly with the Ni and Cu content of the alloy f ≈ 0.92(2Ni + Cu), consistent with the observation that MNSPs contain roughly equal fractions of Ni and Mn + Si, a result which is characteristic of the compositions of nearby G and Γ2 phases.These results are also consistent with CALPHAD