Nuclear Science User Facilities 56 thermodynamic predictions  and with XRD measurements  of the composition and structure of MNSPs. As illustrated in Figure 2b, the square- root total-precipitate volume fraction (√f) scales with the √[2Ni(MNSP) + Cu(CRP)], and the corresponding strengthening contribution scales with the √f in a dispersed-barrier hardening model. Figure 2c compares measured versus predicted (∆σy) for a number of alloys irradiated in various experi- ments, including UCSB-1. Figure 2d shows that in Cu-bearing alloys, the precipitates evolve from Cu-core-Mn- Ni shell structures to Cu-appendage structures, as predicted by a recent model based on the analysis of UCSB-1 APT data .The UCSB-1 data also played a key role in developing a detailed thermo-kinetic model of precipitation hardening and embrittle- ment in Cu-free RPV steels . In summary, while the fluxes and fluences are higher than directly pertinent to extended RPV service, the UCSB-1 library provided tremendous insight on late blooming phase MNSPs. Remaining questions center on the effects of lower flux and fluence and improved predictive embrittlement models.These are being addressed in the ongoing program of studies on the UCSB-2 experiment library. Microstructural and Microstructural Evolutions in Fe-Cr Model Alloys Fe-8Cr to Fe-18Cr tempered martens- itic and ferritic steels are the leading candidate structural alloy system for a wide range of advanced nuclear applications.While more damage resistant than fcc austenitic stainless steels, these bcc alloys experience neutron irradiation that drives complex nm-scale microstructural and microchemical evolutions that lead to hardening and embrittlement, as well as dimensional instabilities like irradiation creep and even swelling at very high fluence.The features that develop under irradiation include dislocation loops, α’ (Cr-rich) precipi- tates, Mn, Ni, Si, Cr, P solute clusters, solute segregation, helium bubbles and, in some cases, growing voids at high fluence.Thus, one objective of developing the UCSB-1 library was to explore a number of these phenomena in a series of model Fe-3Cr to Fe-18Cr binary alloys that were irradiated side- by-side under the same conditions. Figure 3a compares APT examina- tions of Fe-3Cr to Fe-18Cr binary alloys irradiated at 593 K to 1.8 dpa . Precipitation of α’ was observed in all of the alloys containing more than 9% Cr.APT was used to fully quantify the number densities, size distributions, average radii, and mole fraction of α’, and indicated nearly full α’ to α decomposition. Both mole fraction and the matrix Cr composi- tions are in good agreement with the 593 K solvus boundary predicted in a recently published, first-principles guided, Fe-Cr phase diagram (which, notably, deviates substantially from earlier Cr-solubility limits based on classical CALPHAD thermodynamics). The number density and mean radius increase and decrease, respectively, with increasing Cr.These observations are consistent with classical nucleation and growth mechanisms of thermal α’ precipitation, that are greatly acceler- ated by radiation-enhanced diffusion at the low irradiation temperature. Figure 3b shows that in the sub-satu- rated Fe-6Cr alloy, Si and Cr segregate, both in small clusters and visible interstitial dislocation loops .The primarily Si and Cr clusters are likely It is important to emphasize that there are a large number of untapped oppor- tunities remaining in the UCSB-1 library,and what has been accomplished to date can be considered just the tip of the iceberg.