The overarching goal of this research is to advance understanding of, model and predict embrittlement primarily due to hardening by precipitates in Reactor Pressure Vessel (RPV) steels as it pertains to 80-year life extension of fission reactors. The RPV is responsible for primary radioactive containment and pressurizing water in the nuclear reactor. The most immediate materials safety problem facing the RPV steels is neutron irradiation embrittlement, which is the reduction of fracture toughness. Embrittlement is caused by irradiation hardening (Δσy), mainly as a result of precipitation, and manifested as upward ductile-to-brittle transition temperature shifts (ΔT). The excess concentration of defects created under irradiation leads to radiation enhanced diffusion (RED), which greatly accelerates precipitation normally limited by extremely slow kinetics at ≈ 300ᴼC, or at typical reactor operating temperatures. Concomitantly, the radiation induced segregation (RIS) of solute elements leads to enrichment at microstructural features which likely plays a role in the heterogeneous nucleation of precipitates. At lower fluences (ϕt), or displacements per atom (dpa), the hardening features are nm-scale copper rich precipitates (CRPs) and solute defects vacancy complexes at trace impurity levels of > ≈ 0.06 wt% Cu. Effective Cu concentrations are less than ≈ 0.25 wt%. However, at higher ϕt much larger quantities of alloying elements precipitate to form Mn, Ni, Si nm-scale intermetallic precipitates known as MNSPs. Large volume fractions (fv) of MNSPs cause severe hardening and embrittlement. The variables controlling the formation and character of severely embrittling Mn-Ni-Si precipitates (MNSP) under neutron irradiation are neither fully understood nor explicitly treated in regulatory models. There are five primary questions addressed by this work. 1) What is the quantitative effect of temperature (Ti), fluence/dose (ϕt), flux/dose rate (ϕ), particle type, product form and alloy composition on precipitate size, number density, volume fraction, composition and magnetic character of CRPs and MNSPs? 2) Are very high Ni alloys, with high strength and toughness, irradiation tolerant at low Mn; and how does precipitation and hardening occur in steels with a much wider range of Mn, Ni and Si that conventional normalized and tempered bainitic steels? 3) How can charged particle irradiations (CPI) compliment neutron data to gain insight into precipitation mechanisms? 4) Are MNSPs enhanced or induced by irradiation (equilibrium versus non-equilibrium) and can this ongoing controversy by establishing their detailed character, long-time thermal stability, and formation mechanisms? 5) How can these insights help build an improved high fluence, low flux predictive embrittlement model for steels used in in 80-year extended power reactor service? Atom probe tomography (APT), small angle neutron scattering (SANS) and transmission electron microscopy (TEM) show the presence of significant volume fractions of MNSPs in all RPV steels at the high 80-year fluence. Precipitation is reflected in Δσy, as characterized by tensile, shear and microhardness tests. Ni generally plays the strongest role in the formation of MNSPs. In the absence of sufficient Cu and CRPs, and at low to intermediate Ni, the MNSPs homogeneous nucleation rates are negligible in a defect free matrix. Heterogenous precipitation occurs at microstructural features, such as segregated network dislocation and irradiation induced interstitial loop sites. Units of % for compositions refers to atomic percent (%) unless otherwise noted. For typical RPV steel compositions (0.75% to 1.6% Ni with ≥ 0.8% Mn and 0.4 – 1.2% Si) MNSPs compositions are generally similar to G (Mn6Ni16Si7) and Γ2 (Mn2Ni3Si) phases. The formation of these phases is predicted by Calphad and has been confirmed by synchrotron X-Ray and TEM Diffraction measurements in a number of cases. However, in alloys with very low or very high Ni, coupled with variations in Mn and Si, other phase compositions are selected. For example, Ni-silicide type compositions are found in alloys with very low ≤ 0.24 Mn and high ≈ > 3% Ni. Notably at normal levels of > 0.8% Mn, very large MNSP fv form in 3.5% Ni steels at high fluence. The MNSP high Ni fv decreases approximately linearly with the decreasing alloy Mn content. Thus precipitation hardening is much lower in high ≈ 3.5% Ni and ≤ 0.24% Mn steels due to what is described as Mn starvation. For the so-called ATR-2 irradiation condition, which is the focus of this study, the high Ni fv ≈ 2.44 and 0.69% at 1.04 and 0.24% Mn, respectively. The resulting Δσy, which is well correlated with the √fv (as predicted by dispersed barrier hardening models), are 472 and 260 MPa. At high ϕ, a very high N and fv of MNSPs and CRP-MNSPs are observed in low Cu and Cu bearing alloys, respectively. In the latter case, Cu core Mn-Ni-Si shell CRP structures formed at lower dpa evolve into CRP-MNSP appendage co-precipitate features at high dpa. MNSP compositions formed in in rapid and convenient self-ion charged particle irradiation (CPI) are very similar to those found in neutron irradiations (NI). High dpa CPI produce fewer and larger precipitates than in NI. Further, higher dpa are needed to form the same precipitate fv for CPI versus NI conditions. The delayed precipitation is consistent with enhanced recombination of vacancies and SIA defects at the higher CPI dpa rates, which reduces the efficiency of RED. The MNSP grow slowly, but eventually reach large fv at very high dpa. Notably, fv correlates well with the G and Γ2 phase solute product, (Ni16Mn6Si7)(1/29) and (Ni3Mn2Si1)(1/6) and, at high dpa, is close to the equilibrium values, slightly modified by the Gibbs-Thomson effect. However, in steels with very low Mn and high Ni, Ni2-3Si silicide phase type precipitate compositions are observed; and when Ni is low, the precipitate compositions are close to the MnSi phase field. A comparison of dispersed barrier model predictions with measured hardening data suggests that the Ni-Si dominated precipitates are weaker dislocation obstacles than the G phase type MNSPs. Ultimately, fv from the CPI can be used to estimate y (and T) at lower service relevant dpa. While not quantitatively precise, this allows scoping studies of the embrittlement sensitivity of new RPV alloys. Post irradiation annealing (PIA) was used to clarify the irradiation induced versus enhanced controversy regarding dominant nanoscale Mn-Ni-Si precipitate (MNSP) formation mechanisms in pressure vessel steels. Radiation induced, non-equilibrium, MNSPs would dissolve under high temperature PIA, while radiation enhanced precipitates would be stable above a critical radius (rc). A Cu-free, high Ni steel was irradiated with 2.8MeV Fe2+ ions at two temperatures to generate MNSPs with average radii (r ̅) above and below an estimated rc for PIA at 425°Cup to 52 weeks. The complementary APT and Energy Dispersive X-ray Spectroscopy studies show MNSPs with r < rc dissolved, while those with r > rc slightly coarsened, consistent with thermodynamic predictions. Note, the MNSPs would be even more thermodynamically stable at much lower neutron irradiation RPV service temperatures around 290°C. Finally, the ultimate goal of this research is to create and analyze the high ϕt, intermediate ϕ ATR-2 database on both Δσy and microstructural changes in a large number of irradiated alloys. In the final section, ATR-2 results are integrated with a variety of other (mostly UCSB and surveillance) databases to develop a new high ϕt-low ϕ predictive embrittlement chemistry factor. Special emphasis is placed on the Δσy contributions of MNSPs, which are observed in a wide range of RPV steels at high ϕt. This directly informs the Odette Research Group’s development of an advanced embrittlement model which accurately predicts ΔT for low ϕ high ϕt conditions up to 80 full power years of operation.