Large volume fractions of Mn–Ni–Si (MNS) precipitates formed in irradiated light water reactor pressure vessel (RPV) steels cause severe hardening and embrittlement at high neutron fluence. A new equilibrium thermodynamic model was developed based on the CALculation of PHAse Diagrams (CALPHAD) method using both commercial (TCAL2) and specially assembled databases to predict precipitation of these phases. Good agreement between the model predictions and experimental data suggest that equilibrium thermodynamic models provide a basis to predict terminal MNS precipitation over wider range of alloy compositions and temperatures, and can also serve as a foundation for kinetic modeling of precipitate evolution.