Method for installing reactor coolant loop (RCL) piping of a reactor coolant system (RCS) system in a pressurized water reactor (PWR) nuclear power plant by positioning the reactor vessel, steam generator, or both, into position after attaching one or more coolant pumps, surveying coolant loop piping and a plurality of nozzles on the reactor vessel, the steam generator and coolant pump casing, or a combination thereof, with a precise measurement and 3D modeling technology for positioning data, then performing weld prep machining for the coolant loop piping on the reactor vessel side, and machining for the coolant loop piping on the steam generator side and coolant pump side based on the positioning data, and installing temporary supports for coolant loop piping and finishing the fit-up and welding between the coolant loop piping and the reactor vessel, steam generator, and each coolant pump.