A pressurized water nuclear reactor (PWNR) includes a core having a containment shield surrounding a reactor vessel having fuel assemblies that contain fuel rods filled with fuel pellets, and control rods, and a steam generator thermally coupled to the reactor vessel. A flow loop includes the steam generator, a turbine, and a condenser, and a pump for circulating a water-based heat transfer fluid in the loop. The heat transfer fluid includes a plurality of nanoparticles having at least one carbon allotrope or related carbon material dispersed therein, such as diamond nanoparticles.