A nuclear steam supply system having a start-up sub-system for heating a primary coolant. The nuclear steam supply system comprises a reactor vessel with core comprising nuclear fuel, and steam generating vessel fluidly coupled to the reactor vessel. A primary coolant loop formed within the reactor vessel and the steam generating vessel circulates primary coolant through the loop. A steam supply start-up sub-system is fluidly coupled to the primary coolant loop. The start-up sub-system is configured and operable to: (1) extract and receive a portion of the primary coolant from the primary coolant loop; (2) heat the portion of the primary coolant to form a heated portion of the primary coolant; and (3) inject the heated portion of the primary coolant back into the primary coolant loop.