Nuclear steam supply system

A method of heating a primary coolant to a no-load operating temperature in a nuclear steam supply system, the method comprising:

• a) filling a primary coolant loop within a reactor vessel and a steam generating vessel that are fluidly coupled together with a primary coolant;
• b) drawing a portion of the primary coolant from the primary coolant loop and into a start-up sub-system;
• c) heating the portion of the primary coolant within the start-up sub-system to form a heated portion of the primary coolant;
• d) injecting the heated portion of the primary coolant into the primary coolant loop;
• wherein upon injecting the heated portion of the primary coolant into the primary coolant loop, the injection causes the primary coolant to flow through the primary coolant loop which comprises:
• e-1) flowing the primary coolant in a first vertical direction through a riser column within the reactor vessel;
• e-2) flowing the primary coolant in a first lateral direction from the reactor vessel and into a riser pipe within the steam generating vessel;
• e-3) flowing the primary coolant in the first vertical direction through the riser pipe within the steam generating vessel;
• e-4) flowing the primary coolant in a second vertical direction through tubes in the steam generating vessel, the second vertical direction being opposite the first vertical direction;
• e-5) flowing the primary coolant in a second lateral direction from the tubes in the steam generating vessel into a downcomer within the reactor vessel;
• and
• e-6) flowing the primary coolant from the downcomer and into the riser column.